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Number 225                                                                                                    May 2000

U.S. Department of Transportation
National Highway Traffic Safety Administration
400 Seventh Street, S.W., Washington, DC 20590




Findings from the 1996 National Household Survey of Drug Abuse suggest that more than a quarter of the 166 million drivers age 16 and older occasionally drive under the influence of alcohol or a combination of alcohol and marijuana in the United States (See Traffic Tech 185, Dec 1998). The Experimental Psychopharmacology Unit at Maastricht University in the Netherlands has completed the third of a series of studies for the National Highway Traffic Safety Administration (NHTSA) to assess the separate and combined effects of marijuana and alcohol on driving performance in real driving situations (See Traffic Tech 201, June 1999).

In this third study, the purpose was to assess the effects of low doses of marijuana and alcohol, and their combination, on visual search at intersections and general driving proficiency in a City Driving Test. Sixteen subjects between the ages of 21 and 40 (8 male, 8 female) who said they smoked marijuana and drank alcohol at least once a month participated in the study. They were all licensed drivers.

Each participant was dosed with marijuana alone, alcohol alone, a combination of marijuana and alcohol, or neither. Participants began by drinking alcohol or alcohol-placebo. They continued smoking marijuana-placebo or marijuana delivering 9-tetrahydrocannabinol (THC), the primary psychoactive ingredient of marijuana, at 100 µg/kg body weight, a low dose. The marijuana placebo contained marijuana leaf from which the THC had been removed. This level was selected based on an earlier NHTSA study. An initial alcohol dose sufficient to achieve a blood alcohol concentration (BAC) of about 0.04-0.05 g/dl was administered, well below the per se levels in the United States for drivers age 21 and older.

The City Driving Tests were conducted in the evening over a constant route of about 9.3 miles (15 km) within the city of Maastricht. The route went through business and residential areas on two-lane undivided streets and included a 3.1 mile (5 km) four-lane divided segment on a major cross-city thoroughfare. Maneuvers included left and right turns at some intersections and driving through others, left and right lane changes, responding to traffic control devices, and a turn on a residential street. A shortened version of the Royal Dutch Tourists Association Driving Proficiency Test was used to rate drivers' performance on 90 items.

Eye movements were recorded during all tests by a head mounted eye tracking system that accurately measured a freely moving subject's eye line of gaze. A second miniature camera focused on the forward visual scene. Video recordings of participants' eye line of gaze were used to determine their visual search for vehicles on the right at 58 intersections. Two persons accompanied each participant: a licensed driving instructor sitting in the front passenger seat with separate dual controls for safety control and an assistant sitting in the center of the rear seat for data collection.

Mean & Standard Error of Visual Search for
Traffic at Intersections


Visual Search at Intersections Declined for Alcohol and Marijuana Combinations
The graph on the previous page shows the average or mean frequency of time spent during visual search at intersections. The first bar shows that subjects in the placebo condition (neither alcohol nor marijuana), checked for traffic at intersections about 84 percent of the time. Neither alcohol alone nor marijuana alone caused any notable differences in the response. However, the combined treatment with alcohol and THC significantly reduced visual search frequency by about 3 percent. The decline in search frequency was most pronounced in females (7 percent), but the gender effect, as opposed to the overall effect, was not significant.

Perceived Driving Performance and Level of Effort
Each participant scored themselves on their driving performance. The next chart shows that they rated their driving performance significantly lower in the THC condition as compared to those in the placebo condition. They also thought that it took more effort to complete the driving test in the THC condition. Neither alcohol nor alcohol combined with THC significantly altered the participant's ratings of their driving quality and effort.

Mean & Standard Error of Participant's
Perceived Driving Performance

At intersections, participants looked at side streets from the right an average of 84 percent in all cases in the placebo condition. Visual search frequency of these subjects did not change much with either the low dose alcohol or low dose marijuana. Visual search in the combination of both alcohol and marijuana condition, however, dropped by 3 percent.

Low doses of marijuana (100 µg/kg) taken alone, did not impair city driving performance and did not diminish visual search frequency for traffic at intersections in this study. Low doses of alcohol sufficient for producing a BAC of 0.04 g/dl did not impair city driving performance and did not diminish visual search frequency for traffic at intersections in this study. Low doses of marijuana in combination with low doses of alcohol did not affect city driving proficiency, but it did impair peripheral search for traffic in this study. The effects of low doses of marijuana (100 µg/kg) and BAC about 0.04 g/dl on city driving proficiency and visual search were minimal when taken alone, but potentially dangerous for traffic safety when combined. Drivers' perception of their performance did not necessarily match their actual visual search performance.


For a copy of Visual Search and Urban City Driving Under the Influence of Marijuana and Alcohol (26 pages), prepared by the Brain and Behavior Institute of Maastricht University, the Netherlands, write to the Media and Marketing Division, NHTSA, NTS-21, 400 Seventh Street, S.W., Washington, DC 20590, or send a fax to (202) 493-2062.

U.S. Department
of Transportation
National Highway
Traffic Safety

400 Seventh Street, S.W. NTS-31
Washington, DC 20590

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