Legalizing cannabis in Canada next summer could unleash a dangerous wave of stoned drivers on the nation’s roads, warn police, road-safety advocates and opposition politicians.

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Legalizing cannabis in Canada next summer could unleash a dangerous wave of stoned drivers on the nation’s roads, warn police, road-safety advocates and opposition politicians.

The Liberal government is attempting to take precautions by reforming drug-impaired driving laws through Bill C-46, tabled alongside the Cannabis Act last April. When Justice Minister Jody Wilson-Raybould outlined how police would use the powers proposed in the bill to combat drug-impaired driving, it sounded straightforward:

"The police would be authorized to demand an oral fluid sample if they have reasonable suspicion that the driver has drugs in their body," Wilson-Raybould said.

"A positive result would provide them with useful information in determining whether or not they have reasonable grounds to demand either a blood sample or a drug evaluation at the police station."

Those blood samples or drug evaluations could then serve as evidence of impaired driving, just as breathalyzer tests serve as evidence of alcohol impairment.

Experts warn, however, that testing blood samples to determine impairment by cannabis is fraught with scientific complications that undermine the value of blood samples as evidence— and drug evaluations conducted by police officers don’t always hold up in court.

How Bill C-46 would address cannabis-impaired driving

In legal parlance, the proposed limits on blood-drug concentrations are known as "per se" limits. Mirroring the legal approach to blood-alcohol levels, Bill C-46 would enable a quantitative ceiling on the amount of a specific chemical component of cannabis that can be detected in the blood of drivers. Having a blood-drug concentration that exceeds the legal limit would be a criminal offence in and of itself, regardless of whether the driver is objectively impaired.

The specific limits on cannabis blood concentrations aren’t laid out in the legislation itself, and would determined by later regulations. An April background document from Health Canada, however, revealed the exact limits under consideration, which would measure nanograms (billionths of a gram) of THC per millilitre of blood within two hours of driving. (THC refers to tetrahydrocannabinol, the primary psychoactive ingredient in cannabis.)

Joe McHugh / California Highway Patrol</p><p>Officers who are trained as Drug Recognition Evaluators perform darkroom examinations of the eyes looking for signs of drug impairment. These examinations include measuring the size of the pupil in direct light and in near total darkness. Officers also observe the reactions of the pupil under these extreme conditions.</p>

Joe McHugh / California Highway Patrol

Officers who are trained as Drug Recognition Evaluators perform darkroom examinations of the eyes looking for signs of drug impairment. These examinations include measuring the size of the pupil in direct light and in near total darkness. Officers also observe the reactions of the pupil under these extreme conditions.

Drivers found to have between two and five nanograms of THC per millilitre of blood within two hours of driving would face a summary conviction offence with a maximum fine of $1,000, according to the document.

Drivers with five or more nanograms of THC would face a hybrid offence, allowing Crown prosecutors to opt for a summary conviction or proceed to a more serious indictable offence.

Drivers who exceed the limits for both alcohol and THC would also face a hybrid offence. Penalties for both hybrid offences would range from a $1,000 fine for first-time offenders to imprisonment of varying lengths for repeat offenders. The maximum penalty would be up to 10 years in prison, with the possibility of being designated a dangerous offender.

Some U.S. jurisdictions have taken a similar approach. In Colorado, where recreational cannabis is legal, a blood-THC level of 5 ng/ml is enough for police to infer that a driver is under the influence. Washington state similarly set its blood-THC limit at 5 ng. Other states have opted for a 2 ng limit, or for zero-tolerance policies.

A July report from the U.S. National Highway Traffic Safety Administration to Congress specifically called out the 5 ng limit, describing it as "based on something other than scientific evidence."

Why blood limits for THC impairment don’t work

Barry Logan, chief scientist at NMS Labs in Pennsylvania, was commissioned to study the problem for the AAA Foundation for Traffic Safety in the U.S., and published his findings in 2016. Logan and his team analyzed drug testing records of thousands of drivers who were arrested for impaired driving in the U.S. and tested positive for chemical compounds found in cannabis.

"The study found that the number of the concentration of (THC) in the blood sample bore no relationship to the degree to which somebody was exhibiting impairment," said Logan in an interview.

Logan was careful to explain the nuance of his conclusion: It doesn’t mean drivers under with THC in their blood aren’t necessarily impaired. Rather, the study found measuring blood THC levels after the fact doesn’t provide objective, scientifically sound evidence of impairment.

THC molecules are lipophilic, moving quickly from the bloodstream to fatty tissues, Logan explained.

"And so a person’s blood-THC concentration drops precipitously once they stop smoking."

European studies, said Logan, have established a direct correlation between certain blood-THC concentrations and impairment by collecting blood samples from cannabis-using drivers while they drive or immediately after.

