Driving while impaired in any way is never a good idea, yet thanks to recent marijuana legalization initiatives in many states, millions of drivers each year get behind the wheel while stoned. Despite hard data which suggest marijuana use increases one’s risk of being involved in a fatal accident, many cannabis users actually think marijuana makes them a better driver. To help dispel the confusion over the effect marijuana has on driving ability, the Governors Highway Safety Association has released a study claiming that marijuana indeed poses significant and measurable dangers for drivers. Will stricter anti-marijuana laws be on the way?

In states like California or Colorado which have booming marijuana industries, law enforcement agencies and legislators alike are searching for ways to cut back on the dangers associated with stoned driving. This latest study could help spearhead those efforts based on the conclusions it found. In an analysis of federal auto accident data, the Governors Highway Safety Association concluded that as the number of drunk drivers involved in fatal accidents decreased between 2006 and 2016, the amount of drugged or stoned drivers increased. Among those drivers with narcotics in their systems, 41% tested positive for marijuana.

Jim Hedlund, the study’s lead author, spent 22 years as a national highway traffic safety administrator. According to Hedlund, many cannabis users underestimate the drug’s effects on their driving ability:

Drugs can impair. Marijuana can impair. A lot of folks that use marijuana do not think that it can impair. In fact, they think they can drive better when they’re high. That’s not the case. The problem is the active ingredient THC decreases rapidly after smoking, within an hour, but impairment hangs on later.

The Governors Highway Safety Association report doesn’t just single out marijuana, though. Much of the study is focused on opiods and prescription medications and the threats they pose to drivers. The study’s authors point out that research into the effects of marijuana on driving ability is inconclusive due to the difficulties of determining whether or not someone has cannabis in their system and if so, how much.