Marijuana And Actual Driving Performance Executive Summary
National Highway Traffic Safety Administration
By Robbe HWJ, O’Hanlon JF
November 1993
http://www.erowid.org/plants/cannabis/cannabis_driving4.shtml
Abstract
Abstract: This report concerns the effects of marijuana smoking on actual driving performance. It presents the results of one pilot and three actual driving studies. The pilot study’s major purpose was to establish the THC dose current marijuana users smoke to achieve their desired “high”.
From these results it was decided that the maximum THC dose for subsequent driving studies would be 300 mcg / kg (0.3 mg / kg). The first driving study was conducted on a closed section of a primary highway. After smoking marijuana delivering THC doses of 0, 100, 200, and 300 mcg / kg, subjects drove a car while maintaining a constant speed and lateral position.
This study was replicated with a new group of subjects, but now in the presence of other traffic. In addition, a car following test was executed. The third driving study compared the effects of a modest dose of THC (100 mcg / kg) and alcohol )BAC of 0.04 g %) on city driving performance.
This program of research has shown that marijuana, when taken alone, produces a moderate degree of driving impairment which is related to the consumed THC dose. The impairment manifests itself mainly in the ability to maintain a steady lateral position on the road, but its magnitude is not exceptional in comparison with changes produced by many medicinal drugs and alcohol.
Drivers under the influence of marijuana retain insight in their performance and will compensate where they can, for example, by slowing down or increasing effort. As a consequence, THC’s adverse effects on driving performance appear relatively small.
Marijuana Use and Driving
By Robbe HWJ
November 1994
http://www.erowid.org/plants/cannabis/cannabis_driving3.shtml
Abstract
Abstract: This article concerns the effects of marijuana smoking on actual driving performance. It presents the major results of one laboratory and three on-road driving studies. The latter were conducted on a closed section of a primary highway, on a highway in the presence of other traffic and in urban traffic, respectively.
This program of research has shown that marijuana produces only a moderate degree of driving impairment which is related to the consumed THC dose. The impairment manifests itself mainly in the ability to maintain a steady lateral position on the road, but its magnitude is not exceptional in comparison with changes produced by many medicinal drugs and alcohol.
Drivers under the influence of marijuana retain insight into their performance and will compensate where they can (e.g., by increasing distance between vehicles or increasing effort). As a consequence, THC’s adverse effects on driving performance appeared relatively small in the tests employed in this program.
Marijuana, Alcohol and Actual Driving Performance
By Hindrik W. J. Robbe, Ph.D. and James F. O’Hanlon, Ph.D.
1999?
http://www.erowid.org/plants/cannabis/cannabis_driving5.shtml
In a previous series of studies on the effects of THC alone we concluded that THC given in doses up to 300 1lg/kg has “slight” effects on driving performance (Robbe & O’Hanlon, 1993). The results of the present study now compel us to revise that conclusion. The present subjects’ performance was more affected than their predecessors’.
The present subjects showed impaired car following performance after THC 100 1lg/kg whereas the previous ones were not impaired by doses up to 300 1lg/kg. In the present study, road tracking performance after 200 ~g/kg was worse than the performance after 300 ~g/kg in the previous study.
We believe that these differences are attributable to the groups’ respective experience with THC smoking and to driving under the influence of THC. The present group was less experienced and probably had not developed the same degree of behavioral tolerance as their predecessors.
Yet all of the individuals in both groups admitted to having occasionally driven under the influence of THC before entering the studies. Thus, the new data seem no less representative of how drivers normally operate under the influence of THC. The addition of these data to those previously collected merely broadens the range of reactions that might be expected to occur in real life.
That range has not been shown to extend into the area that can rightfully be regarded as dangerous or an obviously unacceptable threat to public safety. Alcohol present in blood concentrations around the legal limit (0.10 g/dl) in most American States is more impairing than anything subjects have shown after THC alone in our studies.
As mentioned, medicinal drugs have had worse effects on psychiatric patients’ driving performance in other studies employing the same test procedures. If not blatantly dangerous, however, the effects of THC alone in this study were certainly more than slight. They were of sufficient magnitude to warrant concern.
Drivers suffering the same degrees of impairment as the present subjects did after THC alone would be less than normally able to avoid collisions if confronted with the sudden need for evasive action. They would probably also be more likely to fall asleep during prolonged vehicle operation.
In short, while the effects of THC alone in doses up to 200 1lg/kg might be categorized as “moderate” in the tests, they could easily become “severe” under exceptional circumstances.
http://www.idmu.co.uk/candrivab.htm
Results: Overall, drug using drivers reported 9% fewer accidents than would be expected from a comparable population group. However, younger drug-using drivers showed increased risks compared to their peers, whereas drivers over 25 reported decreased incidence of accidents. Notable sex differences included drug-using women under 25 or over 40 reporting more accidents than their peers, whereas only males under 20 reported increased risks, with males over 25 reporting markedly lower risk.
Interpretation: Impairment of driving ability from cannabis appears most pronounced among young drivers, suggesting tasks requiring conscious thought or learning may be most affected, whereas “automatic” tasks may be less susceptible to disruption.
The overall effect of cannabis on driving suggests a trade-off between mild psychomotor impairment and improvement in driving behaviour. In males, improved behaviour may outweigh any psychomotor impairment, whereas in females there may be less scope for improvement in behaviour, drug-impairment may outweigh marginal behavioural improvements.
