July 4.

The police are right to be concerned about the pocket breathalyser

(July 1). Not only may some people drink up to the limit but in a series

of experiments funded by the Alcohol Education and Research Council and

conducted along with our colleague, Professor Keith Millar, we have

found that people's inability to drive after alcohol is not well related

to their current blood alcohol reading.

Impairment on tests of ability to react quickly and steer accurately

can occur below the legal limit for driving and can still be present

after someone's reading has dropped below the legal limit. ''Legal''

does not mean ''safe''.

People also vary considerably in how alcohol affects them.

Conventional wisdom is that people who drink more frequently can handle

alcohol better. We have found that this is not entirely the case.

Frequent drinkers learn to compensate for the effects of alcohol by

trying harder, but their increased effort is often at the expense of

other important aspects of the task.

For example, someone may concentrate carefully on steering the car

along the road and arrive home without incident. Unfortunately, bceacuse

they are concentrating harder than normal on steering they will be much

slower to react if something unexpected happens such as a child running

into the road. The many drivers who continue to claim that they drive

all right after a couple of drinks because they are ''careful'' should

be thankful that they have yet to encounter such an emergency.

We might also point out that the defence of unwittingly ingesting

alcohol sufficient to bring one over the legal limit (in fruitcake,

trifle, spiked drinks etc.) is often a fib. In our experiments most

people given enough alcohol to reach the legal limit can tell that they

have had alcohol because they recognise the effects which it has on the

body.

For safety people should not drink at all before driving or operating

other dangerous or demanding machinery. To discourage the game of

guessing how much alcohol is ''safe'' we would favour a reduction of the

current legal limit to 20 milligrams of alcohol per 100ml of blood,

which would provide some leeway for people who have fermented alcohol in

their gut or have genuinely taken a small amount of alcohol in food. To

killjoy is surely better than to kill.

Richard Hammersley

and Frances Finnigan,

Behavioural Sciences Group,

University of Glasgow.