Cannabis is legal for recreational purposes in 11 states around the nation. And along with that legal framework comes rules about how or when you can drive while under the influence of cannabis.

When each state legalized cannabis, many simply tacked cannabis on to their existing rules about driving under the influence of alcohol, with cannabis DUIs garnering the same penalties as alcohol DUIs. 

Like alcohol, cannabis impairs skills commonly associated with safe driving, like quick reflexes and divided attention. At the moment, though, research into how much cannabis impairs a driver and methods for accurately testing how much marijuana a driver is currently under the influence of are very limited.

In fact, a U.S. House of Representatives Appropriations Committee report this year asked for the NHTSA to coordinate with states in an attempt to develop more reliable guidelines, and acknowledged that creating standardized measurements for drug impairment was “unlikely in the near term.”

Now, a study conducted in Canada using toxicology results from trauma centers in British Columbia has given a little more insight into what impact blood marijuana level has on crashes.

Of the 3005 toxicology reports the study received, alcohol was involved in 14.4% of crashes. THC was involved in 8.3%, other drugs in 8.9% and sedating medications in 19.8%. 

The study found there was no increased crash risk in drivers with under 2 nanograms or with 2-5 nanograms of THC per milliliter of blood, and minimal increase in drivers with over 5 nanograms of THC per milliliter. Conversely, there was “significantly increased risk of crash responsibility” in drivers with a blood alcohol concentration of over 0.08%. 

The study speculates about the causes of this. There is evidence, it says, that drivers under the influence of cannabis are aware they are impared and so drive more carefully.