Marijuana and driving ability

        After alcohol, Cannabis is the drug which is found most often in the blood of drivers involved in road accidents.   The effects of THC, the major psychoactive component of Cannabis, on driving ability have been investigated by Dr. H.W.J. Robbe at the Institute of Human Psychopharmacology of the University of Limburg in Maastricht, The Netherlands.
        In his PhD thesis, "Influence of Marijuana on Driving" (available through the IHA), which Robbe defended on May 26, he concludes that marijuana is not a safe drug in traffic.  After using marijuana drivers were less able to drive in a straight line.  However, compared to the effects of alcohol and many medicines (antidepressants and sedatives) on driving ability, the effects of marijuana are not exceptional.
        The use of a moderate amount of marijuana did not affect driving ability, but the drivers thought their ability had been negatively affected.  The use of a moderate amount of alcohol had a negative effect on driving ability, while the drivers thought their performance had not been affected.  The use of alcohol seems to stimulate risky behaviour, whereas the use of marijuana leads to a more prudent driving style.