It seemed straightforward at first.

In 2009, a number of people reported seeing an Ottawa driver cutting off another car, crossing over the centre line and barely missing oncoming traffic, forcing other drivers out of his way, and running into a car driven by an 87-year-old woman. Two separate people called 911 to report him as a drunk driver.

To the police officer who showed up to investigate, Carson Bingley certainly seemed impaired: “swaying from side-to-side, slurred speech, slow and deliberate movements, and (staring) off into the distance during questioning.” He stumbled, his fly was undone, and he had trouble zipping it up again, according to court documents.

READ MORE: Supreme Court to hear case dealing with how to spot drug-impaired drivers

But Bingley didn’t smell of alcohol, and a breathalyzer test showed hardly any alcohol in his system.

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At the station, Bingley admitted to smoking marijuana (later confirmed by a urine test, which also found cocaine), and a police drug expert who administered a series of tests formed the opinion that Bingley was impaired. He was charged with drug-impaired driving.

The traces of THC in Bingley’s system could have been from an exposure much earlier, another expert later testified, though Bingley admitted that night to having taken two Xanax and smoking marijuana in the twelve hours before his arrest.

If all of this sounds more complicated than a traditional drunk driving case, that’s because it is.

It’s also why the courts have been chewing slowly on Carson Bingley v. Her Majesty the Queen, acquitting him twice because of an issue related to whether the police drug expert’s evidence was admissible, ever since the original incident (the Supreme Court will hear the case in mid-October).

(Bingley’s lawyers’ arguments are here.)

Though stoned driving creates many of the same problems as drunk driving, it’s not as easy to define. Without the ability to set a limit, measure it objectively and enforce it, the justice system is forced to fall back on the subjective judgment of police experts who have to determine whether someone was impaired in any given case.

The result is an expensive, cumbersome system that has none of the clarity of our approach to drunk driving. Depending on what the Supreme Court does in the Bingley case, Canada’s approach to drug-impaired driving may turn out to be broken in a way Parliament will have to fix.

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Stoned driving has attracted new attention lately because of the federal government’s plan to legalize marijuana next spring.

On Oct. 2, Ontario laws providing for licence suspensions and vehicle impoundment for drug-impaired drivers went into effect.

Ontario is increasing fines for impaired driving, and trying to tighten laws surrounding using drugs and driving. Dr. Samir Gupta explains what effects marijuana use can have on a person’s ability to drive.

When is a driver who’s been smoking pot impaired, though?

In a Saskatchewan case, a police officer pulled over a driver and found “an overwhelming odour” of marijuana in the car. The driver admitted to smoking pot, and had a roach clip on the car dashboard. (There had been no issue with her driving — she stopped at a drunk driving check stop.)

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She ended up being acquitted — the judge agreed that she had been smoking marijuana, but ruled that there was no evidence that it had caused impairment.

The test used on Bingley involved a cumbersome 11-step evaluation checklist that includes an examination of eye pupil size and muscle tone, and getting the person to stand on one leg.

The testing has to be carried out by officers who’ve completed training that takes over 100 hours and costs over $17,000, and there are only so many of them. (In the Saskatchewan case, the testing took nearly two hours.)

READ MORE: If Canada legalizes marijuana, how will cops combat high drivers?

With all of these time-consuming uncertainties, it’s not surprising that in Canada in 2014 there were 37 impaired charges laid for alcohol for every one laid for drugs.

“When I speak to the police, they say that given the cost, the time, the challenges in these areas, that they only lay drug impaired charges if they are slam-dunk cases,” says Robert Solomon of Western University’s law school. Tweet This

Driving under the influence of drugs has been illegal in Canada since the 1920s, but it wasn’t until 2008 that Parliament put a clear structure in place to deal with impairment by anything other than alcohol. This is the system we still have in place.

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“It was better than nothing,” Solomon says. “It was a step in the right direction.”

The Bingley decision — which may find that police drug recognition experts won’t have their qualifications automatically recognized by the court when they testify, and will have to establish them over and over again — may make things still more complicated. If that happens, “we’re going to see even fewer criminal charges laid for drug-impaired driving.”

“I wouldn’t be surprised if you saw charges only in the most serious cases where there are crashes. It certainly would discourage even more the laying of drug-impaired driving charges.”

WATCH: A “Drugged Driving Suit” created by Ford is aiming to highlight the effects of driving while impaired by drugs.

0:59 “Drugged Driving Suit” gives people idea of effects of driving while impaired by drugs “Drugged Driving Suit” gives people idea of effects of driving while impaired by drugs

The alternative is setting limits for various drugs, licit and illicit, much as we do for alcohol. It’s a much simpler approach — it’s easier to prove that someone had a given level of a substance in their body than that they were impaired. The law assumes impairment from a pre-set level of a drug.

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In 2007, researchers suggested a limit of seven-10 nanograms of THC per millilitre of blood, which they argued was equivalent to a blood-alcohol level of 0.05 per cent, the point at which provincial impaired driving limits start to kick in.

One challenge is that marijuana-related impairment works differently from alcohol, and that traces of THC can be detectable long after there’s any impairment. An Australian driver was acquitted in February after failing a cheek swab test nine days after he last smoked pot.

THC levels can be measured through blood, urine or saliva tests. Saliva is the easiest to collect — cheek swab tests for marijuana are used in Australia — but blood is the most accurate. (On Monday, Conservative senator Claude Carignan introduced a bill calling for saliva-based testing in Canada.)

“Is that somewhat arbitrary?” Solomon asks. “Of course it is. Lots of our laws are arbitrary.”

“We don’t have to have perfect laws.” Tweet This