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A Halifax criminal lawyer says he’s predicting a sharp rise in questionable drug impaired driving charges stemming from “flawed” drug recognition techniques used by police.

Tom Singleton, who handles roughly 50 impaired driving cases a year, predicts police will be stepping up impaired driving enforcement in the province after marijuana is legalized on Wednesday.

His concern is with trained police officers, called drug recognition experts (DRE). Using a 12-step protocol, officers measure a driver’s level of impairment, including pupil size, walk and turn, and finger to nose exercises as well as blood and urine sampling. The practise was first developed in the U.S. and has been used in Canada since 2008.

“It’s controversial in some respects because there are number of studies conducted in the U.S. and the error rate for police officers conducting these kind of a tests is around 17 per cent,” said Singleton. “That means they get it right 83 per cent of the time.

“That’s a bit troublesome when you have a justice system where the Crown is required to prove any criminal offence, which is what being impaired by a drug is, beyond a reasonable doubt. There’s already an error rate in this kind of testing.”

Even with all this evidence the Crown must prove the driver was impaired in court. A recent Statistics Canada report shows they fall short almost half of the time. Suspected impaired drivers get off nearly 40 per cent of the time, twice as often as alcohol-impaired drivers.

A report released by the Canadian Centre of Substance Abuse and Addiction last May says when detecting impairment, DREs have a “modest degree of accuracy,” between 43 and 62 per cent.

That’s in line with Singleton’s own experience, saying he wins roughly 50 per cent of his drugged-driving cases.

Singleton says he’s also concerned about the federal government’s recent move to allow officers to draw a driver’s saliva to test for THC. So far Nova Scotia RCMP, Halifax Regional Police and Cape Breton Regional Police don’t have immediate plans to use the device called Draeger DrugTest 5000, but Singleton says because police have the option raises serious concerns.

“What the government wants to do is make it easier for the police to enforce impaired driving laws in regards to marijuana, so it introduces a saliva test which they will try to claim will give police justifiable grounds to proceed to further testing.

“But what you’re going to see with this new kind of testing are serious court challenges under the Charter of Rights, because, for one, marijuana does not act on a person’s system the same way alcohol does.”

Singleton also anticipates that lawyers specializing in impaired driving cases will be busier after Wednesday. Unlike with drunk driving cases where an illegal breathalyser reading is difficult to overcome, suspected drug-impaired driving cases are far less straightforward, said Singleton.

“When you’re looking at tests that are far from perfect to begin with, it becomes extremely difficult for a lawyer to tell someone you don’t have a chance because they do have a chance when they go to court.”

Phil Star, a Yarmouth-based criminal lawyer, agrees that currently there’s a high degree of subjectivity inherent in drug recognition techniques used by police. He also expects it will take years of court challenges to bring about consistent court decisions.

Meanwhile Jennifer Clarke, Nova Scotia RCMP spokeswoman, said there are 61 drug recognition experts in the province representing both the RCMP and municipal police forces. She said RCMP divisions and provinces are working diligently to increase the number of trained officers.

“The RCMP is working toward training enough RCMP officers in Standardized Field Sobriety Test and DRE to ensure that in the communities the RCMP serves, our members will have access to SFST and/or DRE trained officers 24-7.”

Find more coverage of Canada's legalization of marijuana here.