Marijuana and Car Accident Rates: All the Facts

Most people understand that impaired driving is a bad idea. Schools, parents, hospitals, and activist organizations frequently warn children and young adults about the dangers of drinking alcohol and driving, and for good reason. A report by the National Highway Traffic Safety Administration attempted to quantify the increase in collision risk as the consumption of alcohol increases: at a BAC of just .06, under the legal limit, the risk of an accident goes up more than 100 percent. At a BAC of .19, the risk of an accident increases nearly 2,000 percent.

But alcohol isn’t the only substance that requires consideration these days. Marijuana use has increased significantly over the past few decades, nearly doubling between 1984 and 2015. It’s tempting to blame this increase on the legalization of marijuana, but researchers find it’s actually due to a trend of relaxed attitudes regarding the substance.

If people are smoking more marijuana, does that mean the rate of impaired drivers is also going up? And if so, does being impaired by marijuana come with the same increase in risk as being impaired by alcohol?

Understanding Risk

It’s important to understand the risks here for several reasons. Knowing the full effects of these substances is important to make the right decision if and when partaking in them, and allows people to monitor and take action against friends who might otherwise put themselves (and others) at risk. If involved in a collision, it also gives a person information they can use to determine whether the other party is at fault, and whether it’s worth pursuing a personal injury settlement after the fact.

Marijuana vs. Alcohol

Researchers can’t directly compare the effects of alcohol to the effects of marijuana while driving, especially considering the different effects these substances have on different people. According to a study originally published in the American Journal on Addictions, alcohol and THC (the active ingredient in marijuana) have very different effects, even though both result in impairment.

For example, alcohol has very little effect on conscious decision-making while driving, but highly effects one’s ability to respond automatically to changing conditions. THC has the opposite effect, impairing conscious decision-making but leaving automatic responses less affected. THC users also have a heightened awareness of their impairment level; in other words, drinkers often underestimate how impaired they are, while THC users often overestimate how impaired they are. While driving, this can lead to THC users taking more caution, driving slower, and leaving more room between vehicles.

THC also has a greater variability of effects between users. Similar adults who drink a similar amount of alcohol will show similar signs and effects, but with THC, variables like smoking technique, personal tolerance, and levels of absorption can produce very different effects. This makes the effects of THC much harder to predict than those of alcohol.

The worst effects were seen with a combination of alcohol and marijuana; with this combination, both conscious and unconscious control functions are impaired, and individuals are unable to assess their impairment level.

The Numbers

The effects of marijuana are hard to study, especially when comparing them to alcohol. Instead, it’s important to look at real numbers when evaluating the risk of an accident-but those studies aren’t clear either. For example, one study by the Insurance Institute for Highway Safety found that insurance claims for vehicle collisions increased in states that had recently legalized marijuana by about three percent. However, that study doesn’t prove causality between marijuana and those vehicle crashes.

Conversely, a study found in the American Journal of Public Health studied Colorado and Washington (states where recreational marijuana has been recently legalized), and found that there was no significant increase in vehicle crash fatalities after legalization.

It’s hard to say exactly how much additional risk is faced when driving impaired by the effects of marijuana, or exactly what the legalization of marijuana will do to vehicle collision rates. In any case, the best path forward is one that minimizes risk. Drivers are well-served by limiting their consumption of substances that could impair them, and watching for other people driving erratically.

Melissa Thompson writes about a wide range of topics, revealing interesting things we didn't know before. She is a freelance USA Today producer, and a Technorati contributor.