In a 1996 survey of drivers 16 years and older, more than one fourth reported that they occasionally drove while under the influence of alcohol, marijuana or both. According to the National Highway Traffic Safety Administration (NHTSA), alcohol is the predominant drug factor in fatal crashes and marijuana is the second most common drug factor in crashes. The NHTSA reports on the results of a study of the effects of alcohol and marijuana, alone and in combination, on driving performance.

The two-part study was performed for NHTSA at the Institute for Human Psycho-pharmacology at Maastricht University in the Netherlands. In one study, reaction time, ability to maintain a constant speed and ability to maintain a constant distance while following a moving vehicle ahead were assessed in 18 subjects between 20 and 28 years of age who reported smoking marijuana and drinking alcohol at least once a month. They were exposed to two on-road driving situations after use of marijuana alone, alcohol alone, a combination of marijuana and alcohol, or placebo.

While both alcohol and marijuana alone impaired driving performance, severe impairment was noted in subjects after combined alcohol and marijuana use. The mean reaction time increased from 4.65 seconds at baseline to 6.33 seconds when drivers were under the influence of alcohol and marijuana. This difference represented a 36 percent performance decrement in reaction time.

In another study, the effects of alcohol and marijuana were evaluated in 16 recreational marijuana users in a four-way crossover-design study. While the effects of low doses of marijuana and alcohol were minimal, moderate doses of marijuana and alcohol combined were found to impair driving performance in city traffic situations. The impairment was comparable to that observed with blood alcohol levels of 0.14 g per dL.

In a commentary that accompanied the NHTSA report, Jolly acknowledged the value and veracity of these studies on driver impairment but points out that marijuana use is more difficult than alcohol use to identify in drivers. He notes that the least experienced drivers may have the highest risk of a crash while under the influence of alcohol or other drugs. He advocates that emergency physicians work with law enforcement to evaluate and solve local problems stemming from drug-impaired drivers.