From next month, police in Cleveland, Lancashire, Strathclyde and Sussex will be allowed to stop drivers and, if the motorists consent, administer a drug test. More than 5,000 tests will be conducted.
The moves were announced as the Government released the results of a study into drug-driving. It found that 18 per cent of drivers involved in fatal accidents had taken illegal drugs - including amphetamines, methadone, opiates and cannabis.
Ministers were keen to stress that no action would be taken against any driver who tested positive. "The equipment being used has not been approved," said Baroness Hayman, the roads minister. "The roadside tests are just to check that the equipment works and give officers training to recognise the signs of drug-impairment."
Government sources said that in Germany, police officers decided to prosecute motorists if similar drug roadside tests proved positive - and "only three dozen people came forward in months".
Two devices will be tested. Drugwipe takes a specimen of sweat from the forehead. Any drug traces detected will cause a colour change on a strip visible to the eye.
The Drugwipe test for specific drugs - cannabis, amphetamines including ecstasy, cocaine and opiates - so that if a police officer wishes to check for more than one drug type, a separate device will be required for each test. The cost for testing each drug, according to Drugwipe's director, Ean Lewin, is "about pounds 8.50".
The second device, manufactured by Cozart, requires a saliva specimen. Again, drug traces result in a colour change, which would be displayed electronically - but only one sample needs to be taken. The device can identify five different drug groups. "The machine is 95 per cent accurate," said Philip Hand, a director of Cozart, "which is comparable to conventional laboratory urine tests".
Neither device, however, give indications as to the concentration of the drug present. A looming hurdle for legislators is determining what a "safe" level of drugs in the blood would be for drivers. Baroness Hayman said that the effects of cannabis "probably only lasted for hours".
Keith Hellawell, the UK anti-drugs co-ordinator, said the results could lead to acceptable limits being set for motorists. "We need to know how much of a particular substance affects a particular person," he said.
New drug-driving data, released at a conference organised by the Parliamentary Advisory Committee on Transport Safety, analysed the bodies of 619 fatalities from road crashes.
Of the 284 drivers killed, 27 had smoked cannabis - by far the most prevalent illegal drug found in the bloodstreams of the dead. Very few of the bodies tested showed traces of ecstasy - despite the drug being taken by 500,000 people in the UK.
The figures are based on the first 15 months of a three-year study into the incidence of drugs in road accident victims. The results showed there had been a sixfold increase in the presence of illicit drugs in drivers killed since the last survey - conducted more than a decade ago.Reuse content