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The Impact of Cannabis Decriminalization and Legalization on Road Safety Outcomes: A Systematic Review

  • Sarah B. Windle
    Correspondence
    Address correspondence to: Sarah B. Windle, MPH, Institute for Health and Social Policy, School of Population and Global Health, McGill University, 2001 McGill College Avenue, Suite 1100, Montreal, Quebec H3A 1G1, Canada.
    Affiliations
    Department of Epidemiology, Biostatistics and Occupational Health, School of Population and Global Health, McGill University, Montreal, Quebec, Canada

    Institute for Health and Social Policy, School of Population and Global Health, McGill University, Montreal, Quebec, Canada
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  • Peter Socha
    Affiliations
    Department of Epidemiology, Biostatistics and Occupational Health, School of Population and Global Health, McGill University, Montreal, Quebec, Canada
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  • José Ignacio Nazif-Munoz
    Affiliations
    Programmes d’études et de recherche en toxicomanie, Faculté de médecine et des sciences de la santé, Université de Sherbrooke, Longueuil, Quebec, Canada
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  • Sam Harper
    Affiliations
    Department of Epidemiology, Biostatistics and Occupational Health, School of Population and Global Health, McGill University, Montreal, Quebec, Canada

    Institute for Health and Social Policy, School of Population and Global Health, McGill University, Montreal, Quebec, Canada
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  • Arijit Nandi
    Affiliations
    Department of Epidemiology, Biostatistics and Occupational Health, School of Population and Global Health, McGill University, Montreal, Quebec, Canada

    Institute for Health and Social Policy, School of Population and Global Health, McGill University, Montreal, Quebec, Canada
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Published:September 25, 2022DOI:https://doi.org/10.1016/j.amepre.2022.07.012

      Introduction

      There is substantial debate concerning the impact of cannabis decriminalization and legalization on road safety outcomes.

      Methods

      Seven databases were systematically searched: Embase, MEDLINE, and PsycINFO through Ovid as well as Web of Science Core Collection, SafetyLit, Criminal Justice Database (ProQuest), and Transport Research International Documentation (from inception to June 16, 2021). Eligible primary studies examined group-level cannabis decriminalization or legalization and a road safety outcome in any population.

      Results

      A total of 65 reports of 64 observational studies were eligible, including 39 that applied a quasi-experimental design. Studies examined recreational cannabis legalization (n=50), medical cannabis legalization (n=22), and cannabis decriminalization (n=5). All studies except 1 used data from the U.S. or Canada. Studies found mixed impacts of legalization on attitudes, beliefs, and self-reported driving under the influence. Medical legalization, recreational legalization, and decriminalization were associated with increases in positive cannabis tests among drivers. Few studies examined impacts on alcohol or other drug use, although findings suggested a decrease in positive alcohol tests among drivers associated with medical legalization. Medical legalization was associated with reductions in fatal motor-vehicle collisions, whereas recreational legalization was conversely associated with increases in fatal collisions.

      Discussion

      Increased cannabis positivity may reflect changes in cannabis use; however, it does not in itself indicate increased impaired driving. Subgroups impacted by medical and recreational legalization, respectively, likely explain opposing findings for fatal collisions. More research is needed concerning cannabis decriminalization; the impacts of decriminalization and legalization on nonfatal injuries, alcohol and other drugs; and the mechanisms by which legalization impacts road safety outcomes.

      INTRODUCTION

      There has been a global shift toward cannabis decriminalization and legalization for medical and recreational purposes over the past several decades.
      • Decorte T
      • Lenton S
      • Wilkins C
      Introduction.
      This shift reflects efforts to both align the law with societal values and practices and reduce the harms caused by the criminalization of cannabis.
      • Eastwood N
      Cannabis decriminalization policies across the globe.
      Decriminalization, in which cannabis is not legal but is not considered a criminal offense (e.g., criminal sanctions may be replaced by civil sanctions such as fines or referral to drug rehabilitation), is estimated to have been implemented in >30 countries worldwide.
      • Eastwood N
      Cannabis decriminalization policies across the globe.
      Medical cannabis (e.g., requiring a prescription or other evidence of medical need) is legal in many countries, including Canada, and in more than half of all states in the U.S.

      Historical timeline: history of marijuana as medicine −2900 BC to present. Procon.org. https://medicalmarijuana.procon.org/historical-timeline/. Updated May 12, 2022. Accessed May 4, 2021.

      Uruguay, Canada, and Malta are the only countries to have legalized recreational cannabis on a national scale (in 2013, 2018, and 2021, respectively),

      History of recreational marijuana. Procon.org. https://marijuana.procon.org/history-of-recreational-marijuana/. Updated April 1, 2022. Accessed March 3, 2022.

      and since 2012, 18 U.S. states and the District of Columbia have legalized recreational cannabis.

      Recreational marijuana legality by stat. Procon.org. https://marijuana.procon.org/legal-recreational-marijuana-states-and-dc/. Updated June 6, 2022. Accessed March 3, 2022.

      There is substantial debate concerning the impact of cannabis decriminalization and medical and recreational legalization on potential harms, including drug-impaired driving and related outcomes (e.g., attitudes and beliefs, collisions, injuries).
      Government of Canada
      A framework for the legalization and regulation of cannabis in Canada: the final report of the Task Force on Cannabis Legalization and Regulation.
      Acute cannabis consumption is known to impair driving ability,
      • Busardò FP
      • Pellegrini M
      • Klein J
      • di Luca NM.
      Neurocognitive correlates in driving under the influence of cannabis.
      with meta-analyses estimating that cannabis impairment is associated with a low-moderate increase in collision risk.
      • Rogeberg O
      • Elvik R.
      The effects of cannabis intoxication on motor vehicle collision revisited and revised.
      However the precise economic and societal costs of cannabis-impaired driving are difficult to determine (e.g., because cannabis is frequently consumed with other legal and illegal substances,
      • Capler R
      • Bilsker D
      • Van Pelt K
      • et al.
      Cannabis use and driving: evidence review. Burnaby, British Columbia, Canada.
      and the conduct and reporting of drug testing vary across jurisdictions

      A Berning, DD Smither, Understanding the limitations of drug test information, reporting, and testing practices in fatal crashes. (Traffic Safety Facts Research Note. DOT HS 812 072). Washington, DC: National Highway Traffic Safety Administration. https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/812072#. Published November 2014. Accessed June 21, 2022.

      ).
      The liberalization of cannabis policies may impact drug-impaired driving through different mechanisms, including attitudes, beliefs, and social norms concerning the safety of driving under the influence of cannabis (DUIC)
      • Colonna R.
      Cannabis legalization and driving: exploring young Ontarians’ knowledge, perceptions and attitudes.
      ; the prevalence, frequency, and/or potency of cannabis consumed
      • Myran DT
      • Staykov E
      • Cantor N
      • et al.
      An analysis of the cannabis retail market in Canada 2 years following the legalisation of recreational cannabis.
      • Dragone D
      • Prarolo G
      • Vanin P
      • Zanella G.
      Crime and the legalization of recreational marijuana.
      • Kerr DCR
      • Bae H
      • Koval AL.
      Oregon recreational marijuana legalization: changes in undergraduates’ marijuana use rates from 2008 to 2016.
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      • Pacula RL
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      ; and the consumption of other drugs (e.g., alcohol, opioids), which may be either substitutes or complements to cannabis use.
      • Dragone D
      • Prarolo G
      • Vanin P
      • Zanella G.
      Crime and the legalization of recreational marijuana.
      ,
      • Anderson DM
      • Hansen B
      • Rees DI.
      Medical marijuana laws, traffic fatalities, and alcohol consumption.
      ,
      • Alley ZM
      • Kerr DCR
      • Bae H.
      Trends in college students’ alcohol, nicotine, prescription opioid and other drug use after recreational marijuana legalization: 2008–2018.
      However, assessing the causal impact of cannabis policies on road safety outcomes is challenging owing to a number of factors, including differential policy implementation (e.g., retail sales, home growing) and cointerventions such as cannabis per se driving limits.
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      Jurisdictions that liberalize cannabis policies may also differ from other jurisdictions in terms of time-varying factors, including regulations (e.g., seatbelt laws, graduated licensing, senior licensing, distracted driver laws, speed limits),
      • Anderson DM
      • Hansen B
      • Rees DI.
      Medical marijuana laws, traffic fatalities, and alcohol consumption.
      spillover effects from neighboring jurisdictions with legalization,
      • Lane TJ
      • Hall W.
      Traffic fatalities within U.S. states that have legalized recreational cannabis sales and their neighbours.
      and patterns of substance use
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      The epidemiology of drug abuse.
      ,
      • Freeman TP
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      • et al.
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      and motor-vehicle safety

      Historical fatality trends: car crash deaths and rates. National Safety Council. https://injuryfacts.nsc.org/motor-vehicle/historical-fatality-trends/deaths-and-rates/. Accessed September 24, 2021.

      over time.
      These and other jurisdiction-specific policy, legal, and demographic differences and/or residual biases from methodologic challenges in policy impact evaluation may explain the variability of findings in the literature. However, there is no comprehensive synthesis of the relevant evidence to date. Therefore, a systematic review of the impact of cannabis decriminalization and legalization on road safety outcomes was conducted to inform cannabis-related policy and interventions for the prevention of drug-impaired driving as well as to identify gaps and limitations in the current literature.

      METHODS

      A systematic review was conducted and reported in accordance with the PRISMA statement.
      • Page MJ
      • McKenzie JE
      • Bossuyt PM
      • et al.
      The PRISMA 2020 statement: an updated guideline for reporting systematic reviews.
      The protocol was submitted to the international prospective register of systematic reviews on June 15, 2021, before the completion of preliminary searches (CRD42021261243).

      SB Windle, P Socha, I Nazif-Munoz, S Harper, A Nandi, Impact of cannabis decriminalization and legalization on road safety outcomes. York, United Kingdom: International Prospective Register of Systematic Reviews, National Institute for Health Research. https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42021261243. Published 2021. Accessed July 15, 2021.

      Search Strategy

      Seven databases were systematically searched from inception to June 16, 2021: Embase, MEDLINE, and PsycINFO through Ovid as well as the Web of Science Core Collection, SafetyLit, Criminal Justice Database (ProQuest), and Transport Research International Documentation databases. Search terms included subject headings and text words for cannabis, policies, and road safety outcomes. No limits were placed on language or date of publication. The search strategies (Appendix 1, available online) were peer reviewed by a librarian in accordance with the peer review of electronic search strategies statement.
      • McGowan J
      • Sampson M
      • Salzwedel DM
      • Cogo E
      • Foerster V
      • Lefebvre C.
      PRESS peer review of electronic search strategies: 2015 guideline statement.
      Additional gray literature searches were conducted of relevant conference proceedings and preprint repositories to identify any relevant unpublished literature. The reference lists of included reports were also searched for additional eligible publications.

      Study Selection

      Search results were downloaded and deduplicated in EndNote X9.

      EndNote X9. Philadelphia, PA: Carivate; 2013.

