The AAA Position: Marijuana’s Effect on Driving

The AAA Position: Marijuana’s Effect on Driving

The AAA Position: Marijuana’s Effect on Driving

This election year, voters in five states will decide whether to legalize recreational marijuana use. Among them are California with Proposition 64 and Maine with Question 1. Any states that do will join the four others where the drug is already legal for recreational use. The AAA Foundation for Traffic Safety analyzed cannabis use by drivers in one of those states, Washington, and found that the proportion of drivers involved in fatal crashes who had recently used marijuana more than doubled after Washington legalized the drug for recreational use. In addition, there’s currently no easy way to test whether a driver is impaired by marijuana: Unlike alcohol, it can’t be determined by breath or blood tests.

After alcohol, marijuana is the most common drug found in drivers who have been involved in traffic collisions.Marijuana’s main psychoactive ingredient, active-THC, affects key parts of the brain, which can lead to:

Difficulty paying attention

Difficulty staying in traffic lane

Slower reaction times

Difficulty judging distances

Slower decision-making

Reduced peripheral vision

Reduced coordination

 Research results are mixed, but some studies have found that using marijuana as much as doubles a driver’s risk of crashing.2 Furthermore, research shows that drivers killed in crashes who tested positive for marijuana were 1.29 to 6.6 times more likely to have caused the collision.3

A Worrying Trend in Washington State

In 2012, Washington voters approved Initiative 502 to legalize recreational marijuana use for adults. It took effect in December of that year. AAA examined drug tests and fatal crashes among Washington drivers between 2010 and 2014 and found the following:

Percentage of Washington drivers involved in fatal crashes who had recently used marijuana

8% in 2013

17% in 2014

After legalization, the proportion of fatal crashes that involved marijuana more than doubled.

While the data analyzed for the study did not include enough information to determine which driver was at fault in a given crash, the trend is troubling because the proportion of fatal crashes involving marijuana in Washington had been relatively stable between 2010 and 2013.

Determining Impairment Isn’t Easy

To combat marijuana-related crashes, some states have instituted “per se” legal blood limits on active-THC. Washington instituted one as part of Initiative 502, and four other states have them as well.

Per se limits make it a crime to drive with more than a certain amount of a drug in one’s system. Drunk driving laws are a well-known example: In the U.S., driving with a blood alcohol concentration above 0.08 percent is automatically a crime. That’s because decades of research have established a well-understood relationship between how much alcohol is in someone’s blood and their risk of crashing.

Per se limits work for alcohol, because we can reliably predict crash risk from blood alcohol concentration.

They do not work for marijuana, however, for several reasons:

There’s no evidence that drivers definitively become impaired at a specific level of active-THC in the blood. Some individuals with high blood active-THC levels may not be significantly impaired, whereas others with low levels may still be severely affected.

There’s currently no way to quickly determine active-THC levels. AAA found that it takes more than two hours on average to collect a blood sample, which means high active-THC levels may decline significantly before they can be measured.

Marijuana is metabolized in the body differently from person to person. Frequent users can exhibit persistent blood active-THC long after active use, while occasional users may see their levels decline much more rapidly.

Per se legal limits on marijuana intoxication while driving are well-intentioned, but they’re not supported by scientific evidence. Instead, they’re likely to result in unsafe drivers being cleared and unimpaired drivers being convicted.

AAA’s Position

AAA opposes measures to legalize recreational marijuana use, and recommends a “no” vote on both California’s Proposition 64 and Maine’s Question 1. We have a genuine traffic safety concern related to the legalization of recreational marijuana use. It has taken generations to educate the public about drinking and driving and to strengthen the laws to reduce drunken driving. These measures would create new traffic safety issues and increase the problem of impaired driving.

The AAA Foundation for Traffic Safety’s recent research raises many concerns about whether we are prepared to address the traffic safety risks Proposition 64 and Question 1 pose. More studies are needed before making such a far-reaching policy change that could have unintended, but tragic, consequences for traffic safety.


1 “Trends in alcohol and other drugs detected in fatally injured drivers,” Brady, 2014.
2 “Acute cannabis consumption and motor vehicle collision risk,” Asbridge, Hayden, and Cartwright, 2013.
3 “Cannabis effects on driving skills,” Hartman and Heustis, 2013.

Information taken from “Fatal Road Crashes Involving Marijuana Double After State Legalizes Drug,” May 10, 2016, AAA Foundation for Traffic Safety.

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