Tomorrow's car could save the police a job. It could decide for itself that a motorist may have had too much to drink, deliver an audible warning, or even just stop and refuse to move.

Dilwyn Marple-Horvat, a physiologist at Bristol University, will test his front-seat breathalyser system on the public at a Bristol shopping centre on Friday and Saturday as part of national science week. Using a simulator rally game - and vodka - he has already confirmed that his "personal policeman" can detect the increasing breakdown in coordination of hand and eye with every glass of alcohol.

"Despite regular government advertising campaigns, people still drink and drive and fall asleep at the wheel," he said. "Currently there are no in-car methods of detecting whether a driver is unfit to drive and alerting them to this fact."

The system uses an infra-red camera to track the driver's eye movements, a sensor on the steering wheel to monitor every turn, and a little computer "black box" to compare data from the two.

Drivers with a clear head tend to look in a new direction of travel about three-quarters of a second before moving the hand on the steering wheel. The more a driver drinks, the shorter the delay between these two actions.

Dr Marple-Horvat came up with the idea after studying cerebellar ataxia patients who, because of brain injury, cannot match what they see with what they do.

In the US, tests have taken place on a breath analyser that will refuse to start the car if the driver smells of alcohol.