Bill Alkofer / The Orange County Register files</p><p>A California police officer conducts a field sobriety test on a driver suspected of driving while impaired by marijuana.</p>

Bill Alkofer / The Orange County Register files

A California police officer conducts a field sobriety test on a driver suspected of driving while impaired by marijuana.

For law enforcement, that kind of testing isn’t a realistic option.

"In the real world, by the time a police officer stops somebody, asks them some questions, maybe has them do some field sobriety tests, determines they’re impaired, transports them to hospital, you wait for a phlebotomist, the average time to get a blood sample is about an hour-and-a-half to two hours," Logan said.

By then, Logan explained, the concentration of THC in the blood has dropped so significantly that the concentration of THC "bears no resemblance" to what it was when the person was driving.

"With alcohol, experts, toxicologists can, with some reasonable degree of certainty, extrapolate back," he said, citing decades of research.

"But you can’t do an equivalent calculation for THC to estimate what it might have been 30 minutes or 60 minutes earlier — there’s just so much variability from person to person."

Complicating things further, Logan said, frequent users of cannabis could have more than the legal limit of THC in their blood, even if they haven’t used the drug recently.

"There’s a risk of false positives, if you like, just relying on the number, and the lower that number is set, the greater the risk of these false positives," he said.

The potential for false positives is of particular concern to medical users of cannabis.

"We’re concerned that responsible patients who aren’t impaired will still exceed a per se limit, even when following safe-use guidelines," said Jonathan Zaid, executive director of Canadians for Fair Access to Medical Marijuana.

Zaid is calling for the government to include a special exemption in Bill C-46 for medical cannabis users.

"To be clear, this doesn’t mean patients can drive impaired. Patients still need to be responsible and drive safely," he said.

"But what it does mean is that if patients should exceed a per se limit, but don’t show signs of impairment, then they will be allowed to use a medical defence and get off the per se limit charges."

‘The least objectionable alternative’

The federal government’s proposed blood-THC levels are based, in part, on the findings of the Drugs and Driving Committee of the Canadian Society of Forensic Sciences, according to a Justice Department spokesman.

In that report, released in April, the CSFS described setting per se limits for cannabis as "a controversial exercise for many reasons." The report’s authors reached the same conclusion as the AAA Traffic Safety Foundation Study in the U.S.: Although THC can impair a driver’s ability, the authors wrote, "setting a per se limit does not mean that all drivers below that concentration are not impaired and all drivers above that concentration are impaired."

The idea of recommending a specific limit for blood-THC levels appeared to trouble the authors.

"Determination of a per se limit which addresses both public safety concerns and minimizes the potential for an individual to be ‘wrongly’ convicted of a drugged-driving related offence can be considered an exercise in selecting the least objectionable alternative," said the report.

Passing Bill C-46 as-is would be "a gift to the marijuana lobby," said Pamela McColl, national director of Smart Approaches to Marijuana Canada, who is vigorously opposed to legalization.

"I think it’s a gift to the industry and to lawyers who will get their clients off" for impaired driving charges, McColl said. "There’s so much wiggle room in blood testing, there’s so many ways they can defend their clients."

Ron Ward / The Canadian Press</p>

Ron Ward / The Canadian Press

SAM Canada and its allies in the U.S. anti-legalization movement have asked the Canadian government to rethink its approach to blood per se limits. Ed Wood, an American advocate who favours a tough approach to drug-impaired driving, excoriated the bill during testimony in front of the Standing Committee on Justice and Human Rights on Sept. 20.

"The two-tier structure in Bill C-46 perpetuates the myth that blood levels of THC correlate with levels of impairment, and they don’t," Wood said.

He proposed a stricter approach dubbed "tandem per se." Under that approach, any driver arrested under reasonable suspicion of being impaired would be tested for drugs and alcohol. Detection of "any amount of impairing substance" would render the suspect guilty.

Robert Solomon, a law professor at the University of Western Ontario and national director of legal policy with Mothers Against Drunk Driving Canada, has rebutted Wood’s proposal as "noble, but unrealistic," and said Wood lacks a full understanding of the Canadian criminal-justice system.

"I don’t think that he fully appreciates the problems that we’ve had in Canada attempting to get convictions based on an officer’s unaided observations of a driver’s behaviour," Solomon said.

MADD Canada supports Bill C-46, but Solomon described it as "far from perfect."

"It won’t provide a major deterrent," he said. "It’s better than what we have."

What we have

Currently, proving drug-impaired driving in Canada relies on examinations performed by police officers trained as "drug recognition experts." The DRE program, originally developed in the 1970s by the Los Angeles Police Department, trains officers to perform a series of physical examinations and tests in order to conclude whether a suspect is impaired by specific drugs. The officer’s analysis is then backed up by a blood, urine or saliva sample to prove the presence of the drug.

