MADD Canada wants police officers to screen drivers for drugs using saliva tests, a precaution against impaired driving that now only emphasizes testing motorists for drinking.

Police officers are out in force this time of year, with roadside stops to check that holiday revellers are not drinking and getting behind the wheel.

But few are trained to look for drivers on drugs, and breathalyzers don't detect drug levels. Blood or urine tests that do detect drug traces are too invasive for roadside checks, police say.

A Western University law professor, hired by MADD Canada to examine driving while on drugs, recommends that Canada implement a system of random roadside saliva testing.

In her recent report, Erica Chamberlain urged Canadian lawmakers to restructure their approach to drug-impaired driving.

Driving on drugs 'under-enforced'

Her research showed that drugs are a major problem on the roads, but drivers are rarely checked, she said.

"Of the fatally injured drivers tested for drugs, one-third tested positive for drugs," Chamberlain told CBC News. " So it's a fairly substantial factor."

Police charge about 900 Canadian drivers annually with driving while on drugs, a small fraction of the 60,000 drunk-driving charges each year.

"With drugs, because the enforcement mechanisms in Canada right now are pretty cumbersome and not always available in every detachment, it is under-enforced."

MADD Canada says if police had the right tools to test for drugs — such as a saliva swab test that can detect traces of THC, the main ingredient in marijuana, as well as ecstasy and methamphetamines — the statistics would change.

"I think any tools we can give to the police to enable them to do their job better is a good thing," MADD spokeswoman Christine Taleski told CBC News.

Drug impairment difficult to measure

Chamberlain travelled to Australia to observe how police officers test people for driving while on drugs.

"It was shocking, actually. We were out at about 10:30 on a weekday morning. I was there for about a half-hour and they detected three people who were on meth," she told CBC News.

However, one recent case in Saskatchewan illustrates the difficulty in measuring the level of drug impairment. A woman who admitted to smoking marijuana before getting into her car and failed a series of impairment tests when stopped in June 2011 was acquitted in August of impaired driving.

A urine sample was also taken and confirmed the Saskatchewan woman used marijuana.

However, unlike drunk-driving cases where a blood-alcohol level of .08 per cent is the legal limit in Canada, there's no equivalent for marijuana-impaired driving. Instead, officers must rely on a lengthy list of tests to establish impairment.

The Saskatoon judge in the case said he was not convinced that her ability to operate a vehicle was affected.

In her recommendation, Chamberlain said roadside testing should be coupled with more sophisticated tests tied to drug limits for commonly used illicit drugs.

"This would bring Canada's law more into line with that of most comparable countries and the world leaders in traffic safety," she wrote in her September report.