The Highway Loss Data Institute, an insurance group that studies car accident claim rates and trends, published a study concluding that marijuana legalization leads to increased car accident rates. Their study examined three years' worth of insurance claims—over 2.5 million in all. Their study focused on the car accident rates in three states that have legalized marijuana use for recreational purposes: Colorado, Oregon, and Washington.
After taking into account weather conditions, vehicle types, population density, and other variables that could affect car accidents, the HLDI found that car accident claims saw an increase after marijuana use was legalized. However, measuring the increase isn't easy due to the structure of the study. Rather than looking at a before/after picture of car accident rates, the researchers looked at state-by-state comparisons with each state's immediate neighbors.
Comparing them on a state-by-state basis over the same time period allowed the researchers to get the clearest possible picture of how marijuana use affected drivers that year. If they had compared Colorado's driving trends on a year-by-year basis, changes in car accident rates could be attributed to the economy, driving trends, and other factors unrelated to the study.
Here's the problem with that approach:
It's not entirely clear how much marijuana affected each state's accident rate. For instance, if you compare Colorado with Utah, there's a
21% difference. If you compare Colorado with another neighbor, Wyoming, then you only have a 3% difference. The degree of difference is wildly different, depending on the neighboring state.
That's not the only problem other researchers see with the study.
Correlation vs. Causation: Not an Open-&-Shut Case
Researchers who have been looking at other data believe that marijuana use may not be affecting accident rates at all—for instance, the Colorado State Patrol reported a smaller number of crashes from 2015 to 2016.
Experts from Texas took a look at the crash rates in Washington and Colorado and found that the increase in car accidents was insignificant. Their study and conclusions were published in the American Journal of Public Health in June—a peer-reviewed publication. However, this study used federal data (which only includes fatalities) and information collected by police officers.
It's entirely possible that the difference here is found in minor crashes—while there may not be a significant increase in fatal accidents or major crashes, there might be an increase in small collisions.
The final criticism experts have levied against the insurance group's study is its assumptions about causation and correlation. Specifically, the communications director at the Marijuana Policy Project has said the study doesn't look at the causes of car accidents—it presumes that increase pot sales and increased car accidents are related.
It's already illegal to drive while under the influence of marijuana, so that's not at stake with these policy-shaping studies. The issue is there's no concrete way to measure how much a person is under the influence of marijuana. There's no blood or breath test for smoking pot. Even studies about car accident rates can't reach consensus—at least not yet.
When statutory law remains unclear, it's case law that will steer how marijuana DUI cases are tried. If you're ever arrested, finding an experienced, seasoned defender to handle your case will be more crucial than ever. How they defend your case could determine the rest of your life—especially during a period with little clarity about how marijuana affects driving ability.