      Unique references were transferred to Distiller SR, a systematic review software (Evidence Partners, Ottawa, Canada). Title/abstracts were screened in duplicate by 2 independent reviewers. Reports (including conference proceedings and abstracts) were included if they (1) contained primary data from an experimental, quasi-experimental, or observational study, with or without an external control group; (2) examined group-level cannabis decriminalization, legalization (medical or recreational), or a relevant policy dimension (e.g., retail sales); (3) reported at least 1 road safety outcome (e.g., attitudes concerning DUIC, biochemical measures of cannabis among drivers, traffic-related mortality) in any population; and (4) included a relevant comparator (e.g., pre-exposure for within-group comparisons or unexposed group[s] for between-group comparisons) (Appendix 2, available online). The full text was retrieved for any reference considered potentially eligible by either reviewer. Full texts were likewise screened in duplicate (Appendix 3, available online), with any reference meeting the inclusion criteria included in the review. In the event that a conference abstract or other brief report overlapped entirely with a full-text report, only the full text was included. Non-English language reports were subject to the same selection process.

      Risk of Bias

      Risk of bias of included studies was assessed independently by 2 reviewers using the Risk Of Bias In Non-randomized Studies–of Interventions tool.
      • Sterne JA
      • Hernán MA
      • Reeves BC
      • et al.
      ROBINS-I: a tool for assessing risk of bias in non-randomized studies of interventions.
      Bias was assessed to be low (comparable with a well-performed randomized trial), moderate (good nonrandomized study), serious (important problems), critical (too problematic to contribute useful evidence), or no information in 7 domains: confounding, selection of participants, classification of interventions, deviations from intended interventions, missing data, measurement error, and reporting.

      Data Synthesis

      Data, including publication, policy, and population characteristics; study methodology; and road safety outcomes, were extracted by 1 reviewer and validated by the second reviewer. Given substantial heterogeneity between studies, a narrative synthesis of the findings was conducted.

      RESULTS

      A total of 6,402 records were identified through database searches (Figure 1). After duplicates (n=2,665) were removed, 3,737 records remained for title/abstract screening. Of these, 143 records were considered potentially eligible and were retrieved for full-text review. An additional 10 reports identified from gray literature and citation searches were also assessed for eligibility. A total of 65 reports of 64 studies satisfied the criteria for inclusion.
      Figure 1
      Figure 1Flow diagram of study selection.
      Source: This figure was adapted from Page et al.
      • Page MJ
      • McKenzie JE
      • Bossuyt PM
      • et al.
      The PRISMA 2020 statement: an updated guideline for reporting systematic reviews.
      Included reports (Table 1
      • Anderson DM
      • Hansen B
      • Rees DI.
      Medical marijuana laws, traffic fatalities, and alcohol consumption.
      ,
      • Lane TJ
      • Hall W.
      Traffic fatalities within U.S. states that have legalized recreational cannabis sales and their neighbours.
      ,

      SB Windle, P Socha, I Nazif-Munoz, S Harper, A Nandi, Impact of cannabis decriminalization and legalization on road safety outcomes. York, United Kingdom: International Prospective Register of Systematic Reviews, National Institute for Health Research. https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42021261243. Published 2021. Accessed July 15, 2021.

      ,
      • Adams S
      • Cotti C
      Seatbelt use as a police avoidance strategy: a test using the legality of medical marijuana.
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      Crash fatality rates after recreational marijuana legalization in Washington and Colorado.
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      Fatal crashes in the 5 years after recreational marijuana legalization in Colorado and Washington.
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      Survey of U.S. drivers about marijuana, alcohol, and driving.
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      Marijuana use and driving in Washington State: risk perceptions and behaviors before and after implementation of retail sales.

      C Farmer, S Monfort, A. Woods, Changes in traffic crash rates after legalization of marijuana: results by crash severity. Arlington, VA: Insurance Institute for Highway Safety. https://www.iihs.org/topics/bibliography/ref/2229. Published 2021. Accessed June 23, 2022.

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      ) were published between 2013 and 2021, with more than half (n=35) published in 2019 or later. Most were peer reviewed (n=45), with the remaining being organizational reports (n=13), dissertations (n=3), conference abstracts (n=2), or working papers (n=2), and all were in English language. All studies were observational, including 39 studies that applied quasi-experimental designs (e.g., difference-in-differences, interrupted time series). Apart from a single study conducted using data from Uruguay,
      • Nazif-Munoz JI
      • Oulhote Y
      • Ouimet MC.
      The association between legalization of cannabis use and traffic deaths in Uruguay.
      studies used data from the U.S. only (n=59), Canada only (n=2), or both (n=2). Included studies examined cannabis decriminalization (n=5), medical cannabis legalization and/or retail sales (n=22), and recreational cannabis legalization and/or retail sales (n=50).
      Table 1Characteristics of Included Studies Examining the Impact of Cannabis Policies on Road Safety Outcomes
      First author (year)Study designPolicy intervention(s)Road safety outcome(s)Outcome data source(s)Overall risk of bias
      Assessed using the Risk Of Bias In Non-randomized Studies–of Interventions tool (Appendix 4, available online).
      Adams (2017)
      • Adams S
      • Cotti C
      Seatbelt use as a police avoidance strategy: a test using the legality of medical marijuana.
      Quasi-experimentalMedical cannabis legalizationProportion of drivers, drivers and front seat passengers, or backseat passengers wearing a seatbeltNational Occupant Protection Use Survey (NHTSA)Serious
      Anderson (2013)
      • Anderson DM
      • Hansen B
      • Rees DI.
      Medical marijuana laws, traffic fatalities, and alcohol consumption.
      Quasi-experimentalMedical cannabis legalizationMVC fatalities per 100,000 population; fatalities per 100,000 population resulting from collisions in which (1) at least 1 driver had BAC >0, (2) at least 1 driver had BAC ≥0.1, (3) or all drivers had BAC=0FARS (NHTSA)Moderate
      Aydelotte (2017)
      • Aydelotte JD
      • Brown LH
      • Luftman KM
      • et al.
      Crash fatality rates after recreational marijuana legalization in Washington and Colorado.
      Quasi-experimentalRecreational cannabis legalizationMVC fatalities per billion vehicle miles traveledFARS (NHTSA)Moderate
      Aydelotte (2019)
      • Aydelotte JD
      • Mardock AL
      • Mancheski CA
      • et al.
      Fatal crashes in the 5 years after recreational marijuana legalization in Colorado and Washington.
      Quasi-experimentalRecreational cannabis legalization, recreational cannabis retail salesMVC fatalities per billion vehicle miles traveledFARS (NHTSA)Moderate
      Banta-Green (2016)
      • Banta-Green C
      • Rowhani-Rahbar A
      • Ebel BE
      • Andris LM
      • Qiu Q.
      Cannabis use among drivers suspected of driving under the influence or involved in collisions: analyses of Washington State Patrol data.
      Quasi-experimentalRecreational cannabis legalizationProportion of drivers suspected of DUI or involved in serious collisions who test positive for THCToxicology laboratory, dispatch, and officer activity logs (Washington State Patrol)Serious
      Bartos (2020)
      • Bartos BJ
      • Newark C
      • McCleary R.
      Marijuana medicalization and motor vehicle fatalities: a synthetic control group approach.
      Quasi-experimentalMedical cannabis legalizationAnnual MVC fatalitiesFARS (NHTSA)Moderate
      Benedetti (2021)
      • Benedetti MH
      • Li L
      • Neuroth LM
      • Humphries KD
      • Brooks-Russell A
      • Zhu M.
      Demographic and policy-based differences in behaviors and attitudes towards driving after marijuana use: an analysis of the 2013-2017 Traffic Safety Culture Index.
      Cross-sectional surveyMedical cannabis legalization; recreational cannabis legalizationProportion of respondents who (1) self-reported driving within 1 hour of cannabis use at least once in the last year, (2) said that driving within 1 hour of cannabis use is somewhat or completely acceptable, and (3) somewhat or strongly supported a per se cannabis lawTraffic Safety Culture Index survey (AAA Foundation for Traffic Safety)Serious
      Benedetti (2021)
      • Benedetti MH
      • Li L
      • Neuroth LM
      • Humphries KD
      • Brooks-Russell A
      • Zhu M.
      Self-reported driving after marijuana use in association with medical and recreational marijuana policies.
      Cross-sectional surveyMedical cannabis legalization; recreational cannabis legalizationProportion of respondents who self-reported driving within 1 hour of cannabis use at least once in the last yearTraffic Safety Culture Index survey (AAA Foundation for Traffic Safety)Serious
      Berg (2018)
      • Berg CJ
      • Daniel CN
      • Vu M
      • Li J
      • Martin K
      • Le L.
      Marijuana use and driving under the influence among young adults: a socioecological perspective on risk factors.
      Cross-sectional surveyMedical cannabis retail sales; recreational cannabis retail salesProportion of respondents who self-reported DUIC at least once in the past 30 daysCross-sectional surveySerious
      Calvert (2020)
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      Quasi-experimentalMedical cannabis legalization; recreational cannabis legalization; recreational cannabis retail salesFatal MVCs and fatal pedestrian-involved MVCs per month per 100,000 populationFARS (NHTSA)Moderate
      Chamlin (2021)
      • Chamlin MB.
      An examination of the unintended consequences of the legalization of recreational marijuana on alcohol-related behaviors.
      Quasi-experimentalRecreational cannabis legalizationNumber of fatal MVCs per month that involved alcoholFARS (NHTSA)Serious
      Chung (2019)
      • Chung C
      • Salottolo K
      • Tanner A
      • et al.
      The impact of recreational marijuana commercialization on traumatic injury.
      Quasi-experimentalRecreational cannabis retail salesProportion of patients admitted for MVC-related traumatic injuries who tested positive for THC or other drugsHospital trauma registries (Traumabase, Digital Innovations, Inc.)Serious
      Cook (2020)
      • Cook AC
      • Leung G
      • Smith RA.
      Marijuana decriminalization, medical marijuana laws, and fatal traffic crashes in U.S. cities, 2010-2017.
      Quasi-experimentalCannabis decriminalization; medical cannabis legalizationFatal MVCs per capita; fatal MVCs per capita with BAC ≥0.08FARS (NHTSA)Moderate
      Couper (2014)
      • Couper FJ
      • Peterson BL.
      The prevalence of marijuana in suspected impaired driving cases in Washington State.
      CohortRecreational cannabis legalizationProportion of suspected impaired drivers testing positive for THC or 11-nor-9-carboxy-THCBlood toxicology results submitted by law enforcement officers (Washington

      State)
      Serious
      Cuttler (2018)
      • Cuttler C
      • Sexton M
      • Mischley LK.
      Driving under the influence of cannabis: an examination of driving beliefs and practices of medical and recreational cannabis users across the United States.
      Cross-sectional surveyRecreational cannabis retail salesProportion of respondents who self-reported (1) whether they believed that cannabis impairs their ability to drive safely, (2) whether they drive within 1 hour of using cannabis, and (3) whether they had ever been in an accident or received a ticket while DUICCross-sectional surveySerious
      Delling (2019)
      • Delling FN
      • Vittinghoff E
      • Dewland TA
      • et al.
      Does cannabis legalisation change healthcare utilisation? A population-based study using the healthcare cost and utilisation project in Colorado, USA.
      CohortRecreational cannabis legalization; recreational cannabis retail salesMedical diagnosis of MVC at hospital admissionHealthcare Cost and Utilization Project inpatient databasesSerious
      Eichelberger (2016)
      • Eichelberger AH.
      Survey of U.S. drivers about marijuana, alcohol, and driving.
      Cross-sectional surveyRecreational cannabis legalizationProportion of respondents who: (1) agreed that driving after using cannabis is a problem; (2) self-reported having driven within 2 hours of using cannabis in the past year; or (3) self-reported having driven within 2 hours of using alcohol in the past yearCross-sectional survey from Opinion America Group, LLCSerious
      Eichelberger (2019)
      • Eichelberger AH.
      Marijuana use and driving in Washington State: risk perceptions and behaviors before and after implementation of retail sales.
      Cross-sectional surveyRecreational cannabis retail salesProportion of drivers who tested positive for THC, tested positive for alcohol or both, self-reported the likelihood of cannabis impairing driving, and the risk of being arrested for cannabis-impaired drivingRoadside surveys (NHTSA; Insurance Institute for Highway Safety)Serious
      Farmer (2021)

      C Farmer, S Monfort, A. Woods, Changes in traffic crash rates after legalization of marijuana: results by crash severity. Arlington, VA: Insurance Institute for Highway Safety. https://www.iihs.org/topics/bibliography/ref/2229. Published 2021. Accessed June 23, 2022.

