He drove around for several hours, and at 10 in the morning he fell asleep at the wheel on a motorway

Diana Carpenter Victim's mother

Breathalysers only measure alcohol and so cannot identify drivers who are under the influence of drugs.

While 800,000 drivers were tested for alcohol, only 2,000 were tested for drugs.

Experts believe that at least 4,500 deaths and 135,000 serious injuries are caused in Europe each year by drug-driving

Diana Carpenter's daughter, Amy, was critically injured in an accident involving a driver who had taken ecstasy and amphetamines.

"She was asleep in the back of a car after a party and the driver came along and got in the car and drove off," Mrs Carpenter told BBC News.

"He drove around for several hours, and at 10 in the morning he fell asleep at the wheel on a motorway."

Roadside testing

Mrs Carpenter is backing a pilot scheme launched by nine police forces for road checks to help discover drivers who have taken drugs.

It is estimated that about 88,000 drivers a year escape prosecution for drug-driving because of the absence of roadside testing.

Officers convinced they have persuaded all but a minority of drivers not to drink and drive, hope so-called sobriety tests will deter those under the influence of drugs from taking to the wheel.

They borrow on the experience of officers in the US who, without the breathalyser, have had to rely on them to catch drunken as well as drug abusing drivers.

"The police have been out to America and have been taught how to do this," Mrs Carpenter explained.

Cannabis stays in the bloodstream for up to a month after consumption

Asking motorists to walk in a straight line and tilt their head back, close their eyes and touch the end of their nose will test for balance, co-ordination and depth perception to reveal symptoms of drug taking.

Part of the evaluation will involve an examination to see if the driver's pupil size is constricted or dilated.

Motorists who are suspected of being impaired by drugs could be arrested and examined by a police surgeon, where they may be asked to provide a blood or urine sample.

But some drugs, including cannabis, stay in the bloodstream for up to a month after consumption.

The RAC has also warned that some over-the-counter and prescription drugs can impair driving ability.

It said it wanted a "traffic light" warning system for drugs - with green for those safe to take and drive, amber for caution and red signalling severe adverse effects which would dangerously impair driving.