These are the findings of a major new study by British transport researchers. The unpublished research, seen exclusively by New Scientist, stops well short of condoning driving under the influence of even small amounts of cannabis.

But in a week which has seen renewed debate in Britain surrounding the criminalisation of cannabis, it throws an uncomfortable spotlight on a problem confronting governments everywhere - how to deter the growing numbers of cannabis users from "dope driving".

At present there is no accurate test that can reveal whether a driver has taken cannabis before driving, and developing one will not be easy. But even when this problem is cracked, another will remain - where to set the safety threshold for smoking cannabis.

Advocates of zero tolerance say there should be penalties for drivers caught with any amount of recently smoked cannabis in their body. The new research suggests that would only be credible if governments also adopted zero tolerance on drink driving.

Middle of the road

The new study was undertaken by the Transport Research Laboratory in Crowthorne, Berkshire, and confirms the results of a preliminary study more than a year ago. Researchers at the TRL, led by Barry Sexton, gave 15 volunteers doses of cannabis or alcohol, or a combination of both, before letting them loose on an array of psychomotor tests and a sophisticated driving simulator.

The volunteers were given either enough alcohol to raise alcohol levels in the blood to 50 milligrams per 100 millilitres - about 60 per cent of Britain's legal limit of 80 mg/100 ml - or a specially prepared marijuana joint designed to deliver the same high typically experienced by smokers.

In the study, cannabis significantly affected only one criterion, known as tracking ability. Volunteers found it more difficult to hold a constant speed and follow the middle of the road accurately while driving around a figure-of-eight loop. The TRL researchers point out in their draft report that this test requires drivers to hold their concentration for a short time, a task which is particularly badly affected by the intoxicating effects of cannabis.

Cautious driving

However, volunteers drinking the equivalent of a glass of wine fared worse than those who had smoked a joint. Those who were given both alcohol and cannabis performed worse still, reinforcing the idea that alcohol has a cumulative effect when taken with other drugs.

But the study also found that drivers on cannabis tended to be aware of their intoxicated state, and drove more cautiously to compensate. Indeed, doped-up volunteers often rated themselves as being more impaired than police surgeons brought in to evaluate their sobriety.

Surprisingly, drinking alcohol didn't offset this cautious behaviour, opening up the unproven possibility that a driver who is moderately drunk might be better off under some conditions if they had also smoked.

This cautious behaviour is in line with findings by other researchers. "Whereas alcohol promotes risk taking like fast speeds and close following, cannabis promotes conservative driving, but may cause attention problems and misperceptions of time," says Nicholas Ward, technical adviser to the Immortal project - a three-year European Union trial designed to quantify the crash risk drivers face after taking various drugs and medicines.

Exclusive from New Scientist Print Edition

Source: New Scientist (UK)
Author: Arran Frood
Published: March 19, 2002
Copyright: New Scientist, RBI Limited 2002
Contact: letters@newscientist.com
Website: http://www.newscientist.com/

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WINE 'IS WORSE THAN A JOINT'

A NEWS report that one glass of wine impaired driving performance more than a whole cannabis cigarette has angered those who took part in the government-backed research.

The New Scientist magazine article, by Arran Frood, was based on a leaked report from the Transport Research Laboratory.  However, the lab, in Crowthorne, Berkshire, said that the article was "full of inaccuracies", while declining to say what they were.  A TRL spokeswoman said: "Although the testing is finished, the report the author saw was in draft form and was not yet completed."

Mr Frood, said that the article, in the magazine's latest issue, was "thoroughly checked and the results of the tests corroborate with previous research on the subject."

According to the article, the TRL researchers had found that volunteers who drank 60 per cent of the legal drink-drive limit were less able to drive in a straight line or at a constant speed than those who had smoked a joint.  Unlike the tipsy drivers, they tended to be aware of their state and drove cautiously to compensate, it added.

The results suggested that automatically penalising drivers with smoked cannabis in their bodies would be credible only if a zero-tolerance policy were adopted for drink-driving as well, New Scientist said. 

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Dan B

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Australia: Study Goes to Pot (1998) http://www.mapinc.org/drugnews/v98/n947/a06.html

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I agree there should be impairment tests for driving, but as others have pointed out, blood-level thresholds are not the best way to test for impairment because people vary so much in terms of weight, experience, tolerance, etc. More sophisticated walk-the-line, touch-the-nose, types of tests would actually test impairment which is directly related to driving ability, rather than the amount of intoxicant in the blood, which can be arbitrary. These tests would also be generic and cheap. I'm sure the breathalyzer industry is gearing up their lobbying efforts already.

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