You're standing on the side of the road, with traffic whizzing past.

The police officer who pulled you over suspects you may have smoked the reefer before departing for McDonald's.

But she's in a bit of a quagmire, because, really, there's no reliable way to know for sure. Are you high? If you are high, how high are you, really? Or really did you just want those little cheeseburgers (no ketchup and extra pickles)?

So she does the most logical thing: a field sobriety test. Tried and true. Walk the line. Touch the tip your nose. Can't do it? That's... suspicious. Maybe a night in the clink? Some Canadian cops also have roadside saliva swabs that can be used to test for the presence of drugs, but they are useless, legally speaking (for now.)

Now, had you been quaffing ales before the drive, a breathalyzer — controversial as they can be in terms of accuracy and reliability — would have cleared up the situation pretty quickly.

Of course, no such roadside device exists for cannabis and its psychotropic ingredient THC.

There's growing evidence that cannabis can impair driving by slowing reaction times and encouraging perplexing moves by drivers, like slowing way down and being reluctant to change lanes.

Doctors at Toronto's Centre for Addiction and Mental Health are doing the world's biggest-ever clinical study, asking exactly what causes this behaviour, and how dangerous it is.

'We've seen an increase'

Either way, an innovation war worth billions to the victor has been declared over developing a cannabis breathalyzer.

Legal recreational cannabis is on the way in Canada sometime next year, according to the federal government. Dispensaries are flowering up coast to coast. Canadians, at least a relatively large percentage of them, are smoking the devil's lettuce. There's money to be made.

Last month a UBC researcher claimed to have developed a $15 roadside cannabis breathalyzer that can also test for other drugs, but it's not ready for market. There's also the Vancouver-based Cannabix Technologies (with their slick video), founded by a former RCMP officer, which has raised millions to keep developing their pot breathalyzer.

Then there are the countless American initiatives.

Hard stats are difficult to come by, but according to police, there's been an increase in high drivers, at least in Ontario.

"There's just no doubt about it, we've seen an increase in the number of drug-impaired driving incidents over the past one or two years for sure," says Ontario Provincial Police Sgt. Kerry Schmidt.

"There does seem a willingness to smoke and then drive, but right now the law is the law and you can't do it."

'It's exceptionally complex'

From a cop's perspective, proving someone is high, especially on a small quantity of cannabis, could be more work than it might be worth (though they won't say that). Make the arrest, call in the Drug Recognition Expert officers to figure out if yes, this rabble-rouser is indeed high on cannabis and if they must leave behind a bodily sample.

It's just not as simple as authorities want it to be. — Jeffrey Raber, scientist

A breathalyzer could save a lot of trouble, though they're likely to be fraught with problems. Detecting alcohol is easy because you're looking for ethanol.

But tetrahydrocannabinol (THC) and cannabidiol (CBD), those barely pronounceable molecules that are released during the partial combustion of cannabis, are more complicated. And those are just two of many cannabis derivatives.

"There's so much you could be looking for," says Jeffrey Raber, CEO of The Werc Shop in San Francisco, a laboratory that tests cannabis products. "It's exceptionally complex, there are so many components."

Raber is pretty skeptical about the possibility of developing a cannabis breathalyzer any time soon, especially one acceptable for use in criminal investigations and the courts.

UBC Engineering Prof. Mina Hoorfar, right, says the device she developed to detect marijuana only costs $15 to manufacture. (UBC-O)

"It's just not as simple as authorities want it to be. We'll probably move to automated cars before we figure out how to test for this the right way," he jokes.

There are all kinds of complicating factors. THC can stay in the blood for weeks, so how can you calculate if it played a role in a car crash? Everyone reacts differently to the substances in cannabis. What may send one person for a tour of Andromeda could be a casual smoke for another.

'It's too reductionist'

Of course the big question is: how high is too high to drive? All of the above complications apply, among others. Washington state and Colorado decided to adopt a threshold of five nanograms of THC per millilitre of blood as the legal impairment limit.

"Numbers made up out of thin air," says Raber. "It's too reductionist, it's too simple, and it's just not how cannabis works. There are all kinds of chemicals that are so variable and affect everyone differently."

That perspective is shared by Doug Beirness, an impaired-driving expert with the Canadian Centre on Substance Abuse. While he believes a roadside test will eventually be commonplace, he says our mistake is trying to regulate cannabis like we do alcohol.

"These are very, very different drugs, and they affect people differently. As legalization approaches, we may need a whole new set of strategies to make sure people aren't driving dangerously high."