After medical marijuana became commercially widespread in Colorado in mid-2009, the proportion of fatal crash drivers testing positive for marijuana increased significantly, while no similar change occurred in states without medical marijuana.
Read this if you want to understand how marijuana legalization may affect traffic safety.
Significant increase in marijuana-positive fatal crash drivers after Colorado dispensary expansion (p<0.0001)
What the researchers found
Using federal crash data from 1994 to 2011, researchers found a significant positive trend in the proportion of drivers in fatal crashes who tested positive for marijuana in Colorado after mid-2009, when commercial medical marijuana became widely available. The change in trend was statistically significant (change = 2.16, p<0.0001).
In contrast, 34 states without medical marijuana laws showed no significant changes in marijuana-positive fatal crash drivers during the same period. Neither Colorado nor the comparison states showed significant changes in alcohol-impaired fatal crash proportions.
The study defined the pre-commercial period as 1994 to June 2009 and the post-commercial period as July 2009 to 2011, using 36 six-month intervals.
Why it matters
This was among the first studies to examine the impact of marijuana commercialization on traffic safety using federal crash data. The finding of increased marijuana-positive fatal crashes after dispensary expansion raised important public health and policy concerns about marijuana legalization and driving safety.
The numbers in context
Data: 36 six-month intervals (1994-2011). Change in trend after mid-2009: 2.16 (SE 0.45, p<0.0001) for marijuana-positive drivers in Colorado. No significant change in 34 comparison states. No significant change in alcohol-impaired proportions in either group.
How the study worked
Researchers analyzed data from the Fatality Analysis Reporting System (FARS) across 36 six-month intervals from 1994 to 2011. They compared temporal changes in the proportions of drivers in fatal crashes who were marijuana-positive and alcohol-impaired in Colorado versus 34 non-medical marijuana states. Statistical analysis used interrupted time series methods to detect changes in trends.
What this study cannot tell us
Testing positive for marijuana does not equal impairment at the time of the crash. THC metabolites can be detected days or weeks after use. Testing protocols varied across jurisdictions and over time. Not all fatally injured drivers were tested for marijuana, and testing rates may have increased after commercialization (detection bias). The study could not determine whether marijuana was a causal factor in the crashes.
How to read the evidence
This is a well-designed ecological time series study using federal crash data and comparison states. However, the marijuana-positive test does not confirm impairment at the time of the crash.
When this study was published
Published in 2014 with data through 2011. Colorado subsequently legalized recreational marijuana in 2012, and additional studies have examined the longer-term trends.
The bigger picture
This study contributed to the growing debate about marijuana legalization and traffic safety. While the increase in marijuana-positive drivers in fatal crashes is concerning, interpretation requires caution: testing positive for marijuana does not mean a driver was impaired at the time of the crash, as THC metabolites can persist for weeks.
Questions still open
- Does the increase in marijuana-positive drivers reflect actual impaired driving or increased prevalence of marijuana use? How much of the trend is explained by increased testing? Would recreational legalization (which followed in 2012) amplify this trend?
Common questions
Does testing positive for marijuana mean the driver was high?
Could increased testing explain the trend?
Read the original research
Trends in fatal motor vehicle crashes before and after marijuana commercialization in Colorado.
Drug and alcohol dependence, 140, 137-44
Citation
Salomonsen-Sautel, Stacy; Min, Sung-Joon; Sakai, Joseph T; Thurstone, Christian; Hopfer, Christian. (2014). Trends in fatal motor vehicle crashes before and after marijuana commercialization in Colorado.. Drug and alcohol dependence, 140, 137-44. https://doi.org/10.1016/j.drugalcdep.2014.04.008