DOPING AND DRIVING

You report on an investigation of the relative effects of drinking
and cannabis consumption on driving ability (23 March, p 4). While
the article is right that there is no known test specific for
impairment caused by cannabis, it didn't mention other tests which
have been developed to assess the overall ability to drive, one of
which measures the subject's ability to find and focus on
intermittent lights. This test and others look for impairment from
whatever source.

The opponents of substance testing rightly point out that the effects
of different substances vary between individuals, and in the same
individual over time and in different circumstances. When the
authorities begin testing for impairment rather than the presence of
substances in the body they will be on the right track.