The finding is part of University of Adelaide PhD graduate Dr Marie Longo's study - believed to be the biggest of its type published in the world - of the way four major types of drugs impact on the culpability of drivers involved in crashes.
Dr Longo, from the Department of Clinical and Experimental Pharmacology, analysed the blood sample results from 2500 South Australian drivers injured in road crashes for alcohol, marijuana, stimulants and tranquillisers. She also apportioned fault for crashes by using a consistent scoring method based on police accident reports and taking into account road, vehicle and environmental factors.
Dr Longo, who now works as a Project Officer for the South Australian Government's Drug and Alcohol Services Council, found:
- A significant relationship between having tranquillisers either at the prescribed levels or above and being culpable in crashes;
- Alcohol still overwhelmingly plays the greatest role in road crashes - it is the most frequently detected drug, and produces the greatest effects;
- Conversely, and perhaps controversially, marijuana has a negligible impact on culpability; and
- Stimulants also have little effect on culpability.
"The most interesting result for me was the tranquillisers, because of the ramifications it could have," she says. "What I found was that if a person took tranquillisers like Valium or Serapax at or above prescribed levels, the chance of being culpable in a road crash would increase from the 53% chance if you had no drugs to 70% if you had the tranquillisers.
"The really important thing to emerge from this is that this increase in culpability was not only for having above the prescribed levels of tranquillisers in someone's system, but also at the prescribed levels. I believe this has disturbing implications, with people driving around with prescribed amounts of tranquillisers in their system thinking they are still alright to drive when quite possibly they are not."
However, the number of people in Dr Longo's study who tested positive to tranquillisers was still quite small compared to alcohol, which is still the dominant factor for road crashes in which drugs were involved.
"More than three-quarters of the drivers in crashes I analysed were drug-free. Of the remaining quarter, 55% had alcohol in their system, either by itself or as a combination with the other three drugs I tested for," Dr Longo says. "For the alcohol-alone crashes, the culpability rate was 90% - compared with the culpability rate for drug-free crashes of 53%."