      Quasi-experimentalRecreational cannabis legalization; recreational cannabis retail salesMVCs with injuries per quarter; MVCs with fatalities per quarterState-maintained databasesModerate
      Fedorova (2021)
      • Fedorova EV
      • Ataiants J
      • Wong CF
      • Iverson E
      • Lankenau SE.
      Changes in medical cannabis patient status before and after cannabis legalization in California: associations with cannabis and other drug use.
      CohortRecreational cannabis legalizationSelf-reported DUIC in the past 90 daysCannabis, Health, and Young Adults studySerious
      Fink (2020)
      • Fink DS
      • Stohl M
      • Sarvet AL
      • Cerdá M
      • Keyes KM
      • Hasin DS.
      Medical marijuana laws and driving under the influence of marijuana and alcohol.
      Quasi-experimentalMedical cannabis legalizationSelf-reported DUIC in the past 12 months; self-reported DUI of alcohol in the past 12 monthsNational Longitudinal Alcohol Epidemiologic Survey (1991–1992); National Epidemiologic Survey on Alcohol and Related Conditions (2001–2002); NESARC-III (2012–2013)Serious
      Fowles (2021)
      • Fowles R
      • Loeb PD.
      The association between marijuana and motor vehicle crashes.
      Quasi-experimentalRecreational cannabis legalizationMVC fatalities per vehicle miles traveledFARS (NHTSA)Serious
      Goodman (2020)
      • Goodman S
      • Fischer B
      • Hammond D.
      Lower-risk cannabis use guidelines: adherence in Canada and the U.S.
      Cross-sectional surveyRecreational cannabis legalizationProportion of respondents who reported having ever driven within 2 hours of using cannabisInternational Cannabis Policy study (wave 1)Serious
      Grigorian (2019)
      • Grigorian A
      • Lester E
      • Lekawa M
      • et al.
      Marijuana use and outcomes in adult and pediatric trauma patients after legalization in California.
      CohortRecreational cannabis legalizationProportion of THC–positive patients with trauma whose mechanism of injury is MVC or motorcycle accidentHospital trauma registrySerious
      Guenzburger (2013)
      • Guenzburger GV
      • Masten SV.
      Changes in driver cannabinoid prevalence associated with implementing medical marijuana laws in 14 U.S. states.
      Quasi-experimentalMedical cannabis legalizationProportion of drivers involved in fatal collisions who test positive for cannabinoidsFARS (NHTSA)Serious
      Hake (2019)
      • Hake ML.
      Marijuana Legalization and Traffic Fatalities Involving Cannabinoids [dissertation].
      Quasi-experimentalRecreational cannabis legalizationProportion of drivers involved in fatal collisions who test positive for cannabinoidsFARS (NHTSA)Serious
      Hamzeie (2017)
      • Hamzeie R
      • Thompson I
      • Roy S
      • Savolainen PT.
      State-level comparison of traffic fatality data in consideration of marijuana laws.
      Quasi-experimentalCannabis decriminalization; recreational cannabis legalizationProportion of drivers involved in fatal collisions who test positive for any cannabis-related substanceFARS (NHTSA)Serious
      Hansen (2020)
      • Hansen B
      • Miller K
      • Weber C.
      Early evidence on recreational marijuana legalization and traffic fatalities.
      Quasi-experimentalRecreational cannabis legalizationProportion of fatal MVCs that involve at least 1 driver who is (1) cannabis positive, (2) alcohol positive, or (3) neither; total MVC-related fatalities per billion vehicle miles traveled that involve at least 1 driver who is (1) cannabis positive, (2) alcohol positive, or (3) neitherFARS (NHTSA)Moderate
      Hao (2020)
      • Hao Z
      • Cowan BW.
      The cross-border spillover effects of recreational marijuana legalization.
      Quasi-experimentalRecreational cannabis legalizationDUI arrests per 10,000 populationUniform Crime Reporting Program dataSerious
      Highway Loss Data Institute (2017)
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 34.
      Quasi-experimentalRecreational cannabis retail salesMVC claims per insured vehicle yearsHighway Loss Data InstituteModerate
      Highway Loss Data Institute (2018)
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 35.
      Quasi-experimentalRecreational cannabis retail salesMVC claims per insured vehicle yearsHighway Loss Data InstituteModerate
      Jones (2019)
      • Jones JM
      • Shults RA
      • Robinson B
      • Komatsu KK
      • Sauber-Schatz EK.
      Marijuana and alcohol use among injured drivers evaluated at level I trauma centers in Arizona, 2008–2014.
      Quasi-experimentalMedical cannabis legalizationProportion of drivers evaluated at Level I trauma centers who (1) were THC positive or (2) had BAC exceeding the legal limit for their age (≥0.08 g/dL for drivers aged ≥21 years and any conclusive level [BAC >0g/dL] for drivers aged 16–20 years) or (3) THC positive with any detectable alcoholArizona State trauma registrySerious
      Kamer (2020)
      • Kamer RS
      • Warshafsky S
      • Kamer GC.
      Change in traffic fatality rates in the first 4 states to legalize recreational marijuana.
      Quasi-experimentalRecreational cannabis retail salesMVC fatalities per billion vehicle miles traveledFARS (NHTSA)Serious
      Kim (2016)
      • Kim JH
      • Santaella-Tenorio J
      • Mauro C
      • et al.
      State medical marijuana laws and the prevalence of opioids detected among fatally injured drivers.
      Quasi-experimentalMedical cannabis Retail Sales and/or Allowances for Home CultivationProportion of fatally injured drivers who tested positive for opioidsFARS (NHTSA)Serious
      Lane (2019)
      • Lane TJ
      • Hall W.
      Traffic fatalities within U.S. states that have legalized recreational cannabis sales and their neighbours.
      Quasi-experimentalRecreational cannabis retail salesMVC fatalities per million residentsWide-Ranging Online Data for Epidemiologic Research web application (Centers for Disease Control and Prevention); RoadSafetyBCModerate
      Lee (2018)
      • Lee J
      • Abdel-Aty A
      • Park J.
      Investigation of associations between marijuana law changes and marijuana-involved fatal traffic crashes: a state-level analysis.
      Quasi-experimentalCannabis decriminalization; medical cannabis retail sales; recreational cannabis retail salesProportion of drivers involved in fatal collisions who test positive for THCFARS (NHTSA)Serious
      Lensch (2020)
      • Lensch T
      • Sloan K
      • Ausmus J
      • et al.
      Cannabis use and driving under the influence: behaviors and attitudes by state-level legal sale of recreational cannabis.
      Cross-sectional surveyRecreational cannabis retail salesSelf-reported risky behaviors and attitudes toward cannabis-impaired drivingInternational Cannabis Policy study (Wave 1)Serious
      Leung (2021)
      • Leung G
      • Dutra J.
      Legal access to marijuana and motor vehicle fatalities in the United States, 1990–2019.
      Quasi-experimentalMedical cannabis legalization; recreational cannabis legalizationNumber of MVC fatalitiesFARS (NHTSA)Moderate
      Masten (2014)
      • Masten SV
      • Guenzburger GV.
      Changes in driver cannabinoid prevalence in 12 U.S. states after implementing medical marijuana laws.
      Quasi-experimentalMedical cannabis legalizationProportion of drivers involved in fatal collisions who test positive for cannabinoidsFARS (NHTSA)Serious
      McGinty (2017)
      • McGinty EE
      • Niederdeppe J
      • Heley K
      • Barry CL.
      Public perceptions of arguments supporting and opposing recreational marijuana legalization.
      Cross-sectional surveyRecreational cannabis legalizationProportion of respondents who agree that legalization increases MVCsCross-sectional surveySerious
      Miller (2018)
      • Miller T.
      The Consequences of Legalizing Recreational Marijuana: Evidence From Colorado and Washington [dissertation].
      Quasi-experimentalRecreational cannabis legalization; recreational cannabis retail salesDUI arrest rate per 100,000 population; fatal MVCs involving at least 1 driver with BAC ≥0.08 or higher per 100,000 populationUniform Crime Reports –

      Crime in the United States (Federal Bureau of Investigation); FARS (NHTSA)
      Serious
      Monfort (2018)
      • Monfort SS
      Effect of recreational marijuana sales on police-reported crashes in Colorado.
      Quasi-experimentalRecreational cannabis retail salesMVCs per million passenger vehicle registrationsInsurance Institute for Highway SafetyModerate
      Nazif-Munoz (2020)
      • Nazif-Munoz JI
      • Oulhote Y
      • Ouimet MC.
      The association between legalization of cannabis use and traffic deaths in Uruguay.
      Quasi-experimentalRecreational cannabis legalizationMVC fatalities among light motor vehicles drivers and motorcyclistsNational Road Safety Agency of Uruguay and the Ministry of Transport and Public WorksModerate
      Notrica (2020)
      • Notrica DM
      • Sayrs LW
      • Krishna N
      • Rowe D
      • Jaroszewski DE
      • McMahon LE.
      The impact of state laws on motor vehicle fatality rates, 1999–2015.
      Quasi-experimentalCannabis decriminalization or legalizationAge-adjusted MVC mortality per 100,000 populationFARS (NHTSA)Moderate
      Otto (2016)
      • Otto J
      • Finley K
      • Ward NJ.
      An assessment of traffic safety culture related to driving after cannabis use.
      Cross-sectional surveyMedical cannabis legalization; recreational cannabis legalizationDUIC: behavior, intention, willingness, attitude; perceived norms (injunctive), perceived norms (descriptive)Driving under the Influence of Cannabis SurveySerious
      Pollini (2015)
      • Pollini RA
      • Romano E
      • Johnson MB
      • Lacey JH.
      The impact of marijuana decriminalization on California drivers.
      Cross-sectional surveyCannabis decriminalizationProportion of cannabinoid-positive drivers among drug-tested weekend drivers and among drug-tested fatally injured driversRoadside surveys; FARS (NHTSA)Serious
      Ramirez (2016)

      A Ramirez, A Berning, K Carr, et al. Marijuana, Other Drugs, and Alcohol Use by Drivers in Washington State. Washington, DC: National Highway Traffic Safety Administration Report No. DOT HS 812 299. https://rosap.ntl.bts.gov/view/dot/1977. Published 2016. Accessed June 23, 2022.

