Pubdate: Fri, 10 Feb 2012
Source: Calgary Herald (CN AB)
Copyright: 2012 Canwest Publishing Inc.
Contact: http://www2.canada.com/calgaryherald/letters.html
Website: http://www.calgaryherald.com/
Details: http://www.mapinc.org/media/66
Author: Sharon Kirkey
Bookmark: http://www.mapinc.org/find?224 (Cannabis and Driving)
Referenced: Acute Cannabis Consumption and Motor Vehicle Collision 
Risk: Systematic Review of Observational Studies and Meta-Analysis: 
http://www.bmj.com/content/344/bmj.e536

DRIVING ON POT DOUBLES CRASH RISK, STUDY FINDS

Psychomotor Skills Impaired by Cannabis

Driving under the influence of marijuana nearly doubles the risk of a 
serious or fatal car crash, a Canadian study finds.

Cannabis is the most widely consumed illicit substance in the world, 
and the number of Canadians confessing to driving within an hour of 
using pot is growing, researchers from Dalhousie University write in 
this week's issue of the British Medical Journal.

In addition, "surveys of young drivers have also shown that rates of 
driving under the influence of cannabis have surpassed rates of 
drinking and driving in some jurisdictions," the Halifax team reports.

Not only is cannabis relatively easy to get, "many young people 
really don't believe that cannabis impairs," said lead author Mark 
Asbridge, an associate professor in the department of community 
health and epidemiology at Dalhousie.

Past studies into cannabis and crash risk have been mixed. Some have 
found an increased risk of being involved in a collision after using 
marijuana, while others have found either no association whatsoever, 
or even a lower risk, suggesting people were actually safer driving 
while intoxicated by pot than not.

The Halifax team set out to disentangle the evidence. They performed 
a "systematic" review, scouring the literature for the best-designed 
studies they could find. In the end, they pooled data from nine 
studies that, combined, involved 49,411 drivers from Australia, New 
Zealand, the United States, France and the Netherlands.

All the crashes involved in the analysis took place on public roads 
and involved one or more moving vehicles such as cars, vans, trucks, 
buses and motorcycles.

The researchers found a 92 per cent increased risk -- a near doubling 
- -- of a driver being involved in a collision resulting in serious 
injury or death, to themselves or others, if they used marijuana 
within two to three hours of getting behind the wheel.

The strongest association was with fatal crashes.

The study wasn't designed to answer the question: How much pot does 
it take before the crash risk increases?

Most studies in the analysis used any amount greater than zero as the 
cut-off for a positive test result. But, "for cannabis, there's not 
necessarily a cut-off that we can identify where risk was most 
heightened," Asbridge said. Still, studies have shown that cannabis 
impairs the psychomotor skills needed for safe driving, he said. 
Marijuana affects perception and spatial awareness. Drivers have 
difficulty staying in their lanes, Asbridge said. "There's actually a 
psychological process where people often believe that they're driving 
safer than they really are and they don't recognize that they're 
following too closely, or making these lane violations."

Earlier studies that suggested it might be safer driving under the 
influence of cannabis often relied on urine samples. The problem 
there, Asbridge says, is that markers for cannabis in urine "can stay 
in your body for weeks or even over a month so that's not a measure 
of recent use at all."

His team only included studies that measured active THC metabolites 
from blood samples, which is a more accurate way of measuring whether 
someone has smoke dup within the last few hours.

In 2004, four per cent of Canadian adults reported driving within one 
hour of consuming cannabis, up from 1.9 per cent in 1996-97.

According to the 2009 Canadian Alcohol and Drug Use Survey, 11.4 per 
cent of Canadians overall, and 33 per cent of 15-24 year olds 
reported using marijuana at least once in the previous year, the 
Halifax researchers note in the journal.

Governments in Australia, western Europe and the U.S. have introduced 
roadside testing for cannabis that uses a saliva test, instead of a 
breath test, to detect recent pot use, Waynehall, of the university 
of Queensland Centre for Clinical Research in Australia, writes in an 
accompanying editorial.

In Canada, specially trained police use a 12-step test that looks for 
"biomarkers" -- dilated pupils, for example, or sweaty palms and 
elevated heart rate -- to detect drug-impaired drivers, Asbridge said.

Some jurisdictions in Europe have a zero tolerance approach. "So if 
any measured level is in your system, you're guilty," Asbridge said.

Nearly two-thirds of Canadians are open to the idea of 
decriminalizing or legalizing marijuana, according to the results of 
a recent poll. At its recent biennial policy convention in Ottawa, 
the federal Liberal party voted overwhelmingly to support 
legalization of the drug.
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MAP posted-by: Jay Bergstrom