Do US States Which Have Legalised Cannabis Have Higher Rates Of Car Crashes?

Cannabis use seems to have increased with legalisation

As cannabis proponents rejoice in the increasing global acceptance of cannabis use, opponents find statistics to support arguments against cannabis use. The latest is car crashes, with new attention being given to a supposed increase in automobile accidents in parts of the United States (US) where cannabis use is legal.

Cannabis is legal in 33 US states, either for medical or recreational purposes. This number is expected to increase in the coming years, and could eventually lead to the substance being legal in all states, either continuing with the state-by-state approach or on the federal level.

With legal cannabis being the case in the majority of US states, cannabis use has increased, with 1 in 7 US residents using the substance as of 2017. Such a scenario likely means more drivers on the road who are under the influence of cannabis, and that likelihood is raising concerns.


Cannabis use in the US has increased. Source



Study claims legal cannabis leads to increased auto accidents

The Insurance Institute for Highway Safety (IIHS) and the Highway Loss Data Institute (HLDI) came together on a study comparing US states where cannabis use is legal to states where it isn’t, to determine if there was a higher rate of car crashes in those legal states. The conclusion was that there was, in fact, a higher rate in legal cannabis states, and that the rate increased after those states legalised cannabis.

Specifically, the study focused on data gathered between 2012 and 2016, finding a 6 percent increase in car crashes in the following states after they legalised cannabis use: Colorado, Oregon, Washington, and Nevada.


Claims of an increase in car crashes in US states where cannabis is legal. Source

But this study leaves a lot on the table. In fact, a number of other studies prove otherwise, and the IIHS and HLDI study fails to address a number of questions.



Other studies contradict the IIHS and HLDI conclusions

While the IIHS and HLDI studies states including Washington and Colorado to arrive at their claim of a 6 percent increase in car crashes after legalisation, another study directly contradicts that claim.

A study published in the American Journal of Public Health asked the same question of Washington and Colorado as the IIHS and HLDI did, but focused on data spanning three years after legalisation. The conclusion there was that any evident increases were on par with states that had not legalised cannabis use. 

Another study published on the National Bureau of Economic Research suggests that cannabis legalisation doesn’t have any causal effect on increasing the amount of traffic fatalities. In fact, an author of that study, Benjamin Hansen, actually suggested a decrease in auto fatalities in legal cannabis US states in a previous study. 

Another recent study does cite an increase in auto crashes in legal states, however. That study claims that Colorado, Oregon, and Washington saw an increase of 1.08 traffic fatalities for every 1 million residents. However, that increase was temporary -- it later reduced, going back to normal in about a year. 



IIHS and HLDI fail to answer some critical questions

Despite their claim that legalising cannabis resulted in an increase in car crashes, the IIHS and HLDI could not determine how those laws cause an increase in crashes. Further, increases in populations in those states studied (Colorado saw a 10 percent population increase, for example) weren’t accounted for.

Also, there aren’t any completely reliable methods to determine cannabis use while driving. Therefore, in those states where cannabis isn’t legal, it wouldn’t be possible to determine for certain if car crashes involved cannabis use or not. The same applies to states where the drug is legal -- how do you know if cannabis was truly involved (or not) without a completely reliable mechanism for detecting its use immediately before getting behind the wheel (or while behind the wheel)?



Final words

Nothing in the IIHS and HLDI study proves cannabis legalisation leads to increases in car crashes. As for now, the answer to the question of whether legalisation increases crashes is either No or Unknown. The fact remains that a number of studies contradict the claims of the IIHS and HLDI, and the study avoids some significant questions. For now, more research must be done on the topic before concluding that crashes are a result of legal cannabis.