Tags:  Cannabis decriminalization; medical marijuana laws (MMLs); traffic crashes

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Thumbnail: Medical Marijuana Laws (MMLs) reduce fatal traffic crashes in cities.

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The problem: Cannabis affects cognitive function and can impair driving abilities. As more and more US states legalize cannabis, it is likely that more people will be driving under its influence. Therefore, it is important to examine the effects of cannabis legalization and/ or decriminalization on traffic safety.

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The study: The authors examined the effects of cannabis decriminalization and/or implementation of Medical Marijuana Laws (MMLs) on fatal traffic crashes. They categorized their traffic crash results by age and sex of the driver to look for potential associations with cannabis legalization. The authors used the census data on fatal traffic crashes from 2010-2017 Fatality Analysis Reporting System (FARS). FARS contains traffic crash information from all public roads and includes driver information, crash locations and drug/alcohol presence. They correlated data from ~2500 fatal crashes to actual dates when cannabis decriminalization occurred in that city to identify any spikes in crashes around the time of cannabis liberalization. This was reported as incidence rate ratios (IRRs).

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Main results:

• Following decriminalization, a 13% increase in fatal crashes was observed in young males (ages 18 to 24). Their IRR (95% CI) was 1.13 (1.01, 1.25).

  • This association was strong on weekend nights.

  • No effect was seen on female drivers or older male drivers.

• MMLs, on the other hand, were associated with fewer fatal crashes in both male and female drivers [IRR (95% CI)= 0.91 (0.84, 0.97)].

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Conclusions:  While implementation of MMLs were associated with fewer fatal crashes in cities, cannabis decriminalization resulted in an increase in fatal crashes involving young, male drivers.

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Why this is a good study:

• This study explores an essential but often-ignored aspect of cannabis legalization: traffic safety. It is a comprehensive analysis of crashes that occurred after 2010 (when many states started decriminalizing cannabis).

• The authors incorporated many confounding factors into their analysis- state-level traffic safety laws, driver-texting laws, DUI laws etc.

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Why this isn’t a perfect study:

• The study identified trends in fatal crashes, and not less-severe (non-fatal) crashes. This might affect the final output.

  • Was one demographic group represented more in minor traffic offenses as opposed to major crashes?  This is likely but was not explored in this study.

  • The FARS census data was restricted to major cities, not rural areas. This can also skew the final results.

• Testing procedures to detect cannabis vary from one state to the next. This also contributes to variability.

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What this study adds:

• This is very important study from the public health perspective. Understanding trends in traffic safety that are affected due to cannabis legalization can bring out stricter laws regarding driving following cannabis use.

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What it doesn’t:

• It does not explore the many layers of cannabis laws. For instance, how much cannabis was present in the system? Was it used for medical reasons?

  • The presence of cannabinoids and metabolites don’t necessarily indicate impairment.

• It also does not explore the relationship between alcohol use and cannabis consumption.  It’s possible that one may substitute for the other and/or that they’re consumed together.

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Funder:  No agency funding was reported.

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Author conflicts:  None