On the I-5 interstate, just south of the Canadian border, Washington State Patrol has a problem with pot.

"It smells like marijuana in the car," Trooper Mallorie Baffa says as she shines her flashlight in the face of a 19-year-old man in a Lexus. He caught her attention by taking a wide turn.

Marijuana has been legal in Washington state for the past two-and-a-half years. But the young driver is under the age of 21 so there's zero-tolerance for any amount of the drug in his system. He could lose his licence and have a conviction on his record for the rest of his life.

"He was impaired," says Baffa. "He was under the influence of marijuana. To be impaired by a substance and to be behind the wheel of a car is not safe."

But after his arrest, the young man insisted there was nothing wrong with his driving or his level of impairment.

"It [marijuana] doesn't make me feel any different," he tells CBC News. "After a hard day at work, it's something that calms me down."

The suspect is taken to a hospital to have his blood drawn and tested to find out exactly how much THC is in his system. (Glen Kugelstadt/CBC)

The challenge with THC

The legalization, regulation and taxation of marijuana in Washington State — all things the Trudeau government says it intends to pursue — have unleashed a complexity of challenges for law enforcement and safety advocates as they confront a surge in drivers who've smoked up.

"A third of our [impaired] drivers in the state of Washington are testing positive for marijuana," says toxicologist Brian Capron, who assesses blood samples from some 13,000 drivers every year.

"Obviously legalization started in 2012, so we have had a sharp increase since then."

The challenge is that having THC in your system, the active ingredient in marijuana, may mean someone is too impaired to drive — or it may not.

Washington state toxicology lab manager Brian Capron says 33 per cent of drivers' blood sent to his facility now tests positive for THC. (Glen Kugelstadt/CBC)

Unlike alcohol and the breathalyzer, there is no universally accepted roadside test for marijuana impairment.

Universities and private companies, including some in Canada, are testing a variety of devices that focus on THC in saliva.

Colorado, where recreational marijuana is also legal, is conducting a pilot project with its state patrol. But the test can only detect when marijuana has been smoked, not if it was consumed as an edible or in another form.

The current practice in both states is that officers who suspect marijuana impairment can make an arrest and either ask the driver to submit to a voluntary blood test or petition a judge to order one.

In the latter case, getting a warrant can often take several hours.

Washington and Colorado have both adopted the threshold of five nanograms of THC per millilitre of blood as their legal impairment limit.

It's akin to the .08 blood alcohol content (BAC) limit for drinking. But the science and reliability behind the THC measurement is notably different.

"An individual may be over five and not exhibiting any sign of impairment, and that's why the five is somewhat troublesome," says Capron. "That five hasn't been shown by science to be that one level where everyone is impaired."

Slower, not safer

Alcohol and its effects on the human body have been studied for decades and are fairly predictable, but marijuana varies greatly.

"We're talking incredible variability," says Andrew Spurgin, a research pharmacist with the University of Iowa.

Spurgin and his team operate the National Advanced Driving Simulator in Coralville, Iowa, where they have an exemption from the U.S. Drug Enforcement Administration to test subjects for marijuana impairment.

The ongoing study began in 2011 with more than 100 drivers who agreed to smoke pot and drink alcohol before driving the simulator, the most sophisticated of its kind in North America.

The University of Iowa's National Advanced Driving Simulator, where researchers have been getting drivers high on marijuana and testing their ability to drive. (Chris Corday/CBC)

Just as you often see in stoner movies, Spurgin says those who used marijuana drove much slower than the drunk drivers.

He also says marijuana users were less likely to veer outside of their lanes. But he says that doesn't mean they drive safer.

"It's two very different drugs and two very different time courses in terms of impairment," says Spurgin.

THC levels in blood tend to spike within minutes of smoking and then decline quickly. But THC can still register in blood for hours or even days afterward.

"Without knowing the marijuana dose and time [it was taken], it's a big struggle [to test for impairment]," Spurgin says.

"So as you try to set these limits by what will be measured by law enforcement, it's a much more difficult challenge than with alcohol."

Chronically impaired

Allison Bigelow is among the many medicinal marijuana users in Washington who argue the five-nanogram limit is catching drivers who are perfectly fine to drive. She regularly smokes several joints a day to ease her chronic back pain.

Allison Bigelow, a medical marijuana user and a resident of Mount Vernon, Wash., says the state's THC limit for drivers is flawed on many levels. (Chris Corday/CBC)

"I've used quite a lot of it actually," says Bigelow, who lives in Mount Vernon, Wash., about 45 minutes south of the Canadian border.

Four years ago, she was stopped by police for a broken tail light, and a suspicious police officer sent her off for a blood test. She came back at over 20 nanograms and was charged with DUI.

Her defence lawyer challenged the conviction, claiming her built-up tolerance to the drug meant there was no evidence she was unsafe.

Bigelow says she was independently tested by a doctor, who found she likely has chronically high levels of THC in her system.

"My last toke was at midnight. At about 10 in the morning — so it was about 10 hours [later] — I was at 12.5 nanograms without medicating," she says.

"For those heavy users who have developed a tolerance, it's like persecution."

Gregg Thomson lost his 18-year-old son, Stan, in a drugged-driving crash. (CBC)

Canadian father's 'mission moment'

Canada will have to weigh the merits of providing courts with a legal-impairment limit, as with alcohol, or instead relying on the observations of drug-recognition officers performing roadside tests.

Gregg Thomson, a resident of Merrickville, Ont., and a long-time advocate with Mothers Against Drunk Driving (MADD), argues it's better to adopt Washington's model of five nanograms per millilitre.

"It makes sense when you think about it," he says.

"Court processes need something, and the message is easy to communicate."

In 1999, Thomson's 18-year-old son Stan was among five teenagers killed outside Perth, Ont., in a collision involving a young driver who had used marijuana beforehand.

Stan Thomson was among five teenagers killed in 1999 outside Perth, Ont., in a collision involving a young driver who had used marijuana beforehand. (CBC)

This was the scene of the June 27, 1999 crash that killed Stan and four other teens. (CBC)

"That became my mission moment," says Thomson.

Thomson says while he believes legalizing marijuana has the potential to improve controls and safety messaging around the drug, he's not impressed with what he's heard so far from the Trudeau government.

"I'm scared to death. We had a government that based its platform on legalizing marijuana and never mentioned safety once."

Liberal MP and former Toronto police chief Bill Blair has been tasked with developing the rules around legalized marijuana, but there's no timeline yet or any indication of who will sit on an advisory panel.

Clearly though, more research on the effects of marijuana, including on driving performance and on those with a built-up tolerance will need to be part of the effort.

"Where we are today with drugs is where we were 25 years ago with alcohol," says Thomson. "That's how far behind the curve we are."