Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study

Sarah B. Windle; Mark J. Eisenberg; Pauline Reynier; Josselin Cabaussel; Brett D. Thombs; Roland Grad; Carolyn Ells; Crystal Sequeira; Kristian B. Filion

doi:10.9778/cmajo.20200155

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RE: Cannabis legalization and motor vehicle fatalities.
Alex Jiang, Shannon Erdelyi, John A Staples, Lulu X Pei, Herbert Chan and Jeffrey R Brubacher [MD]

Posted on: 25 June 2021
Duration of cannabis impairment: Word of Caution
Charl Els, Mace Beckson [MD, DFAPA] and Sebastian Straube [BM BCh, MA (Oxon), DPhil]

Posted on: 17 May 2021

Posted on: (25 June 2021)
Page navigation anchor for RE: Cannabis legalization and motor vehicle fatalities.
RE: Cannabis legalization and motor vehicle fatalities.
Alex Jiang, Data Analyst, University of British Columbia
Other Contributors:
Shannon Erdelyi, Statistician

John A Staples, Internal Medicine

Lulu X Pei, Statistician

Herbert Chan, Epidemiologist

Jeffrey R Brubacher, Associate Professor, Emergency Physician
The association between cannabis legalization and population-level motor vehicle fatality rates remains uncertain and we read the recent study by Windle et al. with interest.

Windle et al. analyzed U.S. Fatality Analysis Reporting System (FARS) data to estimate changes in fatal collisions and associated deaths following recreational cannabis legalization in 11 U.S. states. They used Poisson regression adjusted for calendar year. State-level results were meta-analyzed using inverse-variance weighted random-effects models to report that recreational cannabis legalization was associated with a 15% increase in fatal collisions and a 16% increase in associated deaths.

We were surprised by the reported magnitude of harm. Windle et al.’s estimates are heavily weighted by findings from California, the largest state to legalize cannabis. To understand these findings, we repeated their analysis of California FARS data (2007-2018) and also found that legalization was associated with a substantial increase in motor vehicle fatalities (IRR=1.23;95%CI=1.19-1.28). This estimate is slightly higher than the IRR of 1.20 reported by Windle et al., likely because we accessed FARS data in May 2021 (v.s. Sept. 2020). However, the temporal trend in pre-legalization death rates did not follow a log-linear pattern so predictions using Poisson regression adjusted for calendar year were a poor fit for the data (Figure 1). Thus, we restricted the analysis to data from 2010 onward when t...
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The association between cannabis legalization and population-level motor vehicle fatality rates remains uncertain and we read the recent study by Windle et al. with interest.

Windle et al. analyzed U.S. Fatality Analysis Reporting System (FARS) data to estimate changes in fatal collisions and associated deaths following recreational cannabis legalization in 11 U.S. states. They used Poisson regression adjusted for calendar year. State-level results were meta-analyzed using inverse-variance weighted random-effects models to report that recreational cannabis legalization was associated with a 15% increase in fatal collisions and a 16% increase in associated deaths.

We were surprised by the reported magnitude of harm. Windle et al.’s estimates are heavily weighted by findings from California, the largest state to legalize cannabis. To understand these findings, we repeated their analysis of California FARS data (2007-2018) and also found that legalization was associated with a substantial increase in motor vehicle fatalities (IRR=1.23;95%CI=1.19-1.28). This estimate is slightly higher than the IRR of 1.20 reported by Windle et al., likely because we accessed FARS data in May 2021 (v.s. Sept. 2020). However, the temporal trend in pre-legalization death rates did not follow a log-linear pattern so predictions using Poisson regression adjusted for calendar year were a poor fit for the data (Figure 1). Thus, we restricted the analysis to data from 2010 onward when the trend appeared roughly log-linear (Figure 2). This modified analysis found no change in Californian motor vehicle fatality rates associated with legalization (IRR=1.02;95%CI=0.98-1.06).

Auto Regressive Integrated Moving Average (ARIMA) is another analytic approach to evaluate the impact of policy changes on population event rates. ARIMA can account for non-linear temporal trends, seasonality, and autocorrelation; and is appropriate when the outcome is not rare. We fit a linear regression model with an indicator variable for legalization and an ARIMA error structure to the log-transformed fatality rate (2007-2018). We found no change in motor vehicle fatality rate in associated with cannabis legalization in California (Figure 3; IRR=0.95;95%CI=0.85-1.06). Sensitivity analyses yielded similar results for ARIMA analyses with a variety of date ranges.

Our analyses suggest that legalization of recreational cannabis in the U.S. may not have been associated with the large increase in fatal collisions implied by Poisson regression models that adjusted only for annual linear trends. We applaud Windle et al. for using publicly-available data as doing so facilitates replication and scientific transparency. We agree that the impact of cannabis legalization on motor vehicle collisions in Canada should be closely monitored. Unfortunately, Canadian collision research is limited by lack of robust, openly available and timely collision data.

