How high is too high to drive?

On the I-5 interstate, just south of the Canadianborder, WashingtonState Patrol has a problem with pot.

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

Marijuana has been legalin Washingtonstate forthe past two-and-a-half years.But the young driver isunder theage of 21so there's zero-tolerance for any amount of thedrug in his system.He could lose his licence and have a conviction on hisrecord for the rest of hislife.

"He was impaired," says Baffa. "He was under the influence of marijuana.To be impaired by any 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 tellsCBC News."After a hard day at work, it's something that calms medown."

The challenge with THC

The legalization,regulation and taxation of marijuana inWashington Stateall things the Trudeau government says it intendstopursuehaveunleashed a complexity of challenges for lawenforcement and safety advocates as they confront a surge indrivers who've smoked up.

"A third of our [impaired]drivers in the state of Washington aretesting positive for marijuana," says toxicologist Brian Capron, whoassesses blood samples from some13,000 driversevery year.

"Obviouslylegalization 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 todriveor it may not.

Unlike alcohol and the breathalyzer, there is no universallyaccepted 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 wasconsumed as an edible or in another form.

The current practice in both states is thatofficers who suspectmarijuana impairment can make an arrest and either ask the driver to submit to a voluntary blood test or petitiona judge to order one.

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

Washington and Coloradohave both adopted the threshold of fivenanograms ofTHC per millilitre of blood as their legal impairment limit.

It's akin to at the .08 blood alcohol content (BAC) limitfor drinking.But the science and reliability behindtheTHCmeasurement is notably different.

"An individual may be over fiveand not exhibiting any sign ofimpairment, and that's why the fiveis somewhat troublesome,"saysCapron.

"That fivehasn't been shown by science to be that onelevel where everyone is impaired."

Slower, not safer

Alcohol and its effects on the human body havebeen studied for decades and are fairly predictable, butmarijuana
varies greatly.

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

Spurgin and his team operate the National Advanced DrivingSimulator in Coralville, Iowa, where they have an exemption fromthe U.S. Drug Enforcement Administration to test subjectsfor marijuana impairment.

Theongoing studybegan in 2011 with more than 100 driverswho agreed to smoke pot and drink alcohol before driving the simulatorthe most sophisticatedof its kind in North America.

Just as you often seein stoner movies, Spurgin says those who usedmarijuana drove much slower than the drunk drivers.

He also says marijuana userswereless likely to veer outside of theirlanes.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," saysSpurgin.

THC levels in blood tend to spike within minutes of smokingand then decline quickly.ButTHC 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 bylaw enforcement, it's a much more difficult challenge than withalcohol."

Chronically impaired

Allison Bigelow is among the many medicinalmarijuana users inWashington who argue the five-nanogram limit is catching drivers whoare perfectly fine to drive.She regularly smokes several joints a day to ease her chronicback pain.

"I've used quite a lot of it actually," saysBigelow, 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 withDUI.

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

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

"My last toke was at midnight.At about 10 in the morning so it wasabout 10 hours [later] I was at 12.5 nanogramswithout medicating," she says.

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

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 relyingon the observations of drug-recognition officersperforming roadside tests.

Gregg Thomson, a resident ofMerrickville, Ont., and a long-time advocate withMothers Against Drunk Driving (MADD), argues it's better to adoptWashington's modelof five nanograms per millilitre.

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

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

In 1999, Thomson's 18-year-old son Stanwas among five teenagers killedoutside Perth, Ont., in a collision involving a young driver who had usedmarijuana beforehand.

"That became my mission moment," saysThomson.

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

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

Liberal MP and former Toronto police chief Bill Blair has been tasked withdeveloping the rules around legalized marijuana, but there's notimeline 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 tolerancewill need to be part of the effort.

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