traffic accidents (RTAs) account for about
25% of all deaths by injury worldwide, which
compares to 6% for wars (Peden et al., 2004).
Traffic deaths have been declining somewhat
in western countries, but RTAs continue
to be a major epidemic of modern times.
Consequently, all contributory factors must
be evaluated thoroughly. The take-home message
from this review by Penning and co-authors
is that drugs of abuse alone or in combination
with alcohol are an important and increasing
cause of impaired driving. The research
that they review shows clear representation
of these drugs amongst driving related deaths
and injuries. Some of the facts they present
are startling - did you know that over 2
million Europeans use cannabis daily and
drive (Coullé et al., 2008).
of course, is the archetypal impairing factor
in driving, and the methodology adopted
to assess the effects of other drugs on
driving generally follows that established
for alcohol. Penning et al. identify three
main types of evidence: (1) actual driving
performance; (2) laboratory studies of performance
on tasks assumed to assess driving-related
skills and processes; and (3) epidemiology.
While actual driving studies have a direct
connection to the target performance that
is attractive, they have several problems.
Carrying out studies of impairing drugs
on the public roads is not permitted in
many countries, and such studies are expensive,
both in resource costs, and in the time
a driver volunteer must spend accumulating
data on a single or small group of measures.
Laboratory studies avoid some of the legal
problems, and allow more detailed measures
of performance impairments to be made. The
final problem is common to both real driving
and laboratory measures, that there is no
direct way of quantifying the impact of
the observed performance decrement on accident
risk. A 20% increase in the standard deviation
of lateral position (weaving) is obviously
a bad thing, but how much increase in the
risk of a collision does it mean? Epidemiological
studies are needed here. They can assess
the risk due to alcohol, but it is then
much harder to allocate this to specific
components of driving performance.
the three methods complement one another,
and contribute to different aspects of our
understanding of the impact of drugs on
driving. And all three methods clearly show
the dangers of alcohol: blood alcohol concentrations
above about 0.04-0.05%, are associated with
rapidly increasing accident risk, and impairment
to a broad range of performance measures.
Lower concentrations can produce detectable
impairments for some measures, but the picture
is not so clear-cut, and 0.05% seems to
be the evidence-based legal limit that is
the most widely accepted in Europe.
illicit drugs, only cannabis has convincing
results from all three types of evidence,
and this drug is clearly a hazard, especially
in combination with alcohol. Penning et
al. cite the work of Ramaekers and colleagues
(2009) showing that alcohol at a ‘legal’
level (0.04% BAC) combined with cannabis
(THC) had an additive effect increasing
lateral deviation above that reported for
0.1% BACs. Such a driver might thus be seriously
impaired, but still pass a breathalyser.
et al's review concerned only drugs of abuse,
but we should not forget that some widely-used
prescription drugs can impair driving and
increase accident risk (e.g. Skegg et al.,
1979). Some of these, such as the benzodiazepines
and opiates, are of course also drugs of
for other drugs is less conclusive, but
impairments are commonly found. In some
cases, for example stimulant drugs, aspects
of performance may be improved. This is
reason why some lorry drivers (or truckers)
use stimulants to reduce fatigue and avoid
falling asleep at the wheel - which is itself
a major cause of deaths and accidents for
this group (Alford, 2009). However this
should not reassure us too much, as there
is evidence both from experimental research,
police reports and user interviews that
these drugs can lead to overconfidence and
risk-taking behaviour. Such a 'risky shift'
can, of course, also occur with alcohol.