Figures: https://rsph.med.ubc.ca/figures/
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Competing Interests: None declared.
References
Sarah B. Windle, Mark J. Eisenberg, Pauline Reynier, et al. Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study. CMAJO 2021;9:E233-E241.
Schaffer AL, Dobbins TA, Pearson S-A. Interrupted time series analysis using autoregressive integrated moving average (ARIMA) models: a guide for evaluating large-scale health interventions. BMC Medical Research Methodology. 2021;21(1):58.
Posted on: (17 May 2021)
Page navigation anchor for Duration of cannabis impairment: Word of Caution
Duration of cannabis impairment: Word of Caution
Charl Els, Clinical Professor, University of Alberta
Other Contributors:
Mace Beckson, Clinical Professor

Sebastian Straube, Professor and Division Director
It is with great interest that we read the research suggesting a small but significant increase in cannabis-associated driving fatalities - as many as 308 annually [1]. Although this study focuses on driving outside the workplace, cognitive and other skills required for driving overlap with those required for performing safety-sensitive work tasks. Motor vehicle crash risk has been used as a proxy to inform fitness for d...
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It is with great interest that we read the research suggesting a small but significant increase in cannabis-associated driving fatalities - as many as 308 annually [1]. Although this study focuses on driving outside the workplace, cognitive and other skills required for driving overlap with those required for performing safety-sensitive work tasks. Motor vehicle crash risk has been used as a proxy to inform fitness for duty considerations in safety-sensitive positions [2]. Hence, this research on non-occupational driving fatalities indicates increased safety -sensitive occupational risk associated with cannabis use.
The extrapolated risk is consistent with the Occupational and Environmental Medical Association of Canada's (OEMAC's) position statement [3] (also adopted by the Canadian Society of Addiction Medicine, or CSAM), which suggests: "Irrespective of the source of procurement, the use of cannabis can lead to impairment, which may adversely impact the performance of individuals at work," and "[i]t is recognized that the timing and duration of cannabis impairment is variable and that more research is needed in this regard".
Health Canada [4] recognizes that cannabis use may result in impairment of mental alertness and physical coordination, suggesting the effects may last up to 24 hours. The College of Family Physicians of Canada [5] suggests that for non-occupational operation of vehicles, physicians should advise patients to wait at least 6 hours before driving after inhaled cannabis use, and at least 8 hours before driving after oral consumption while acknowledging that the duration of impairment may well be longer. Complex man-machine interactions in high cognitive-demand situations and emergencies may manifest otherwise more subtle impairment.
A meta-analysis by McCartney and coworkers (2021) [4] finds that delta-9-THC impairs aspects of driving performance, varying by dose, route, and frequency of use. They state that there is no universal answer to the question of how long to wait before driving. Nevertheless, the uncertainty about the duration of impairment in any individual requires that general recommendations about driving and resuming safety-sensitive work should err on the side of caution, while still being reasonable.
Applying the precautionary principle in the context of uncertainty about the exact duration of impairment in any one individual worker using cannabis (which likely is variable depending upon various factors), the above-mentioned OEMAC position statement recommends that until more definitive evidence is available, it is not advisable to operate motor vehicles or equipment, or engage in other safety-sensitive tasks, for at least 24 hours after cannabis use, or for longer if the risk of impairment persists.
There is no expectation for occupational physicians evaluating individual employees to wait for empirical studies to be conducted in every conceivable safety-sensitive workplace setting before taking reasonable harm-reduction steps to mitigate risk as it relates to known impairing substances. Hence, extrapolation of road safety data and experimental research findings to the safety-sensitive occupational context, combined with an abundance of caution, is reasonable and recommended.
References:
1. Windle SB, Eisenberg JM, Reynier P, et al. (2021) Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study. CMAJ Open 2021. DOI:10.9778/cmajo.20200155.
2. Hegmann K, Weiss M, Bowden M, DuBrueler, et al. (2014) Opioids and Safety-sensitive Work: Reply to Swotinsky. JOEM; 56:e134-5
3. Position statement on the Implications of cannabis use for safety- sensitive work. Occupational and Environmental Medical Association of Canada (OEMAC). URL: https://secureservercdn.net/198.71.233.129/i0r.9ce.myftpupload.com/wp- content/uploads/2018/09/Position-Statement-on-the-Implications-of-cannabis -use.pdf
4. Health Canada. (2013). Information for health care professionals: cannabis (marihuana, marijuana) and the cannabinoids. URL: from: http://www.hc-sc.gc.ca/dhp-mps/alt_formats/pdf/marihuana/med/infoprof- eng.pdf
5. McCartney D, Arkell TR, Irwin C, McGregor IS (2021). Determining the magnitude and duration of acute delta-9-tetrahydrocannabinol (delta-9- THC)-induced driving and cognitive impairment: A systematic and meta- analytic review. Neuroscience and Biobehavioral Reviews 126:175-193.
6. Guidance in Authorizing Cannabis Products Within Primary Care. Mississauga, ON: College of Family Physicians of Canada; 2021.
Conflict of Interest:
CE and SS are among the authors of the referenced OEMAC position statement; research that was funded by an Occupational Health & Safety Innovation & Engagement Grant from the Government of Alberta.
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Competing Interests: None declared.