Marijuana use had little effect on simulated driving skills, according to a Hartford Hospital study, but test subjects were more easily distracted when under the influence of the drug.
Investigators from Hartford Hospital and the University of Iowa Carver College of Medicine assessed the simulated driving performance of 50 male and 35 female subjects in a double-blind, placebo-controlled trial. All 85 subjects reported having used marijuana from one to 10 times per month previously.
The study was published in the March issue of the Journal of Psychoactive Drugs.
During the study, some subjects were given actual marijuana cigarettes, and some were given a placebo, with neither the investigator nor the subject knowing which they had smoked. Another administrator kept track of who was given which type of cigarette.
The marijuana was supplied by the National Institute of Drug Abuse and the University of Mississippi, the only legal source of the drug in the U.S.
Subjects drove a high-tech simulator that was very realistic, said Beth Anderson, an investigator in the study. "It was an actual car with parts replaced by computers."
Participants then drove down a simulated country road for 15 minutes, first in an "uneventful" simulation, and then in collision-avoidance and distracted-driving simulations, the study states.
In the collision-avoidance portion, drivers reacted to simulated events such as another driver entering an intersection illegally, a changing traffic light, and a dog running into the road.
The researchers found no signifcant difference between the study groups in the collision-avoidance tests.
During the distracted-driving segment, participants solved "mental math" problems while driving, Anderson said. Subjects answered aloud simple math problems that were provided by a recording.
Speed and steering variability, as well as the number of errors made in the math portion of the test, were used to determine how impaired subjects were, according to Anderson.
"The study didn't find a lot of impairment," Anderson said. "[Subjects] slowed down. It looked like they were trying to compensate. Compensation would only take you so far."
The study states that "participants receiving active marijuana decreased their speed more so than those receiving the placebo cigarette during a distracted section of the drive."
Anderson stressed that the findings do not mean that driving high is harmless.
For instance, researchers noted that in the distracted-driving tests, "participants under the influence of marijuana failed to benefit from prior [driving] experience … as evidenced by a decrease in speed and a failure to show expected practice effects."
"The results do not imply that it is safe to drive under the influence of marijuana, especially because we know people aren't just smoking marijuana," Anderson said. "They do it while drinking. They do this when others are in the car, listening to music, talking on cellphones or texting. These behaviors distract drivers and are even more dangerous when someone has been using marijuana."
Anderson said the study showed that the effects of marijuana on driving need to be studied further.
"We need to know what marijuana does to the brain. We need to understand the ramifications. To create public policy and to keep people safe, you need to know what's really happening in the brain," Anderson said. "You have to have the facts."
A 2004 fact sheet published by the National Highway Traffic Safety Administration said that marijuana has been shown to adversely affect driving.
"Decreased car handling performance, increased reaction times, impaired time and distance estimation, inability to maintain headway, lateral travel, subjective sleepiness, motor incoordination, and impaired sustained vigilance have all been reported," the fact sheet states.
"Some drivers may actually be able to improve performance for brief periods by overcompensating for self-perceived impairment. The greater the demands placed on the driver, however, the more critical the likely impairment."