Approximately 600 Canadian police officers — 202 of them Mounties — were trained as drug recognition experts as of this month, according to the RCMP. There are 43 drug recognition experts in Manitoba.

"DRE takes about two hours from end to end," Solomon said. "It requires police to record 100 separate pieces of information, it costs $17,000 to train each officer, the cost of maintaining their certification is is high, (and) the courts have not welcomed the DRE evidence."

A late 2015 ruling by Manitoba provincial court Judge Cynthia Devine illustrates Solomon’s point; DRE evidence doesn’t always fare well in court.

In November 2013, Tyler Manaigre was arrested in Steinbach and charged with impaired driving by drugs. Arresting RCMP Cpl. Terry Sundell followed Manaigre after observing him driving 80 km/h in a 100 km/h zone on the outskirts of town. (In her ruling, Devine was careful to note that the speed limit reduced to 80 km/h just 250 meters down the road, and that winter weather conditions may have merited driving slower than the speed limit.)

Adrian Wyld / The Canadian Press files</p><p>Justice Minister Jody Wilson-Raybould says legislation will provide police with tools to combat impaired driving.</p>

Adrian Wyld / The Canadian Press files

Justice Minister Jody Wilson-Raybould says legislation will provide police with tools to combat impaired driving.

Even though Manaigre was driving in a normal manner, Sundell testified, he pulled the driver over to check his sobriety, licence and registration. After some questioning, Manaigre said he had smoked "just a little bit of pot" earlier in the evening.

Sundell, who had had been certified as a drug recognition expert in Florida within a month of stopping Manaigre, performed a standard field sobriety test to determine whether there were reasonable grounds to arrest him for impaired driving. (Such tests, which include walking a straight line and turning, standing on one leg and an eye examination, don’t qualify as admissible evidence of impaired driving, but can be used to find grounds for an arrest and subsequent DRE test.)

Sundell testified Manaigre performed poorly on the tests. He made the arrest and took him back to the local RCMP detachment for a full drug evaluation. After concluding Manaigre was driving while impaired by cannabis, Sundell demanded a urine sample, which also tested positive for cannabis. An RCMP toxicologist testified the urinalysis confirmed only whether Manaigre consumed cannabis at some point in the previous month, and that consuming a small amount of marijuana may not impair someone’s ability to drive.

In spite of the officer’s evaluation, the positive urine sample and even Manaigre’s admission that he used cannabis before getting behind the wheel, Devine was unwilling to find Manaigre guilty of driving while impaired.

"There was nothing about the driving to suggest impairment by a drug," she wrote in her decision, noting that Manaigre also acted appropriately during his encounter with police.

"I am satisfied beyond a reasonable doubt that the test results indicate marijuana consumption," ruled Devine.

"But even though I accept that these tests are standardized and utilized across Canada and the United States, I am not satisfied beyond a reasonable doubt that they demonstrated impairment, even to a slight degree, in this case… it is unknown how performance on the tests is correlative of impairment."

The blood limits in Bill C-46 appear to be designed to address that kind of reasonable doubt about cannabis impairment by establishing an objective test, but the scientific consensus is clear: blood testing for THC content can’t definitively prove impairment.

Manaigre’s attorney, Michael Dyck, raised another problem with the legal limits on blood-THC content: people who use cannabis and wait until they no longer feel impaired before driving will have no way of testing themselves to ensure they’re below the limit, unlike alcohol users who can use keychain breathalyzers.

"The reality is that if people are allowed to legally purchase and consume marijuana, and people are allowed to legally operate motor vehicles, we have to think about where they can do those things — not simultaneously — but where they can do those things and still live their lives," Dyck said in an interview.

"(If) a person consumes marijuana and they’re not impaired and they’re driving on the road, should we have a problem with that as a society? I’d say no, because they’re not doing anything dangerous. Once they’re impaired, that’s when they become a danger. But those two things don’t necessarily go hand-in-hand."

Between the Canadian Society of Forensic Sciences report and testimony presented to to the Standing Committee on Justice and Human Rights, the Justice Department appears to be well aware of the limitations of blood testing for cannabis impairment. Regardless, the department says it is trying to err on the side of public safety.

"The current scientific evidence shows that setting legal limits for drugs is more complex than alcohol," a Justice spokesman said in an email.

"The proposed levels for THC — particularly for the low-level, fine-only drug offence — reflects a precautionary approach. In the absence of clear evidence of what is the safe level of THC to drive, the best way forward is to say that there is no safe level.... At this time, the minister has no plans to alter Bill C-46 as it relates to blood testing for the proposed new drug-impaired driving offences."