Study: Marijuana Doesn’t Affect Driving Performance


Photo: The Inspiration Room

‚ÄčIt’s something most seasoned pot smokers already know, but still it’s nice to get more scientific confirmation: Marijuana doesn’t make you wreck your car.

Subjects show almost identical driving skills just before and just after smoking marijuana, according to a study published in the March issue of the Journal of Psychoactive Drugs.

‚ÄčInvestigators from Hartford Hospital in Connecticut and the University of Iowa Carver College of Medicine measured the simulated driving performance of 85 people in a double-blind, placebo controlled trial.

Photo: West Coast Leaf

Volunteers responded to various simulated events associated with auto crash risk, such as avoiding a driver who was entering an intersection illegally, deciding to stop or go through a changing traffic light, responding to the presence of emergency vehicles, avoiding colliding with a dog who entered into traffic, and maintaining safe driving during a secondary (in-the-car) sound distraction.
Test subjects performed the tests sober, and then again 30 minutes after smoking a joint containing either 2.9 percent THC or a placebo joint with no THC.
The volunteers performed virtually the same after smoking marijuana as they did sober and/or after smoking bunk pot. “No differences were found during the baseline driving segment (and the) collision avoidance scenarios,” the scientists reported.
One reason no increase in accidents is associated with marijuana may be the well known “little old lady” syndrome, in which pot-smoking drivers slow down and drive more cautiously to compensate for any slight impairment that may occur.
“Participants receiving active marijuana decreased their speed more so than those receiving placebo cigarette during (the) distracted section of the drive,” the study reported.
Authors suggested that test subjects’ reduction in speed on this task could mean that they were compensating for perceived impairment.
“No other changes in driving performance were found,” researchers concluded.
A 2008 driving simulator study published in the scientific journal Accident, Analysis and Prevention also found that drivers who smoked marijuana were more likely to decrease their driving speed.
“Average speed was the most sensitive driving performance variable affected by both THC and alcohol but with an opposite effect,” investigators reported. “Smoking THC cigarettes caused drivers to drive slower in a dose-dependent manner, while alcohol caused driers to drive significantly faster than in ‘control’ conditions.”
The federal government’s Department of Transportation (DOT) did research with a fully interactive simulator on the effects of alcohol and marijuana, alone and in combination, on driver behavior and performance (“The Effects of Alcohol on Driver-Controlled Behavior in a Driving Simulator, Phase I,” DOT-HS-806-414). The study found that alcohol consistently and significantly caused impairment — but that marijuana only had an occasional effect.
A more recent federal study found that “THC [the active ingredient in marijuana]is not a profoundly impairing drug… It apparently affects controlled information processing in a variety of laboratory tests, but not to the extent which is beyond the individual’s ability to control when he is motivated and permitted to do so in driving” (“Marijuana and Actual Performance,” DOT-HS-808-078).
The federal study says that “It appears not possible to conclude anything about a driver’s impairment on the basis of his/her plasma concentrations of THC… determined in a single sample.”