Largest study on cannabis and driving finds only a low increased risk

BY CHRIS FOWLIE

  • Several other recent studies find no increased risk
  • burdon is on officials to justify roadside testing

The largest study of its kind has found drivers under the influence of cannabis are far less likely to be culpable in traffic accidents than drunk drivers, while several other recent studies have found no increased risk at all.

In an epidemiological study of approximately 8,000 accidents published in the British Medical Journal, researchers at the French National Institute for Research on Transportation and Safety found that alcohol intoxication and speeding were nearly ten times more likely to be an attributing factor in traffic fatalities than the use of cannabis.

Overall, researchers estimated that psychomotor impairment caused by cannabis was similar to that exhibited by drivers with blood alcohol concentrations (BAC) ranging from 0.02 to 0.05 per
cent. The legal limit in New Zealand for a driver over 20 is a BAC of 0.08

The relative risk for causing a fatal accident (where 1 = no increased risk) was 1.8-2.2 for cannabis. The risk factor was ten times that – about 20 – for alcohol above a BAC of 0.05, and also for speeding.

The study results have been provoking embarrassment among French government officials as they always claimed drugs are responsible for more deaths than speeding.

A recent study in Sweden found that the introduction of zero-concentration limits for THC and other drugs in the blood of drivers did not result in a reduction of driving under the influence of drugs (DUID). Researchers also noted that “the spectrum of drugs identified in blood samples from DUID suspects has not changed much since the zero-limit law was introduced” in 1999.

In another recent study, by researchers at the University of Maryland, the use of cannabis was not associated with the risk to cause a traffic accident. Researchers looked at the presence of
alcohol and illegal drugs in 6,581 drivers who were hospitalized at a shock trauma center from 1997 to 2001. Crash culpability was strongly associated with alcohol use. In contrast, this study did not find an association between crash culpability and cannabis use. Since only urine tests on cannabinoids were performed, it is not known whether drivers were actually under the influence of
cannabis – and since cannabis lingers long after use this makes the association even weaker.

In another recent study, researchers of the University of Victoria (Canada) investigated whether clients in treatment for problems related to the use of alcohol, cannabis, cocaine, or various combinations of these substances, had a higher risk to drive while impaired compared to a control group. 445 drivers under treatment were included. In the 8 years before treatment, every drug group except the “cannabis only” group had significantly more convictions for driving while impaired than controls.

UK Police use computer games to test impairment

Last year UK Police introduced traditional coordination tests to check for drivers impairment by drugs. Now the Home Office has turned to computer games. The traditional test included such tasks as instructing drivers to walk in a straight line, stand on one leg or touch their nose with their index finger. The Home Office believes the tests are too subjective, so has loaded a laptop with  several “games” designed to assess the reactions of a motorist.

Drivers would be tested not only on their speed and dexterity but also the accuracy with which they performed the task. This is because while cannabis can slow reaction time, amphetamines quicken it – but also make individuals more prone to error.

Both these sorts of tests – traditional and computer – are more accurate than urine or saliva screens, as they measure actual impairment rather than past exposure to drugs.

A urine test may pick up cannabis residues that are several weeks old, while doing nothing to  detect drivers impaired by fatigue or prescription drugs. Coordination- or computer-based tests are also less invasive, cheaper and quicker to operate.

Sources:
Libération, 3 October 2005;
Jones AW. Traffic Inj Prev 2005;6(4):317-22;
Soderstrom CA, et al. Annu Proc Assoc Adv Automot Med 2005;49:315-30;
Macdonald S, et al. Traffic Inj Prev
2005;6(3):207-11.

(NORML News Summer 2006)

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