      Cross-sectional surveyRecreational cannabis retail salesProportion of drivers positive for THC, alcohol, other drugs only, or medicationsRoadside surveysSerious
      Rotermann (2020)
      • Rotermann M.
      What has changed since cannabis was legalized?.
      Cross-sectional surveyRecreational cannabis legalizationProportion of respondents with a driver's license who reported having driven within 2 hours of cannabis use in the past 3 months; proportion of respondents who reported having been a passenger in a vehicle driven by someone who had consumed cannabis within 2 hours in the past 3 monthsNational Cannabis SurveySerious
      Salomonsen-Sautel (2014)
      • Salomonsen-Sautel S
      • Min SJ
      • Sakai JT
      • Thurstone C
      • Hopfer C.
      Trends in fatal motor vehicle crashes before and after marijuana commercialization in Colorado.
      Quasi-experimentalMedical cannabis retail salesProportion of drivers involved in fatal collisions who (1) test positive for cannabis or (2) have BAC ≥0.08FARS (NHTSA)Serious
      Santaella (2017)
      • Santaella J
      • Mauro C
      • Wall MM
      • Kim JH
      • Martins SS.
      Reductions in traffic fatalities rates across states with operational dispensaries of marijuana.
      Quasi-experimentalMedical cannabis retail salesMVC fatalities per 100,000 populationFARS (NHTSA)Moderate
      Santaella-Tenorio (2017)
      • Santaella-Tenorio J
      • Mauro CM
      • Wall MM
      • et al.
      U.S. traffic fatalities, 1985–2014, and their relationship to medical marijuana laws.
      Quasi-experimentalMedical cannabis legalization; medical cannabis retail salesMVC fatalities per 100,000 populationFARS (NHTSA)Moderate
      Santaella-Tenorio (2020)
      • Santaella-Tenorio J
      • Wheeler-Martin K
      • Dimaggio CJ
      • et al.
      Association of recreational cannabis laws in Colorado and Washington state with changes in traffic fatalities, 2005–2017.
      Quasi-experimentalRecreational cannabis retail salesMVC fatalities per billion vehicle miles traveledFARS (NHTSA)Moderate
      Sevigny (2018)
      • Sevigny EL.
      The effects of medical marijuana laws on cannabis-involved driving.
      Quasi-experimentalCannabis decriminalization; medical cannabis legalization, home cultivation, and retail sales; recreational cannabis legalizationProportion of drivers involved in fatal MVCs who are THC positiveFARS (NHTSA)Serious
      Slater (2016)
      • Slater ME
      • Castle IJP
      • Logan BK
      • Hingson RW.
      Differences in state drug testing and reporting by driver type in U.S. fatal traffic crashes.
      CohortMedical cannabis legalizationProportion of drivers involved in fatal MVCs who are tested for drugsFARS (NHTSA)Serious
      Steinemann (2018)
      • Steinemann S
      • AGalanis D
      • Nguyen T
      • Biffl W.
      Motor vehicle crash fatalaties and undercompensated care associated with legalization of marijuana.
      CohortMedical cannabis legalizationProportion of fatally injured drivers who are positive for (1) THC, (2) methamphetamine, or (3) alcoholFARS (NHTSA); Hawaii State trauma registrySerious
      Tefft (2016)
      • Tefft BC
      • Arnold LS
      • Grabowski JG.
      Prevalence of marijuana involvement in fatal crashes: Washington, 2010–2014.
      CohortRecreational cannabis legalizationProportion of drivers involved in fatal MVCs who are THC positiveWashington State FARS (Washington Traffic Safety Commission)Serious
      Tefft (2020)

      Tefft BC, Arnold LS. Cannabis use among drivers in fatal crashes in Washington State before and after legalization Washington, DC: AAA Foundation for Traffic Safety; 2020. https://aaafoundation.org/wp-content/uploads/2020/01/19-0637_AAAFTS-WA-State-Cannabis-Use-Among-Drivers-in-Fatal-Crashes_r4.pdf.6. Published 2020. Accessed June 23, 2022.

      CohortRecreational cannabis legalizationProportion of drivers involved in fatal MVCs who are THC positiveWashington State FARS (Washington Traffic Safety Commission)Serious
      Temple (2020)
      • Temple C
      • Lai J
      • Carreiro S
      • Neavyn M.
      Traffic fatalities before and after legalization of adult-use cannabis.
      CohortRecreational cannabis retail salesMVC-related fatalities per monthFARS (NHTSA)Serious
      Vogler (2017)
      • Vogler J
      State marijuana policies and vehicle fatalities.
      Quasi-experimentalMedical cannabis legalization; recreational cannabis legalizationMVC-related fatalities per 100,000 population (quarterly): (1) overall, (2) no alcohol but cannabis positive; proportion of vehicle fatalities involving drunk drivers (without evidence of cannabis); proportion of drivers testing positive for both alcohol and cannabis; proportion of drivers testing positive for cannabisFARS (NHTSA)Moderate
      Wadsworth (2018)
      • Wadsworth E
      • Hammond D.
      Differences in patterns of cannabis use among youth: prevalence, perceptions of harm and driving under the influence in the USA where non-medical cannabis markets have been established, proposed and prohibited.
      Cross-sectional surveyRecreational cannabis legalization; recreational cannabis retail salesAmong youth who had ever used cannabis: ever driven within 2 hours of use; all respondents: ever been a passenger of a driver who used cannabis within 2 hours, do you think driving a vehicle within 2 hours of using cannabis increases accident risk, how likely are drivers to get caught by the policeInternational Tobacco Control Policy Evaluation Project tobacco and youth e-cigarette survey (wave 1)Serious
      Windle (2021)
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      Quasi-experimentalRecreational cannabis legalization; recreational cannabis retail salesFatal MVCs and MVC fatalities per 100,000 person-yearsFARS (NHTSA)Serious
      Winstock (2021)
      • Winstock AR
      • Lynskey MT
      • Maier LJ
      • Ferris JA
      • Davies EL.
      Perceptions of cannabis health information labels among people who use cannabis in the U.S. and Canada.
      Cross-sectional surveyRecreational cannabis legalizationRespondents were asked to respond to 3 measures related to a DUIC message: newness, believability, and potential to change behaviorGlobal Drug Survey 2019Serious
      Woo (2019)
      • Woo Y.
      The Effects of Cannabis and the Legalization of Marijuana on Fatal Crashes in Washington State [dissertation].
      Quasi-experimentalRecreational cannabis legalization; recreational cannabis retail salesFatal MVCs per month; fatal MVCs per month in which drivers were THC positiveWashington State FARS (Washington Traffic Safety Commission); police accident report filesSerious
      Woods-Fry (2019)
      • Woods-Fry H
      • Vanlaar WGM
      • Lyon C
      • Brown S
      • Robertson RD.
      Road safety monitor 2019: trends in marijuana use among Canadian drivers.
      Cross-sectional surveyRecreational cannabis legalizationProportion of respondents who reported (1) having driven a motor vehicle within 2 hours of using cannabis in the past 12 months, (2) having driven a motor vehicle within 2 hours of using cannabis and alcohol in the past 12 months, or (3) agreement that cannabis does not affect one's driving as much as alcoholRoad Safety Monitor (Traffic Injury Research Foundation; Beer Canada; Desjardins)Serious
      Woods-Fry (2020)
      • Woods-Fry H
      • Robertson RD
      • Vanlaar WGM.
      Road safety monitor 2020: trends in marijuana use among Canadian drivers.
      Cross-sectional surveyRecreational cannabis legalizationProportion of respondents who reported (1) having driven a motor vehicle within 2 hours of using cannabis in the past 12 months, (2) having driven a motor vehicle within 2 hours of using cannabis and alcohol in the past 12 months, (3) agreement that cannabis does not affect one's driving as much as alcohol, or (4) concern about drugged driversRoad Safety Monitor (Traffic Injury Research Foundation; Beer Canada; Desjardins)Serious
      BAC, blood alcohol concentration; DUI, driving under the influence; DUIC, driving under the influence of cannabis; FARS, Fatality Analysis Reporting System; MVC, motor-vehicle collision; NHTSA, National Highway Traffic Safety Administration; THC, tetrahydrocannabinol.
      a Assessed using the Risk Of Bias In Non-randomized Studies–of Interventions tool (Appendix 4, available online).
      The overall risk of bias of included reports was assessed to be moderate (n=19; moderate or low risk of bias in all the 7 domains) or serious (n=46; serious risk of bias in at least 1 domain) using the Risk Of Bias In Non-randomized Studies–of Interventions tool (Appendix 4, available online).
      • Sterne JA
      • Hernán MA
      • Reeves BC
      • et al.
      ROBINS-I: a tool for assessing risk of bias in non-randomized studies of interventions.
      Studies were assessed to be at either moderate (n=26) or serious (n=39) risk of confounding depending on the extent to which they addressed potential confounding (e.g., owing to time-varying factors). Risk of bias in the measurement of outcomes was assessed to be low (n=21), moderate (n=19), serious (n=23), or no information (n=2), with the most common sources of potential measurement bias related to drug-testing practices and self-reported outcomes. Selection of participants into the study was assessed to be low (n=32), moderate (n=7), or serious (n=26), often owing to selection that was potentially related to the policy and outcomes of interest. Given the absence of routine protocol registration for nonexperimental studies, most included studies were assessed to be at moderate risk of bias for selection of the reported results (n=60) if they clearly defined their outcome measurements and analyses, and these were internally consistent across the methods and results of the report. Risk of bias owing to missing data was largely considered low (n=30) or no information (n=30), and assessed risk of bias because of classification of interventions and deviations from intended interventions were low for all reports.

      Cannabis Decriminalization

      Five studies examined cannabis decriminalization (Appendix 5, available online).
      • Cook AC
      • Leung G
      • Smith RA.
      Marijuana decriminalization, medical marijuana laws, and fatal traffic crashes in U.S. cities, 2010-2017.
      ,
      • Hamzeie R
      • Thompson I
      • Roy S
      • Savolainen PT.
      State-level comparison of traffic fatality data in consideration of marijuana laws.
      ,
      • Lee J
      • Abdel-Aty A
      • Park J.
      Investigation of associations between marijuana law changes and marijuana-involved fatal traffic crashes: a state-level analysis.
      ,
      • Pollini RA
      • Romano E
      • Johnson MB
      • Lacey JH.
      The impact of marijuana decriminalization on California drivers.
      ,
      • Sevigny EL.
      The effects of medical marijuana laws on cannabis-involved driving.
      Four (all serious risk of bias) found increases in positive cannabis tests (referred to as cannabis positivity for a positive cannabis test in any bodily fluid in the remaining text) associated with decriminalization among drivers involved in fatal collisions (range of ORs: 1.10–1.67).
      • Hamzeie R
      • Thompson I
      • Roy S
      • Savolainen PT.
      State-level comparison of traffic fatality data in consideration of marijuana laws.
      ,
      • Lee J
      • Abdel-Aty A
      • Park J.
      Investigation of associations between marijuana law changes and marijuana-involved fatal traffic crashes: a state-level analysis.
      ,
      • Pollini RA
      • Romano E
      • Johnson MB
      • Lacey JH.
      The impact of marijuana decriminalization on California drivers.
      ,
      • Sevigny EL.
      The effects of medical marijuana laws on cannabis-involved driving.
      The fifth study (moderate risk of bias) found no difference in fatal motor-vehicle collisions (MVCs) (incidence rate ratio=1.02; 95% CI=0.95, 1.10) or fatal MVCs involving alcohol (incidence rate ratio=0.95; 95% CI=0.79, 1.15) associated with decriminalization in the U.S.
      • Cook AC
      • Leung G
      • Smith RA.
      Marijuana decriminalization, medical marijuana laws, and fatal traffic crashes in U.S. cities, 2010-2017.

