New Study Examines Medical Marijuana's Effects on Drivers

The University of California at San Diego’s Center for Medicinal Cannabis Research is conducting a two-year study on the effects of driving while stoned.
The University of California at San Diego’s Center for Medicinal Cannabis Research is conducting a two-year study on the effects of driving while stoned.
Courtesy Dr. Tom Marcotte

With just a few short weeks until the Nov. 8 election, opponents and supporters of Proposition 64 continue to make the case for and against recreational marijuana.

Polls suggest a majority of Californians supports legalizing marijuana for recreational use — 58 percent, to be exact, according to a recent Los Angeles Times poll. The Automobile Club of Southern California (part of the national American Automobile Association, or AAA) is not among Proposition 64's proponents.

Last week, AAA announced its opposition to legalizing cannabis in both California and Maine, which is also voting on recreational pot this year. The Auto Club says weed-related traffic deaths are up in Washington after the state legalized cannabis in 2012.

“The AAA Foundation for Traffic Safety 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 is 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,” AAA says on its website.

The club goes on to cite “marijuana’s effects on driving,” including difficulty paying attention, slower reaction times, slower decision-making and reduced coordination. “Research results are mixed," AAA admits, "but some studies have found that using marijuana as much as doubled a driver’s risk of crashing.”

Driving under the influence will remain illegal even if Proposition 64 passes. There is no legal limit in California for how much THC can be in your system while behind the wheel, but lawmakers in Sacramento have pondered the question in the past and are likely to revisit the issue again. In Colorado, Washington and other states where weed is nearly legal, drivers are allowed up to five nanograms. But some experts are skeptical of the quantification.

“It’s a real challenge,” says Dr. Tom Marcotte, co-director of the University of California at San Diego’s Center for Medicinal Cannabis Research. “Most people would say [five nanograms] isn’t a very compelling number. Unlike alcohol, where there is clear correlation and linear effect — you drink more alcohol, your blood alcohol content goes up, and you’re more impaired and probably not safe to drive — with cannabis you smoke it, your THC levels in the blood go up really quickly and then they drop really quickly.”

THC is a fat-soluble substance that travels through water systems, such as blood, and then settles into fatty environments, like the brain. This makes roadside testing especially complicated. Blood, saliva and urine tests can only measure current THC levels, which aren’t necessarily a good indicator of a driver’s impairment. Until roadside tests can on the spot analyze brain tissue, testing for drugged driving is nearly impossible. Then there are edibles. Food does not travel through the blood, and it can take hours before it's digested and peak THC levels are reached.

UCSD is in the early stages of a driving study that could help fill in some of these gray areas. Test subjects will smoke either a placebo or a THC joint, and then Marcotte’s team will follow them throughout the day. Their driving performance will be monitored using a simulator in a controlled lab. Blood, urine and breath samples will be collected, but the key to understanding impaired driving could be the cognitive tests administered by researchers and state-sanctioned drug-recognition experts, or DREs. These DREs are law enforcement officials specially trained to evaluate if someone is under the influence of drugs other than alcohol. They also will evaluate test subjects in order to develop on-the-spot cannabis-impairment guidelines.

“With alcohol, you see people staggering, but you don’t see that with cannabis,” Marcotte says, adding that California officials are really interested in knowing whether the current tests they have can be applied to cannabis use.

In a different study, Marcotte compared the effects of medicinal cannabis on driving in patients with multiple sclerosis, and found that the amount people swerved in that study was not substantially different that what had been reported in another study of individuals the morning after taking Ambien. But this study, and others like it, didn’t take into account chronic smokers whose tolerance has built up over time. This last variable also is key to determining when someone is too impaired to drive and could hold the key to developing accurate roadside testing.


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