ALCOHOL IMPAIRS DRIVING MORE THAN MARIJUANA

Exclusive From New Scientist Print Edition

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 The unpublished research, seen exclusively by New Scientist,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".

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.