      Medical Cannabis Legalization

      A total of 23 reports of 22 studies examined the impact of medical cannabis legalization or retail sales on road safety outcomes (Appendix 6, available online). Reported outcomes included self-reported attitudes, beliefs, and behaviors concerning driving under the influence; positive cannabis, alcohol, and other drug tests among drivers; seat belt use; and MVCs.
      Self-reported attitudes, beliefs, and behaviors concerning driving under the influence. All studies that examined these outcomes (n=5) were assessed to be at serious risk of bias. Two studies found no difference in attitudes and beliefs concerning DUIC between residents of U.S. states with medical cannabis legalization and those without.
      • Benedetti MH
      • Li L
      • Neuroth LM
      • Humphries KD
      • Brooks-Russell A
      • Zhu M.
      Demographic and policy-based differences in behaviors and attitudes towards driving after marijuana use: an analysis of the 2013-2017 Traffic Safety Culture Index.
      ,
      • Otto J
      • Finley K
      • Ward NJ.
      An assessment of traffic safety culture related to driving after cannabis use.
      Three studies found an association between legalization and greater self-reported DUIC,
      • Benedetti MH
      • Li L
      • Neuroth LM
      • Humphries KD
      • Brooks-Russell A
      • Zhu M.
      Demographic and policy-based differences in behaviors and attitudes towards driving after marijuana use: an analysis of the 2013-2017 Traffic Safety Culture Index.
      ,
      • Benedetti MH
      • Li L
      • Neuroth LM
      • Humphries KD
      • Brooks-Russell A
      • Zhu M.
      Self-reported driving after marijuana use in association with medical and recreational marijuana policies.
      ,
      • Fink DS
      • Stohl M
      • Sarvet AL
      • Cerdá M
      • Keyes KM
      • Hasin DS.
      Medical marijuana laws and driving under the influence of marijuana and alcohol.
      with 1 study noting that the finding was not consistent across all time periods and states,
      • Fink DS
      • Stohl M
      • Sarvet AL
      • Cerdá M
      • Keyes KM
      • Hasin DS.
      Medical marijuana laws and driving under the influence of marijuana and alcohol.
      whereas a fourth study found lower self-reported DUIC associated with retail sales.
      • Berg CJ
      • Daniel CN
      • Vu M
      • Li J
      • Martin K
      • Le L.
      Marijuana use and driving under the influence among young adults: a socioecological perspective on risk factors.
      Positive cannabis, alcohol, and other drug tests among drivers and patients with trauma. Seven studies examined the association between medical legalization and/or retail sales and cannabis positivity. All studies found increases after medical legalization or retail sales in pooled states
      • Sevigny EL.
      The effects of medical marijuana laws on cannabis-involved driving.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      or in at least 1 jurisdiction, including Arizona,
      • Jones JM
      • Shults RA
      • Robinson B
      • Komatsu KK
      • Sauber-Schatz EK.
      Marijuana and alcohol use among injured drivers evaluated at level I trauma centers in Arizona, 2008–2014.
      California,
      • Guenzburger GV
      • Masten SV.
      Changes in driver cannabinoid prevalence associated with implementing medical marijuana laws in 14 U.S. states.
      ,
      • Masten SV
      • Guenzburger GV.
      Changes in driver cannabinoid prevalence in 12 U.S. states after implementing medical marijuana laws.
      Colorado,
      • Salomonsen-Sautel S
      • Min SJ
      • Sakai JT
      • Thurstone C
      • Hopfer C.
      Trends in fatal motor vehicle crashes before and after marijuana commercialization in Colorado.
      Connecticut,
      • Lee J
      • Abdel-Aty A
      • Park J.
      Investigation of associations between marijuana law changes and marijuana-involved fatal traffic crashes: a state-level analysis.
      Hawaii,
      • Guenzburger GV
      • Masten SV.
      Changes in driver cannabinoid prevalence associated with implementing medical marijuana laws in 14 U.S. states.
      ,
      • Masten SV
      • Guenzburger GV.
      Changes in driver cannabinoid prevalence in 12 U.S. states after implementing medical marijuana laws.
      ,
      • Steinemann S
      • AGalanis D
      • Nguyen T
      • Biffl W.
      Motor vehicle crash fatalaties and undercompensated care associated with legalization of marijuana.
      and Washington.
      • Guenzburger GV
      • Masten SV.
      Changes in driver cannabinoid prevalence associated with implementing medical marijuana laws in 14 U.S. states.
      ,
      • Masten SV
      • Guenzburger GV.
      Changes in driver cannabinoid prevalence in 12 U.S. states after implementing medical marijuana laws.
      One study found decreases in cannabis positivity after retail sales in Arizona and New Jersey, relative to decreases in states with cannabis prohibition.
      • Lee J
      • Abdel-Aty A
      • Park J.
      Investigation of associations between marijuana law changes and marijuana-involved fatal traffic crashes: a state-level analysis.
      In the study assessed to have the lowest risk of bias (moderate), Vogler
      • Vogler J
      State marijuana policies and vehicle fatalities.
      (2017) found a 5.5% increase in cannabis positivity among drivers involved in fatal collisions after medical legalization in 22 pooled states, although the estimate was imprecise.
      Six studies examined alcohol positivity among drivers. Three studies (all serious risk of bias) did not find changes in alcohol positivity associated with medical legalization in Arizona
      • Jones JM
      • Shults RA
      • Robinson B
      • Komatsu KK
      • Sauber-Schatz EK.
      Marijuana and alcohol use among injured drivers evaluated at level I trauma centers in Arizona, 2008–2014.
      or Hawaii
      • Steinemann S
      • AGalanis D
      • Nguyen T
      • Biffl W.
      Motor vehicle crash fatalaties and undercompensated care associated with legalization of marijuana.
      nor associated with retail sales in Colorado.
      • Salomonsen-Sautel S
      • Min SJ
      • Sakai JT
      • Thurstone C
      • Hopfer C.
      Trends in fatal motor vehicle crashes before and after marijuana commercialization in Colorado.
      However, 3 other studies (all moderate risk of bias) that used pooled data from U.S. states or cities found decreases in alcohol positivity associated with medical legalization (range= –6% to –28%).
      • Anderson DM
      • Hansen B
      • Rees DI.
      Medical marijuana laws, traffic fatalities, and alcohol consumption.
      ,
      • Cook AC
      • Leung G
      • Smith RA.
      Marijuana decriminalization, medical marijuana laws, and fatal traffic crashes in U.S. cities, 2010-2017.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      Two studies examined other drug positivity among fatally injured drivers. One study observed an increase in methamphetamine positivity before versus after medical legalization in Hawaii (4.8% vs 7.3%),
      • Steinemann S
      • AGalanis D
      • Nguyen T
      • Biffl W.
      Motor vehicle crash fatalaties and undercompensated care associated with legalization of marijuana.
      whereas another estimated a 21% decrease in the odds of opioid positivity (OR=0.79; 95% CI=0.61, 1.03) after retail sales/home cultivation in pooled U.S. states.
      • Kim JH
      • Santaella-Tenorio J
      • Mauro C
      • et al.
      State medical marijuana laws and the prevalence of opioids detected among fatally injured drivers.
      Finally, a single study compared state drug-testing rates, finding no differences by state legalization status or by driver survival or fault.
      • Slater ME
      • Castle IJP
      • Logan BK
      • Hingson RW.
      Differences in state drug testing and reporting by driver type in U.S. fatal traffic crashes.
      Seatbelt use. One study found a reduction in seatbelt use among drivers of passenger vehicles, particularly male drivers between the ages of 25 and 69 years, in states with medical legalization versus those without.
      • Adams S
      • Cotti C
      Seatbelt use as a police avoidance strategy: a test using the legality of medical marijuana.
      Motor-vehicle collisions. A total of 8 studies (all moderate risk of bias) examined the impact of medical cannabis legalization and/or retail sales on fatal MVCs or related mortality. All studies found reductions in fatal MVCs or related mortality associated with medical legalization and/or retail sales.
      • Anderson DM
      • Hansen B
      • Rees DI.
      Medical marijuana laws, traffic fatalities, and alcohol consumption.
      ,
      • Bartos BJ
      • Newark C
      • McCleary R.
      Marijuana medicalization and motor vehicle fatalities: a synthetic control group approach.
      ,
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,
      • Cook AC
      • Leung G
      • Smith RA.
      Marijuana decriminalization, medical marijuana laws, and fatal traffic crashes in U.S. cities, 2010-2017.
      ,
      • Leung G
      • Dutra J.
      Legal access to marijuana and motor vehicle fatalities in the United States, 1990–2019.
      ,
      • Santaella J
      • Mauro C
      • Wall MM
      • Kim JH
      • Martins SS.
      Reductions in traffic fatalities rates across states with operational dispensaries of marijuana.
      ,
      • Santaella-Tenorio J
      • Mauro CM
      • Wall MM
      • et al.
      U.S. traffic fatalities, 1985–2014, and their relationship to medical marijuana laws.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      In pooled U.S. jurisdictions, reductions associated with legalization ranged from –3.5% to –9.3%.
      • Anderson DM
      • Hansen B
      • Rees DI.
      Medical marijuana laws, traffic fatalities, and alcohol consumption.
      ,
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,
      • Cook AC
      • Leung G
      • Smith RA.
      Marijuana decriminalization, medical marijuana laws, and fatal traffic crashes in U.S. cities, 2010-2017.
      ,
      • Leung G
      • Dutra J.
      Legal access to marijuana and motor vehicle fatalities in the United States, 1990–2019.
      ,
      • Santaella-Tenorio J
      • Mauro CM
      • Wall MM
      • et al.
      U.S. traffic fatalities, 1985–2014, and their relationship to medical marijuana laws.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      Santaella-Tenorio et al.
      • Santaella-Tenorio J
      • Mauro CM
      • Wall MM
      • et al.
      U.S. traffic fatalities, 1985–2014, and their relationship to medical marijuana laws.
      (2017) found no association between medical retail sales and fatal MVCs, despite finding a reduction associated with medical legalization; however, CIs were wide. Calvert and Erickson
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      (2020) also examined pedestrian-involved fatal collisions specifically and found no conclusive immediate or continuing effects. In studies that reported subgroups by sex, 1 study found similar reductions in fatal collisions associated with medical legalization among males compared with that among females (–9% vs –11%),
      • Cook AC
      • Leung G
      • Smith RA.
      Marijuana decriminalization, medical marijuana laws, and fatal traffic crashes in U.S. cities, 2010-2017.
      whereas 2 other studies suggested greater reductions in fatalities among males than among females (–10.8% vs –6.9%,
      • Anderson DM
      • Hansen B
      • Rees DI.
      Medical marijuana laws, traffic fatalities, and alcohol consumption.
      –4.2% vs –2.5%
      • Vogler J
      State marijuana policies and vehicle fatalities.
      ). Five studies also reported subgroups by age; however, categorical cut offs selected varied, and there was no clear pattern across studies that suggested effect measure modification by age group.
      • Santaella J
      • Mauro C
      • Wall MM
      • Kim JH
      • Martins SS.
      Reductions in traffic fatalities rates across states with operational dispensaries of marijuana.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.

