NEW drug-driving laws could end up wasting police resources by targeting thousands of motorists with small amounts in their bodies, including those on prescription medication, a motoring group has warned.

The Institute of Advanced Motorists (IAM) said UK Government legislation due to be unveiled this week should include roadside testing equipment and be based on impairment – the actual effect that narcotics have on driving – rather than on the mere presence of drugs in the system. It warned a failure to include this test would result in drivers being penalised for having trace amounts of drugs, including prescription medication, even if they took them several days before getting behind the wheel.

It said not having an impairment test risked diverting police attention away from the hard core of offenders who posed the biggest risk of causing accidents and could turn public opinion against the measures.

Simon Best, chief executive of the IAM, welcomed the proposals for a new drug-driving law and plans to equip police with roadside drug detection equipment later this year.

However, he said: "The introduction of a 'drugalyser' type test needs to be backed up by some measure of impairment.

"Without this, the test could simply catch those people who have used drugs at some point, but are not necessarily still impaired by them. Impairment as the key factor is also essential in tackling drivers who may have used over the counter or prescription drugs which, while legal, can have an equal impact on driving ability as illegal ones."

A spokesman for the Department for Transport said the question of impairment would be tackled by a technical advisory panel when the new law was drafted.