Study links cannabis and crashes

Drivers who regularly use cannabis are at high risk of causing a serious crash, but not because they have just smoked a joint, a study has found.

The Auckland University study of more than 1000 drivers found that habitual users - who on average smoked at least once a week - had a nearly 10-fold higher risk of having a serious crash.

Drivers who had smoked a joint within the last three hours initially appeared to be at increased risk too, but this link disappeared when factors such as their alcohol consumption and driving speed were taken into account.

The researchers, whose paper has been published in the British journal Addiction, do not know why habitual users are at increased risk. But they do advise against roadside testing of drivers for cannabis use because of the small numbers who have just had a toke and the virtual absence of a link between that and serious crashes.

Only 0.5 per cent of drivers in the randomly selected, roadside arm of the study had smoked within the previous three hours, though most of those who had were habitual users.

The paper says targeting groups in the population at high risk for cannabis use may therefore be more cost-effective than roadside testing.

The Government is keen to introduce roadside drug testing if overseas research produces a reliable system, but acknowledges the difficulties.

"There is some road-testing in Australia in one or two states," Associate Health Minister Jim Anderton said last night. "The police here have been evaluating their results and running some trials.

"The problem we've got is that there's a multiplicity of drugs ... so you would need to have different tests."

The Auckland University researchers interviewed the drivers of 571 vehicles involved in crashes in which one or more occupants was hospitalised or killed and those of 588 randomly selected cars they flagged down in the Auckland region.

Ten per cent of the serious-crash drivers were habitual cannabis users, compared with 0.9 per cent of the other group.

The journal paper suggested it was unlikely that the increased crash risk among habitual users was due to brain damage from the drug.

"It's much more likely to be things about habitual marijuana smokers, the way they drive, the type of people they are, than anything else, but we don't know," said one of the researchers, Dr Jennie Connor.