When Rhonda Fields co-sponsored House Bill 1325 in the Colorado senate back in 2013, she set out to ensure the state’s roads would remain safe once marijuana could be legally sold in licensed stores the following year.

The state legislation set a blood limit of five nanograms of THC, at which someone driving a vehicle might be impaired.

“We just knew that we did not want people driving on our streets and putting folks at risk because they were driving while they were stoned,” Fields recalled in an interview. “There’s been a norm that’s established when people drink and drive. You don’t want to get behind the wheel. Now that we have this DUI for those who are under the influence of drugs, we’re trying to create that same level of self-regulation.”

After Ottawa rolled out its plans for marijuana legalization last month, some experts say testing potentially impaired drivers for pot is not as simple, nor as universally accepted a process, as it is for alcohol.

The proposed legislation would fine those with two to five nanograms of THC in their blood, within two hours of driving, up to $1,000.

Those caught with five nanograms of THC or more, or with a combination of THC and alcohol in their blood, could face maximum penalties mirroring those already in place for impaired driving. At minimum, such drivers would face mandatory $1,000 fines for a first offence, 30 days imprisonment for a second offence and 120 days for a third.

But while limits are usually an accurate indicator of impairment when it comes to alcohol, that’s not always the case with marijuana, according to Doug Beirness, a senior researcher for the Canadian Centre on Substance Abuse, who has studied cannabis use and driving.

“What you’re really looking for is that point at which people above that limit are impaired and people below that limit are not,” said Beirness. “We’ve played that game with alcohol over the years but there’s scientific evidence to support the 80 (milligram) limit. Finding that point (with marijuana) is really, really difficult because we don’t have the evidence to support it.”

Under the government’s proposal, an officer would be able to demand a driver provide an oral fluid sample, such as a saliva test, if they reasonably suspect the driver has taken drugs. If it yields a positive reading, the officer could then demand the driver take a drug evaluation test at a police station, or a blood sample at a hospital.

Beirness said Canada needs a marijuana impairment standard, similar to what already exists for alcohol, based on physiological factors and qualitative observations, which would prove somebody is impaired.

With marijuana, he said, numbers simply aren’t always reliable.

“It gets into the blood very quickly and then disappears very quickly,” said Beirness. “If you’re an hour and a half, two hours from the time you’re stopped, by the time you get a blood sample drawn, you may very well be below that limit.”

In Colorado, there is a “reasonable inference” limit which allows somebody charged with impaired driving to argue in court that at five nanograms, they may not have been impaired, due to factors such as a higher tolerance for cannabis.

Fields said it was important to take into account that not everybody reacts to THC in their system the same way, when trying to determine if five nanograms was the right number.

“Everyone has their own level of tolerance so it’s kind of hard to know when someone has had too much,” she said. “The science is still evolving.”

In Washington State, where marijuana became legal in 2012, a five-nanogram limit also exists. However, while the number is identical to Colorado’s limit, it holds more weight.

“The THC result is a matter for the prosecution and not a matter for arrest,” said Shelly Baldwin of the Washington Traffic Safety Commission.

“Obviously an officer on the roadside has no idea what somebody’s blood level is. They’re arresting based on impairment. But once that person’s arrested, if they get a blood draw from them, then if they have five nanograms of active THC in their blood then they can be guilty.”

Some experts worry that the government’s commitment to legalizing marijuana by Canada Day 2018 has already changed perceptions about the dangers of driving under the influence of pot. This includes the misconception that marijuana makes drivers safer, said Robyn Robertson, president and CEO of the Traffic Injury Research Foundation.

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A State Farm Canada survey conducted in February found that nearly half of respondents who admitted to driving high in the past believed they could do so safely, an increase of five per cent from a similar survey last year.

In Washington, where alcohol-related road fatalities have been on decline for the past decade, deaths involving use of alcohol and drugs, or a mix of various drugs, have “starkly” increased, according to Baldwin. There were 149 such fatalities in 2015, up from 78 in 2011.

“It’s almost what I think of as unexpected impairment,” she said. “Now that marijuana’s legal, somebody might use a little bit of marijuana whereas maybe they didn’t before and then go to happy hour and have a couple drinks and now all of a sudden they’re much more impaired than they were expecting.”

Fields said she believes her bill has helped make Colorado’s streets safer since its implementation.

Given the complex variables at play when it comes to detecting — and limiting — stoned drivers on the road, she said it’s important that jurisdictions like Canada working to legalize pot consider as many different perspectives as possible.

“When you’re trying to craft good public policy, you need to have all the stakeholders at the table,” said Fields. “For us in the state of Colorado, we listened very keenly to the cannabis industry and we listened to the people who use cannabis. It’s important that we don’t dismiss their part of the story.”

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QUICKFACTS:

What’s THC?: THC, or tetrahydrocannabinol, is the chemical responsible for most of marijuana’s psychological effects.

How much usage now?: In 2012, an estimated 10.4 million trips were made by Canadians who had used cannabis within two hours prior, according to data from a federal survey. But there were only about 51,000 impaired driving charges in 2014, of which 1,355, or 2.6 per cent, were for drug-impaired driving. Mothers Against Drunk Driving Canada says it’s safe to assume half of those charges were pot-related.

How many fatalities?: From 2000 to 2012, the number of people killed while driving in Canada who tested positive for marijuana increased from 13 to 20 per cent, according to Traffic Injury Research Foundation.

SOURCES: Livescience.com, Traffic Injury Research Foundation, MADD Canada

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