A single glass of wine will impair your driving more than smoking a joint. And under certain test conditions, the complex way alcohol and cannabis combine to affect driving behaviour suggests that someone who has taken both may drive less recklessly than a person who is simply drunk.

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 The unpublished research, seen exclusively by New Scientist, stops well short of condoning driving under the influence of even small amounts of cannabis.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”.

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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.