With cannabis legalization happening in many parts of the US, researchers have been given the opportunity to study the effects of the drug on a large scale. That's how we know, for instance, that when cannabis is legalized, ice cream and cookie sales go up. 

In a new study, researchers from Temple University and the University of Arkansas measured the possible effects of cannabis legalization on road safety by analyzing automobile insurance data. The team was concerned that most of the literature on cannabis and traffic safety comes from the study of fatal crashes.

"Using fatality data is a significant shortcoming," the team wrote in their paper, published in the journal Health Economics. "In 2016, only 37,461 of the approximately 7,277,000 auto accidents reported to the police involved a fatality."

"The existing literature misses over 99.5% of auto crashes."

By looking instead at insurance premiums between 2014 and 2019, they hoped they would be able to get a clearer overall view of traffic accidents, and how they were affected by the legalization of weed.

"Auto insurers cover 67% of all medical and property damage from automobile accidents," they wrote. "Through this lens, we paint a more comprehensive picture."

"We find that the legalization of medical cannabis leads to a decrease in auto insurance premiums on average of $22 per policy per year," the team wrote in their study. 

"The effect is stronger in areas directly exposed to a dispensary, suggesting increased access to cannabis drives the results." 

Additionally, the team found that areas with relatively high drunk driving rates prior to the legalization of medical cannabis experienced large declines in their premiums following legalization. They estimate that policyholders in the states which legalized weed saved around $500 million in premiums.

"Using a ratio of attributable expenses from premiums," they add, "the annual savings attributable to reduced medical expenses is roughly $220 million."

The results may seem counter-intuitive – more access to drugs, you would think, would not lead to safer roads and fewer accidents. However, the team has a number of theories of possible contributing factors, including that fewer people are driving under the influence of alcohol. 

However, they caution against assuming that the effect is only because people are simply substituting weed for alcohol. It could be a matter of where people go to get high versus where they go to get drunk.

"Bar-equivalents do not typically exist for cannabis, and current medical cannabis laws stipulate that consumption occurs in a private residence," the team concluded in their paper. "Thus, joint consumption of cannabis and alcohol is likely to take place in the home."