As with drink, so with drugs. There is no excuse for anyone who attempts to drive under the influence of alcohol or of narcotics. Anything that assists the police in catching such people is to be welcomed.

The problem hitherto has been that no simple and effective test has existed for drugged drivers. Under existing rules officers who believe a motorist is under the influence of a drug have to arrest suspects before escorting them to a police station and asking for a blood or urine sample.

Undoubtedly this has meant that some drug users have escaped and that there have been more accidents than there might have been. New road-side devices to be tested by four police forces, including Strathclyde, should remedy that situation.

Nevertheless, it is open to question whether this should mean an increase in random testing. The legal position affecting drugs and alcohol is different. It is not against the law to use or possess alcohol, only to attempt to drive with too much of it in one's bloodstream.

Drugs falling within defined classifications, in contrast, are simply illegal. Equally, traces of drugs such as cannabis stay in the bloodstream for much longer than alcohol. Thus it is possible that random tests will catch out someone who is driving perfectly well but who may have smoked cannabis a fortnight previously. In other words, a road safety campaign will become confused, to say the least, with action against drugs.

What of it? Drugs are illegal, are they not? Yet surely the reason for sanctioning yet another intrusion on civil liberties - think of all the perfectly ordinary people inconvenienced or embarrassed at the roadsides - -is to crack down on driving under the influence. Are we really to expend a large amount of police resources in order to discover that someone has used cannabis 30 days ago - as is medically possible? Surely impairment through drug use while driving should be the only point in this case?