      Recreational Cannabis Legalization

      A total of 50 included studies examined recreational cannabis legalization and/or retail sales (Appendix 7, available online). Reported road safety outcomes included self-reported attitudes, beliefs, and behaviors concerning driving under the influence; positive cannabis, alcohol, and other drug tests among drivers; and MVCs.
      Self-reported attitudes, beliefs, and behaviors concerning driving under the influence. Several studies (all serious risk of bias) examined attitudes and beliefs concerning DUIC (n=11),
      • Benedetti MH
      • Li L
      • Neuroth LM
      • Humphries KD
      • Brooks-Russell A
      • Zhu M.
      Demographic and policy-based differences in behaviors and attitudes towards driving after marijuana use: an analysis of the 2013-2017 Traffic Safety Culture Index.
      ,
      • Cuttler C
      • Sexton M
      • Mischley LK.
      Driving under the influence of cannabis: an examination of driving beliefs and practices of medical and recreational cannabis users across the United States.
      ,
      • Eichelberger AH.
      Survey of U.S. drivers about marijuana, alcohol, and driving.
      ,
      • Eichelberger AH.
      Marijuana use and driving in Washington State: risk perceptions and behaviors before and after implementation of retail sales.
      ,
      • Lensch T
      • Sloan K
      • Ausmus J
      • et al.
      Cannabis use and driving under the influence: behaviors and attitudes by state-level legal sale of recreational cannabis.
      ,
      • McGinty EE
      • Niederdeppe J
      • Heley K
      • Barry CL.
      Public perceptions of arguments supporting and opposing recreational marijuana legalization.
      ,
      • Otto J
      • Finley K
      • Ward NJ.
      An assessment of traffic safety culture related to driving after cannabis use.
      ,
      • Wadsworth E
      • Hammond D.
      Differences in patterns of cannabis use among youth: prevalence, perceptions of harm and driving under the influence in the USA where non-medical cannabis markets have been established, proposed and prohibited.
      ,
      • Winstock AR
      • Lynskey MT
      • Maier LJ
      • Ferris JA
      • Davies EL.
      Perceptions of cannabis health information labels among people who use cannabis in the U.S. and Canada.
      ,
      • Woods-Fry H
      • Vanlaar WGM
      • Lyon C
      • Brown S
      • Robertson RD.
      Road safety monitor 2019: trends in marijuana use among Canadian drivers.
      ,
      • Woods-Fry H
      • Robertson RD
      • Vanlaar WGM.
      Road safety monitor 2020: trends in marijuana use among Canadian drivers.
      or self-reported DUIC (n=12)
      • Benedetti MH
      • Li L
      • Neuroth LM
      • Humphries KD
      • Brooks-Russell A
      • Zhu M.
      Demographic and policy-based differences in behaviors and attitudes towards driving after marijuana use: an analysis of the 2013-2017 Traffic Safety Culture Index.
      • Benedetti MH
      • Li L
      • Neuroth LM
      • Humphries KD
      • Brooks-Russell A
      • Zhu M.
      Self-reported driving after marijuana use in association with medical and recreational marijuana policies.
      • Berg CJ
      • Daniel CN
      • Vu M
      • Li J
      • Martin K
      • Le L.
      Marijuana use and driving under the influence among young adults: a socioecological perspective on risk factors.
      • Cuttler C
      • Sexton M
      • Mischley LK.
      Driving under the influence of cannabis: an examination of driving beliefs and practices of medical and recreational cannabis users across the United States.
      • Eichelberger AH.
      Survey of U.S. drivers about marijuana, alcohol, and driving.
      • Fedorova EV
      • Ataiants J
      • Wong CF
      • Iverson E
      • Lankenau SE.
      Changes in medical cannabis patient status before and after cannabis legalization in California: associations with cannabis and other drug use.
      • Goodman S
      • Fischer B
      • Hammond D.
      Lower-risk cannabis use guidelines: adherence in Canada and the U.S.
      • Lensch T
      • Sloan K
      • Ausmus J
      • et al.
      Cannabis use and driving under the influence: behaviors and attitudes by state-level legal sale of recreational cannabis.
      • Rotermann M.
      What has changed since cannabis was legalized?.
      • Wadsworth E
      • Hammond D.
      Differences in patterns of cannabis use among youth: prevalence, perceptions of harm and driving under the influence in the USA where non-medical cannabis markets have been established, proposed and prohibited.
      • Woods-Fry H
      • Vanlaar WGM
      • Lyon C
      • Brown S
      • Robertson RD.
      Road safety monitor 2019: trends in marijuana use among Canadian drivers.
      • Woods-Fry H
      • Robertson RD
      • Vanlaar WGM.
      Road safety monitor 2020: trends in marijuana use among Canadian drivers.
      or driving under the influence of alcohol (n=3).
      • Eichelberger AH.
      Survey of U.S. drivers about marijuana, alcohol, and driving.
      ,
      • Woods-Fry H
      • Vanlaar WGM
      • Lyon C
      • Brown S
      • Robertson RD.
      Road safety monitor 2019: trends in marijuana use among Canadian drivers.
      ,
      • Woods-Fry H
      • Robertson RD
      • Vanlaar WGM.
      Road safety monitor 2020: trends in marijuana use among Canadian drivers.
      For all outcomes, the estimated direction of effect was inconsistent across studies.
      Positive cannabis, alcohol, and other drug tests among drivers and patients with trauma. Reported outcomes included positive cannabis (n=15), alcohol (n=5), or other drug (n=3) tests among drivers involved in fatal collisions or patients presenting with MVC-related trauma. Nine studies found an increase in cannabis positivity associated with recreational legalization or retail sales in Colorado
      • Lee J
      • Abdel-Aty A
      • Park J.
      Investigation of associations between marijuana law changes and marijuana-involved fatal traffic crashes: a state-level analysis.
      ; Washington
      • Couper FJ
      • Peterson BL.
      The prevalence of marijuana in suspected impaired driving cases in Washington State.
      ,
      • Eichelberger AH.
      Marijuana use and driving in Washington State: risk perceptions and behaviors before and after implementation of retail sales.
      ,
      • Lee J
      • Abdel-Aty A
      • Park J.
      Investigation of associations between marijuana law changes and marijuana-involved fatal traffic crashes: a state-level analysis.
      ,

      A Ramirez, A Berning, K Carr, et al. Marijuana, Other Drugs, and Alcohol Use by Drivers in Washington State. Washington, DC: National Highway Traffic Safety Administration Report No. DOT HS 812 299. https://rosap.ntl.bts.gov/view/dot/1977. Published 2016. Accessed June 23, 2022.

      ,
      • Tefft BC
      • Arnold LS
      • Grabowski JG.
      Prevalence of marijuana involvement in fatal crashes: Washington, 2010–2014.
      ,

      Tefft BC, Arnold LS. Cannabis use among drivers in fatal crashes in Washington State before and after legalization Washington, DC: AAA Foundation for Traffic Safety; 2020. https://aaafoundation.org/wp-content/uploads/2020/01/19-0637_AAAFTS-WA-State-Cannabis-Use-Among-Drivers-in-Fatal-Crashes_r4.pdf.6. Published 2020. Accessed June 23, 2022.

      ,
      • Woo Y.
      The Effects of Cannabis and the Legalization of Marijuana on Fatal Crashes in Washington State [dissertation].
      ; Colorado and Washington combined
      • Hamzeie R
      • Thompson I
      • Roy S
      • Savolainen PT.
      State-level comparison of traffic fatality data in consideration of marijuana laws.
      ; or Alaska, Colorado, Oregon, and Washington combined.
      • Chung C
      • Salottolo K
      • Tanner A
      • et al.
      The impact of recreational marijuana commercialization on traumatic injury.
      ,
      • Grigorian A
      • Lester E
      • Lekawa M
      • et al.
      Marijuana use and outcomes in adult and pediatric trauma patients after legalization in California.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      Five studies found no difference in Colorado
      • Chung C
      • Salottolo K
      • Tanner A
      • et al.
      The impact of recreational marijuana commercialization on traumatic injury.
      ,
      • Grigorian A
      • Lester E
      • Lekawa M
      • et al.
      Marijuana use and outcomes in adult and pediatric trauma patients after legalization in California.
      ,
      • Hansen B
      • Miller K
      • Weber C.
      Early evidence on recreational marijuana legalization and traffic fatalities.
      or Washington.
      • Banta-Green C
      • Rowhani-Rahbar A
      • Ebel BE
      • Andris LM
      • Qiu Q.
      Cannabis use among drivers suspected of driving under the influence or involved in collisions: analyses of Washington State Patrol data.
      ,
      • Hake ML.
      Marijuana Legalization and Traffic Fatalities Involving Cannabinoids [dissertation].
      ,
      • Hansen B
      • Miller K
      • Weber C.
      Early evidence on recreational marijuana legalization and traffic fatalities.
      A single study found a decrease in Colorado and Washington.
      • Sevigny EL.
      The effects of medical marijuana laws on cannabis-involved driving.
      There were 2 studies at the lowest risk of bias (moderate): Vogler (2017) found a pooled increase of 31.4% in MVC fatalities in which the driver was cannabis positive, and alcohol was not involved in Arkansas, Colorado, Oregon, and Washington compared with that in control states.
      • Vogler J
      State marijuana policies and vehicle fatalities.
      Hansen and colleagues
      • Hansen B
      • Miller K
      • Weber C.
      Early evidence on recreational marijuana legalization and traffic fatalities.
      (2020) estimated that compared with MVC fatalities in synthetic controls, MVC fatalities in which at least 1 driver was cannabis positive increased by >60% after legalization in Colorado and Washington, with 45%–60% of the increase attributable to legalization, although state-specific estimates were imprecise.
      Five studies examined alcohol positivity, with 1 study finding an increase before versus after recreational legalization in Colorado,
      • Chamlin MB.
      An examination of the unintended consequences of the legalization of recreational marijuana on alcohol-related behaviors.
      and 2 studies suggesting a decrease in alcohol positivity before versus after recreational retail sales in Washington.
      • Eichelberger AH.
      Marijuana use and driving in Washington State: risk perceptions and behaviors before and after implementation of retail sales.
      ,

      A Ramirez, A Berning, K Carr, et al. Marijuana, Other Drugs, and Alcohol Use by Drivers in Washington State. Washington, DC: National Highway Traffic Safety Administration Report No. DOT HS 812 299. https://rosap.ntl.bts.gov/view/dot/1977. Published 2016. Accessed June 23, 2022.

