U.S. DEPARTMENT OF TRANSPORTATION

NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION

DOT HS 808 078 NOVEMBER 1993

MARIJUANA AND ACTUAL DRIVING PERFORMANCE

CONCLUSIONS

The major conclusions from the present program are summarized as follows:

* Current users of marijuana prefer THC doses of about 300 ug/kg to achieve their desired "high".

* It is possible to safely study the effects of marijuana on driving on highways or city streets in the presence of other traffic.

* Marijuana smoking impairs fundamental road tracking ability with the degree if impairment increasing as a function of the consumed THC dose.

* Marijuana smoking which delivers THC up to a 300 ug/kg dose slightly impairs the ability to maintain a constant headway while following another car.

* A low THC dose (100 ug/kg) does not impair driving ability in urban traffic to the same extent as a blood alcohol concentration (BAC) of 0.04g%.

* Drivers under the influence of marijuana tend to over-estimate the adverse effects of the drug on their driving quality and compensate when they can; e.g. by increasing effort to accomplish the task, increasing headway or slowing down, or a combination of these.

* Drivers under the influence of alcohol tend to under-estimate the adverse effects of the drug on their driving quality and do not invest compensatory effort.

* The maximum road tracking impairment after the highest THC dose (300 ug/kg) was within a range of effects produced by many commonly used medicinal drugs and less than that associated with a blood alcohol concentration (BAC) of 0.08g% in previous studies employing the same test.

* It is not possible to conclude anything about a driver's impairment on the basis of his/her plasma concentrations of THC and THC-COOH determined in a single sample.