More drivers use marijuana, but link to crashes is murky

May 12, 2015


There are fewer alcohol-impaired drivers on U.S. roads than ever before, but the proportion of drivers testing positive for marijuana and other illegal drugs is on the rise, results of the latest National Roadside Survey of Alcohol and Drug Use by Drivers indicates. At the same time, an in-depth federal study found no link between marijuana use and driver crash risk after controlling for driver demographic factors and alcohol use.

The National Highway Traffic Safety Administration (NHTSA) in February released results of the 2013-14 roadside survey, a nationally representative survey of nighttime weekend drivers. The voluntary, anonymous survey includes data collected from more than 9,000 drivers at a representative sample of 300 roadside sites nationwide. This was the second time that the survey collected information about driver use of illegal and legal drugs in addition to alcohol. Both saliva and blood samples were used to detect drugs, including cannabinoids, stimulants, sedatives, antidepressants and narcotic analgesics. For marijuana, samples were screened for THC and its active metabolite, 11-OH-THC.

The survey found a large increase in the proportion of weekend nighttime drivers testing positive for marijuana or other illegal drugs compared with the 2007 survey, which was the first one to screen for drug use. About 1 in 5 weekend nighttime drivers tested positive for at least one legal or illegal drug, NHTSA reports. Marijuana showed the greatest increase in prevalence among illegal drugs. The percentage of weekend nighttime drivers testing positive for marijuana use increased from 8.6 percent in 2007 to 12.6 percent in 2013-14.

In contrast to the rise in drug use, the roadside survey found a third fewer drivers with alcohol in their system in 2013-14 compared with 2007. About 8.3 percent of weekend nighttime drivers tested positive for alcohol, and about 1.5 percent of drivers had a blood alcohol concentration (BAC) of 0.08 percent or higher. The proportion of weekend nighttime drivers at or above 0.08 percent BAC declined 32 percent from 2007 and plunged 80 percent from 1973 when the first roadside survey was conducted.

NHTSA in its research summary notes, "Changes in state policy on marijuana use, including medical and recreational use, may have contributed to an increase in marijuana use by drivers. However, the survey does not permit a state-by-state comparison. The change in use may reflect the emergence of a new trend in the country that warrants monitoring."

Voters in Colorado and Washington approved the legalization of the recreational use of marijuana for adults 21 and older in 2012, while Alaska, Oregon and the District of Columbia followed suit in 2014. Efforts are under way in a number of state legislatures to legalize marijuana or decriminalize possession. Medical marijuana use is legal in 23 states and D.C.

Evidence is mixed on the effects of marijuana use on crash risk. Laboratory studies indicate that marijuana use degrades driving skills, but crash-data research hasn't been as definitive. Some studies have found that using the drug could more than double crash risk, while others have failed to find a link between marijuana use and crashes.

NHTSA drug study

Results of the first large-scale case-control study in the U.S. to examine the crash risk associated with driver drug use help to broaden researchers' understanding of the issue.

Conducted in Virginia Beach, Va., during a 20-month period ending in 2012, the NHTSA-sponsored study gathered data from more than 3,000 drivers who were involved in police-reported crashes, plus a comparison group of 6,000 drivers who didn't crash. Research teams responded to crashes 24 hours a day, seven days a week and screened drivers for a large number of potentially impairing legal and illegal drugs using blood and saliva samples. NHTSA released a summary of the Virginia Beach study in early 2015, along with results of the national roadside survey.

The drug most frequently used by drivers was marijuana. THC, the main psychoactive substance in marijuana, was detected in 7.6 percent of the crash-involved drivers and 6.1 percent of the control drivers. In comparison, based on breath tests, alcohol was detected in 5 percent of the crash-involved drivers and 2.7 percent of the control drivers. After marijuana, the most frequently detected drugs were opiates (e.g., heroin, oxycodone) and stimulants (e.g., amphetamines, cocaine). About 3 percent of the crash-involved drivers tested positive for more than one class of drug; 2.1 percent of the control drivers tested positive for more than one drug.

Although marijuana-positive drivers were overrepresented in the crash-involved population, when researchers controlled for demographic factors (age, gender, ethnicity) and alcohol use, they didn't find an increase in crash risk associated with marijuana use. The analyses were unable to examine the crash risk associated with different amounts of the drug.

In contrast, driver alcohol use was associated with an elevated risk of a crash, both before and after controlling for demographic factors, and crash risk increased as BACs increased. Drivers with a BAC of 0.08 percent or higher had about 4 times the risk of crashing as sober drivers. Drivers with a BAC of 0.15 percent had 12 times the risk of crashing.

In announcing results of the research, NHTSA Administrator Mark Rosekind said, "The combined message of these two surveys is that our work to understand and combat drunk driving is paying off, but that we have much to learn about how illegal drugs and prescription medications affect highway safety — and that developing that knowledge is urgent, because more and more drivers have these drugs in their systems."

NHTSA notes that studies using driving simulators and test tracks have found that marijuana at sufficient dosage levels impairs driving functions. The agency is conducting more studies on the impact of drugged driving, including a roadside survey in Washington, where marijuana use is legal, as well as a simulator study with the National Institute on Drug Abuse to assess the performance of drivers under the influence of drugs.

Fewer weekend nighttime drivers have illegal BACs...

...but more drivers test positive for marijuana, other drugs

The 2007 national roadside survey was the first to include driver screening for illegal and legal drugs.

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