      The 2 studies at the lowest risk of bias (moderate) found no difference before and after legalization in Colorado or Washington compared with that in synthetic controls
      • Hansen B
      • Miller K
      • Weber C.
      Early evidence on recreational marijuana legalization and traffic fatalities.
      or in 4 pooled states compared with that in control states.
      • Vogler J
      State marijuana policies and vehicle fatalities.
      Studies examining other drug positivity
      • Chung C
      • Salottolo K
      • Tanner A
      • et al.
      The impact of recreational marijuana commercialization on traumatic injury.
      ,
      • Couper FJ
      • Peterson BL.
      The prevalence of marijuana in suspected impaired driving cases in Washington State.
      ,

      A Ramirez, A Berning, K Carr, et al. Marijuana, Other Drugs, and Alcohol Use by Drivers in Washington State. Washington, DC: National Highway Traffic Safety Administration Report No. DOT HS 812 299. https://rosap.ntl.bts.gov/view/dot/1977. Published 2016. Accessed June 23, 2022.

      or arrests for driving under the influence
      • Hao Z
      • Cowan BW.
      The cross-border spillover effects of recreational marijuana legalization.
      ,
      • Miller T.
      The Consequences of Legalizing Recreational Marijuana: Evidence From Colorado and Washington [dissertation].
      were all assessed to be at serious risk of bias, and the estimated direction of effect was mixed across studies.
      Motor-vehicle collisions. Twenty studies examined the impact of recreational cannabis legalization (n=13) and/or retail sales (n=13) on MVCs or related morbidity, mortality, or healthcare utilization. For Colorado, most studies found an increase in MVCs or related outcomes associated with recreational legalization
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,
      • Delling FN
      • Vittinghoff E
      • Dewland TA
      • et al.
      Does cannabis legalisation change healthcare utilisation? A population-based study using the healthcare cost and utilisation project in Colorado, USA.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      or retail sales,
      • Lane TJ
      • Hall W.
      Traffic fatalities within U.S. states that have legalized recreational cannabis sales and their neighbours.
      ,
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,
      • Delling FN
      • Vittinghoff E
      • Dewland TA
      • et al.
      Does cannabis legalisation change healthcare utilisation? A population-based study using the healthcare cost and utilisation project in Colorado, USA.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 34.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 35.
      ,
      • Monfort SS
      Effect of recreational marijuana sales on police-reported crashes in Colorado.
      ,
      • Santaella-Tenorio J
      • Wheeler-Martin K
      • Dimaggio CJ
      • et al.
      Association of recreational cannabis laws in Colorado and Washington state with changes in traffic fatalities, 2005–2017.
      ,
      • Temple C
      • Lai J
      • Carreiro S
      • Neavyn M.
      Traffic fatalities before and after legalization of adult-use cannabis.
      ,
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      with 2 studies finding no difference.
      • Hansen B
      • Miller K
      • Weber C.
      Early evidence on recreational marijuana legalization and traffic fatalities.
      ,
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      Among the studies assessed to be at the lowest risk of bias (moderate), fatal collisions or MVC fatalities were estimated to have increased from 3.6% to 5.9% after legalization
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      or by 0.4 fatalities per billion vehicle miles travelled
      • Hansen B
      • Miller K
      • Weber C.
      Early evidence on recreational marijuana legalization and traffic fatalities.
      ; after retail sales, MVCs were estimated to have increased from 2.5% to 13.9%
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 34.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 35.
      ,
      • Monfort SS
      Effect of recreational marijuana sales on police-reported crashes in Colorado.
      or by 0.83 and 1.46 fatalities per million residents or billion vehicle miles traveled, respectively.
      • Lane TJ
      • Hall W.
      Traffic fatalities within U.S. states that have legalized recreational cannabis sales and their neighbours.
      ,
      • Santaella-Tenorio J
      • Wheeler-Martin K
      • Dimaggio CJ
      • et al.
      Association of recreational cannabis laws in Colorado and Washington state with changes in traffic fatalities, 2005–2017.
      For Washington, most studies found an increase in fatal collisions or MVC fatalities associated with recreational legalization
      • Vogler J
      State marijuana policies and vehicle fatalities.
      ,
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      ,
      • Woo Y.
      The Effects of Cannabis and the Legalization of Marijuana on Fatal Crashes in Washington State [dissertation].
      or retail sales.
      • Lane TJ
      • Hall W.
      Traffic fatalities within U.S. states that have legalized recreational cannabis sales and their neighbours.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 34.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 35.
      ,
      • Monfort SS
      Effect of recreational marijuana sales on police-reported crashes in Colorado.
      ,
      • Santaella-Tenorio J
      • Wheeler-Martin K
      • Dimaggio CJ
      • et al.
      Association of recreational cannabis laws in Colorado and Washington state with changes in traffic fatalities, 2005–2017.
      ,
      • Temple C
      • Lai J
      • Carreiro S
      • Neavyn M.
      Traffic fatalities before and after legalization of adult-use cannabis.
      ,
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      ,
      • Woo Y.
      The Effects of Cannabis and the Legalization of Marijuana on Fatal Crashes in Washington State [dissertation].
      Several studies suggested no difference
      • Hansen B
      • Miller K
      • Weber C.
      Early evidence on recreational marijuana legalization and traffic fatalities.
      or were inconclusive
      • Vogler J
      State marijuana policies and vehicle fatalities.
      ,
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      owing to wide CIs. Among the studies at the lowest risk of bias (moderate), fatal collisions or MVC fatalities were estimated to have increased from 1.4% to 3.6%
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,
      • Monfort SS
      Effect of recreational marijuana sales on police-reported crashes in Colorado.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      or by 0.7 fatalities per billion vehicle miles traveled after legalization
      • Hansen B
      • Miller K
      • Weber C.
      Early evidence on recreational marijuana legalization and traffic fatalities.
      ; estimates for retail sales ranged from −0.14% to 9.7%
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 34.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 35.
      ,
      • Monfort SS
      Effect of recreational marijuana sales on police-reported crashes in Colorado.
      or increased by 0.94 and 0.08 fatalities per million residents or billion vehicle miles traveled, respectively.
      • Lane TJ
      • Hall W.
      Traffic fatalities within U.S. states that have legalized recreational cannabis sales and their neighbours.
      ,
      • Santaella-Tenorio J
      • Wheeler-Martin K
      • Dimaggio CJ
      • et al.
      Association of recreational cannabis laws in Colorado and Washington state with changes in traffic fatalities, 2005–2017.
      Two studies that combined data from Colorado and Washington estimated increases in MVC fatalities per billion vehicle miles traveled of 0.2 (95% CI= –0.4, 0.9) and 1.2 (95% CI= –0.6, 2.1) for recreational legalization and 1.8 (95% CI=0.4, 3.7) for retail sales.
      • Aydelotte JD
      • Brown LH
      • Luftman KM
      • et al.
      Crash fatality rates after recreational marijuana legalization in Washington and Colorado.
      ,
      • Aydelotte JD
      • Mardock AL
      • Mancheski CA
      • et al.
      Fatal crashes in the 5 years after recreational marijuana legalization in Colorado and Washington.
      For Oregon, most studies likewise found an increase in MVCs related to recreational legalization
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      ,
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      or retail sales,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 34.
      ,
      • Monfort SS
      Effect of recreational marijuana sales on police-reported crashes in Colorado.
      ,
      • Temple C
      • Lai J
      • Carreiro S
      • Neavyn M.
      Traffic fatalities before and after legalization of adult-use cannabis.
      ,
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      although 2 studies found no difference
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 35.
      or were inconclusive owing to wide CIs.
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      Among the studies assessed to be at the lowest risk of bias (moderate), fatal collisions or MVC fatalities were estimated to have increased from 1.5% to 20.5% after legalization.
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      For retail sales, estimates ranged from 0.7% to 4.5%
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 34.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 35.
      ,
      • Monfort SS
      Effect of recreational marijuana sales on police-reported crashes in Colorado.
      ; Lane and Hall
      • Lane TJ
      • Hall W.
      Traffic fatalities within U.S. states that have legalized recreational cannabis sales and their neighbours.
      (2019) found that Oregon had an increase of 1.40 (95% CI=0.68, 2.12) in MVC fatalities per million residents associated with their neighbor Washington's legalization of retail sales, compared with a 0.85 (95% CI= –1.57, 3.26) increase associated with Oregon's own retail sales 15 months later. Several studies were suggestive of an increase in MVC fatalities in Alaska after recreational legalization
      • Vogler J
      State marijuana policies and vehicle fatalities.
      ,
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      or retail sales
      • Temple C
      • Lai J
      • Carreiro S
      • Neavyn M.
      Traffic fatalities before and after legalization of adult-use cannabis.
      ,
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      ; however, the estimates were imprecise.
      Among studies that pooled data from >2 U.S. states, most found point estimates suggesting an increase in MVCs or related outcomes associated with recreational legalization
      • Calvert C
      • Erickson D.
      An examination of relationships between cannabis legalization and fatal motor vehicle and pedestrian-involved crashes.
      ,

      C Farmer, S Monfort, A. Woods, Changes in traffic crash rates after legalization of marijuana: results by crash severity. Arlington, VA: Insurance Institute for Highway Safety. https://www.iihs.org/topics/bibliography/ref/2229. Published 2021. Accessed June 23, 2022.

      ,
      • Notrica DM
      • Sayrs LW
      • Krishna N
      • Rowe D
      • Jaroszewski DE
      • McMahon LE.
      The impact of state laws on motor vehicle fatality rates, 1999–2015.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      ,
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      or retail sales,
      • Lane TJ
      • Hall W.
      Traffic fatalities within U.S. states that have legalized recreational cannabis sales and their neighbours.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 34.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 35.
      ,
      • Kamer RS
      • Warshafsky S
      • Kamer GC.
      Change in traffic fatality rates in the first 4 states to legalize recreational marijuana.
      ,
      • Leung G
      • Dutra J.
      Legal access to marijuana and motor vehicle fatalities in the United States, 1990–2019.
      ,
      • Monfort SS
      Effect of recreational marijuana sales on police-reported crashes in Colorado.
      ,
      • Windle SB
      • Eisenberg MJ
      • Reynier P
      • et al.
      Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
      whereas 1 study was inconclusive for retail sales.

      C Farmer, S Monfort, A. Woods, Changes in traffic crash rates after legalization of marijuana: results by crash severity. Arlington, VA: Insurance Institute for Highway Safety. https://www.iihs.org/topics/bibliography/ref/2229. Published 2021. Accessed June 23, 2022.

      Among the studies at the lowest risk of bias (moderate), there was an estimated 1.4%‒7.8% increase in MVC fatalities associated with legalization

      C Farmer, S Monfort, A. Woods, Changes in traffic crash rates after legalization of marijuana: results by crash severity. Arlington, VA: Insurance Institute for Highway Safety. https://www.iihs.org/topics/bibliography/ref/2229. Published 2021. Accessed June 23, 2022.

      ,
      • Leung G
      • Dutra J.
      Legal access to marijuana and motor vehicle fatalities in the United States, 1990–2019.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      and a 1.4%‒6.0% increase in MVCs associated with retail sales.

      C Farmer, S Monfort, A. Woods, Changes in traffic crash rates after legalization of marijuana: results by crash severity. Arlington, VA: Insurance Institute for Highway Safety. https://www.iihs.org/topics/bibliography/ref/2229. Published 2021. Accessed June 23, 2022.

      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 34.
      ,
      Highway Loss Data Institute
      Recreational Marijuana and Collision Claim Frequencies. Bulletin 35.
      ,
      • Monfort SS
      Effect of recreational marijuana sales on police-reported crashes in Colorado.
      One study using data from Uruguay found that recreational legalization was associated with a 52.4% increase (95% CI=11.6, 93.3) in light motor-vehicle drivers’ fatality rate, but this was not observed for motorcyclists.
      • Nazif-Munoz JI
      • Oulhote Y
      • Ouimet MC.
      The association between legalization of cannabis use and traffic deaths in Uruguay.

      DISCUSSION

      In this systematic review of 64 observational studies that primarily used data from the U.S., there was mixed evidence regarding the impacts of medical and recreational cannabis legalization on attitudes, beliefs, and self-reported DUIC. However, reported outcomes, populations, and study designs varied widely across studies. Nevertheless, cannabis decriminalization, medical legalization, and recreational legalization were found to be associated with increases in cannabis positivity among drivers, although many studies examining this outcome were assessed to be at serious risk of bias because of potential confounding and measurement error. This systematic review also found an association between medical cannabis legalization and retail sales and reductions in fatal MVCs across studies. In contrast, recreational cannabis legalization and retail sales were associated with increases in fatal MVCs. Many studies that reported collisions and/or fatalities were quasi-experimental in design and were assessed to be at lower risk of bias than studies that examined other outcomes.
      Increases in positive cannabis tests among drivers may reflect increases in the prevalence, frequency, and/or potency of cannabis use; however, the detection of cannabis does not in itself indicate driving impairment.
      • Windle SB
      • Sequeira C
      • Filion KB
      • et al.
      Impaired driving and legalization of recreational cannabis.
      Unlike alcohol, for which blood alcohol concentration is a consistent indicator of the degree of impairment from alcohol, tetrahydrocannabinol passes quickly from the blood and into other organs where its metabolites can remain for extended periods.
      • Woods-Fry H
      • Robertson RD
      • Vanlaar WGM.
      Road safety monitor 2020: trends in marijuana use among Canadian drivers.
      Therefore, despite consistent evidence from meta-analyses indicating that acute cannabis use is associated with increased risk of MVCs,
      • Rogeberg O
      • Elvik R.
      The effects of cannabis intoxication on motor vehicle collision revisited and revised.
      it remains difficult to correlate specific biochemical levels of tetrahydrocannabinol with the degrees of driving impairment across individuals.
      • Arkell TR
      • Spindle TR
      • Kevin RC
      • Vandrey R
      • McGregor IS.
      The failings of per se limits to detect cannabis-induced driving impairment: results from a simulated driving study.
      Although all policies were associated with increases in cannabis positivity, medical legalization and retail sales were associated with reductions in fatal MVCs, whereas recreational legalization and retail sales were associated with increases in fatal MVCs. These opposing findings are likely owing to the subgroups primarily impacted by medical and recreational legalization, respectively. Individuals who use medical cannabis are older and in poorer health than those who use recreational cannabis,
      • Lin LA
      • Ilgen MA
      • Jannausch M
      • Bohnert KM
      Comparing adults who use cannabis medically with those who use recreationally: results from a national sample.
      and the most common reason for seeking medical cannabis certification is chronic pain.
      • Mahabir VK
      • Smith CS
      • Vannabouathong C
      • Merchant JJ
      • Garibaldi AL.
      Comparing medical cannabis use in 5 U.S. states: a retrospective database study.
      The substitution of cannabis for alcohol
      • Anderson DM
      • Hansen B
      • Rees DI.
      Medical marijuana laws, traffic fatalities, and alcohol consumption.
      ,
      • Cook AC
      • Leung G
      • Smith RA.
      Marijuana decriminalization, medical marijuana laws, and fatal traffic crashes in U.S. cities, 2010-2017.
      ,
      • Santaella-Tenorio J
      • Mauro CM
      • Wall MM
      • et al.
      U.S. traffic fatalities, 1985–2014, and their relationship to medical marijuana laws.
      ,
      • Lucas P
      • Reiman A
      • Earleywine M
      • et al.
      Cannabis as a substitute for alcohol and other drugs: a dispensary-based survey of substitution effect in Canadian medical cannabis patients.
      and/or opioids
      • Lucas P
      • Reiman A
      • Earleywine M
      • et al.
      Cannabis as a substitute for alcohol and other drugs: a dispensary-based survey of substitution effect in Canadian medical cannabis patients.
      • Wendelboe AM
      • Mathew R
      • Chongsuwat T
      • et al.
      Is there less opioid abuse in states where marijuana has been decriminalized, either for medicinal or recreational use? A Clin-IQ.
      • Stith SS
      • Vigil JM
      • Adams IM
      • Reeve AP.
      Effects of legal access to cannabis on scheduled II-V drug prescriptions.
      among individuals who use medical cannabis may explain the observed reductions in fatal MVCs associated with medical legalization because cannabis is known to impair psychomotor skills and cognitive functions to a lesser degree than alcohol and opioids.
      • Asbridge M
      • Hayden JA
      • Cartwright JL.
      Acute cannabis consumption and motor vehicle collision risk: systematic review of observational studies and meta-analysis.
      In this review, data were limited concerning alcohol and other drug positivity among drivers; however, the findings were suggestive of a decrease in alcohol positivity associated with medical legalization. Although the review was inconclusive regarding an association between recreational legalization and alcohol and other drug positivity among drivers, other studies have found no changes in alcohol use
      • Calvert CM
      • Erickson D.
      Recreational cannabis legalization and alcohol purchasing: a difference-in-differences analysis.
      or suggested that individuals who use recreational cannabis may use alcohol as a complement to cannabis.
      • Gunn R
      • Jackson K
      • Borsari B
      • Metrik J.
      A longitudinal examination of daily patterns of cannabis and alcohol co-use among medicinal and recreational veteran cannabis users.
      • Kim JH
      • Weinberger AH
      • Zhu J
      • Barrington-Trimis J
      • Wyka K
      • Goodwin RD.
      Impact of state-level cannabis legalization on poly use of alcohol and cannabis in the United States, 2004–2017.
      • Lira MC
      • Heeren TC
      • Buczek M
      • et al.
      Trends in cannabis involvement and risk of alcohol involvement in motor vehicle crash fatalities in the United States, 2000-2018.
      Overall, this review has a number of policy and research implications. First, there are few data available concerning the impact of cannabis decriminalization on road safety outcomes, despite this being the most common cannabis liberalization policy globally.
      • Eastwood N
      Cannabis decriminalization policies across the globe.
      Second, cannabis policies may have opposing effects on road safety outcomes, depending on the population subgroups most impacted (e.g., medical versus recreational use). Third, the impact of cannabis decriminalization and legalization on road safety outcomes may be modified by a number of factors, including heterogeneity in policy, legal, and demographic characteristics between legalizing jurisdictions. Allowance of retail sales may be an important effect measure modifier owing to increased visibility, affordability, and access to cannabis compared with legalization alone.
      • Pacula RL
      • Powell D
      • Heaton P
      • Sevigny EL.
      Assessing the effects of medical marijuana laws on marijuana use: the devil is in the details.
      ,
      • Hall W
      • Lynskey M.
      Assessing the public health impacts of legalizing recreational cannabis use: the U.S. experience.
      This review did not find clear differences between legalization only and retail sales, although most studies selected either one exposure or the other, making direct comparisons difficult. Likewise, few studies considered other policy dimensions (e.g., home growing). Cointerventions such as per se cannabis limits or zero tolerance laws may also modify the effect of cannabis policies on road safety outcomes; however, their effectiveness in deterring DUIC is unclear.
      • Watson TM
      • Mann RE.
      International approaches to driving under the influence of cannabis: a review of evidence on impact.
      ,
      • Razaghizad A
      • Windle SB
      • Gore G
      • et al.
      Interventions to prevent drugged driving: a systematic review.
      Previous studies have also suggested that the impact of cannabis policies is likely to differ between population subgroups.
      • Pacula RL
      • Powell D
      • Heaton P
      • Sevigny EL.
      Assessing the effects of medical marijuana laws on marijuana use: the devil is in the details.
      Few studies in this review reported results by sex or age group, although 2 studies suggested that the impact of legalization on MVC-related outcomes may be greater among males.
      • Anderson DM
      • Hansen B
      • Rees DI.
      Medical marijuana laws, traffic fatalities, and alcohol consumption.
      ,
      • Vogler J
      State marijuana policies and vehicle fatalities.
      Finally, future research should consider quasi-experimental designs (e.g., difference-in-differences), which were typically assessed to have a lower risk of bias than other observational study designs in this review. Likewise, outcomes that were not self-reported nor depended on drug-testing rates/reporting (e.g., fatal MVCs) were the least likely to be subject to measurement error. Future work should consider methodologic solutions for underreported and/or frequently mismeasured outcomes (e.g., nonfatal injuries, drug testing) to advance the understanding of the impact of cannabis policies on road safety outcomes.

      Limitations

      First, few conclusions could be drawn concerning the impact of cannabis decriminalization on road safety outcomes because there were few studies available. Second, data were limited concerning nonfatal injuries from MVCs; the impact of legalization on MVC fatalities may or may not have direct correspondence to nonfatal injuries. Third, the assessed risk of bias for all included studies was moderate or serious (because no study could be considered at low risk of bias given the potential for confounding). However, this was considered when drawing conclusions. Fourth, the overall findings may not apply to specific jurisdictions, given the heterogeneity between policy, legal, and demographic characteristics. Finally, nearly all the available data were from the U.S., which limits the generalizability of the findings to other jurisdictions globally. Likewise, more than half of the included studies used data from the U.S. Fatality Analysis Reporting System; this database includes data on collisions that occurred on public roadways and resulted in at least 1 death within 30 days.

      Fatality Analysis Reporting System (FARS) encyclopedia. Washington, DC: United States Department of Transportation. https://www-fars.nhtsa.dot.gov/Main/index.aspx. Accessed June 23, 2022.

      Although the incidence of fatal collisions is likely to be accurately reported, there are known limitations regarding the drug-testing data in this database

      A Berning, DD Smither, Understanding the limitations of drug test information, reporting, and testing practices in fatal crashes. (Traffic Safety Facts Research Note. DOT HS 812 072). Washington, DC: National Highway Traffic Safety Administration. https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/812072#. Published November 2014. Accessed June 21, 2022.

      that were considered during the risk of bias assessment for studies reporting these outcomes.

      CONCLUSIONS

      This systematic review found that medical cannabis legalization was associated with reductions in fatal MVCs, whereas recreational cannabis legalization was associated with increases in fatal MVCs. There was also evidence that cannabis decriminalization, medical legalization, and recreational legalization may be associated with increases in cannabis positivity among drivers. There were mixed impacts of medical and recreational legalization on attitudes, beliefs, and self-reported DUIC. Evidence was limited concerning the impact of medical and recreational legalization on nonfatal injuries and road safety outcomes related to alcohol and other drugs, although the findings were suggestive that medical cannabis legalization may decrease alcohol positivity among drivers. More research is needed concerning the impact of cannabis decriminalization on road safety outcomes and the mechanisms by which legalization impacts road safety outcomes.

      CREDIT AUTHOR STATEMENT

      Sarah B. Windle: Conceptualization, Methodology, Investigation, Writing – original draft. Peter Socha: Validation, Investigation, Writing – review & editing. José Ignacio Nazif-Munoz: Methodology, Writing – review & editing. Sam Harper: Methodology, Writing – review & editing. Arijit Nandi: Methodology, Writing – review & editing, Supervision.

      ACKNOWLEDGMENTS

      The authors thank Ms. Genevieve Gore, MLIS for her peer-review of the systematic review search strategies.
      Sponsors had no role in the study design; collection, analysis, and interpretation of data; writing of the report; and the decision to submit the report for publication.
      SBW is supported by a Canada Graduate Scholarship in Honour of Nelson Mandela from the Canadian Institutes of Health Research and a Doctoral Training Award from the Fonds de recherche Québec – Santé. PS is supported by a Master's Training Award from the Fonds de recherche Québec – Santé.
      No financial disclosures were reported by the authors of this paper.

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