The federal government faces a critical challenge as it moves to legalize marijuana: It will be much harder to detect and charge people who toke and drive than people who drink and drive. Some researchers fear that it may be impossible.

Tetrahydrocannabinol (THC), the chemical in marijuana that creates the buzz, acts very differently from alcohol. American states that have legalized marijuana have struggled to come up with a legally enforceable equivalent of .08 for alcohol consumption. There is no marijuana equivalent to a modern breath-analysis machine, and it could be years, if ever, before one reaches the market.

If the American experience is any guide, Canadians may have to accept that, once marijuana is legal for recreational use, traffic deaths will increase.

Story continues below advertisement

"If nothing is done, this is a major concern that could lead to increased death and injuries," warns Andrew Murie, chief executive officer of Mothers Against Drunk Driving Canada.

But he adds that there are steps that governments can take to reduce the risk, and the Liberal federal government says it is committed to taking them. The question is whether they will be enough.

According to a 2015 report by the Colorado State Police, marijuana-related traffic deaths increased by 32 per cent in 2014, the year it became legal for businesses to sell marijuana for recreational use. Marijuana was a factor in 20 per cent of traffic fatalities, double the number of five years before.

Although some of the data may be influenced by the fact that police and researchers are testing for THC more than in the past, surveys show that people of every age in Colorado smoke more pot than they used to, now that it is legal. In Canada, annual school-based surveys by the Centre for Addiction and Mental Health (CAMH) reveal that teenagers are twice as likely to toke and drive as they are to drink and drive (10 per cent versus 5 per cent).

We've been here before, and we've fought back. In the 1970s and 1980s, federal and provincial governments successfully combined tough new laws – the .08 limit, roadside checks, the breath test – and major public awareness campaigns to deter rampant drinking and driving. It worked. Nationally (excluding British Columbia), 1,094 people died in accidents involving a drinking driver in 1995. In 2012, the figure was 563, less than half.

With marijuana, the tools of behaviour modification – the ads, the school programs, the stories in media and such – will also be available. But legal sanctions will be tougher to set and enforce.

The problem, according to Erin Holmes, a research director at the Washington-based Foundation for Advancing Alcohol Responsibility (Responsibility.org), is that research into the effect of alcohol on the body preceded policy and legislation. But, with legalized marijuana, the law is getting ahead of the research.

Story continues below advertisement

"There just aren't the studies to say: Here is the nanogram level at which every person would be so impaired that they could not safely operate a motor vehicle," Ms. Holmes said in an interview. "We're just not there with the research. And forensic toxicologists have gone so far as to say we many never get there."

Washington and Colorado have set a limit of five nanograms of THC per millilitre of blood in your system, although some experts consider that level arbitrary.

However, new data will soon be available from Canada. At CAMH, based in Toronto, Robert Mann is overseeing one of the most extensive studies ever undertaken to determine how marijuana impairs driving ability.

For the past three years, moderate marijuana users 19 to 25 years old have been asked to smoke as much as they would in a normal situation. The volunteers were then subjected to a series of tests, including driving in a highly accurate simulator.

"Some people say that 'I'm aware of the effects, I can compensate for them, so I'm just as good or even maybe a safer driver'" after smoking pot, Dr. Mann says.

The study will determine the truth of that assertion. Interim results, Dr. Mann reports, suggest that distractions, such as counting backward by threes while driving after smoking, can be particularly problematic. Final results could be ready by the end of the year.

Story continues below advertisement

Another, possibly even bigger, problem is measuring THC levels in the blood. There is no breath-analysis machine that would allow a roadside test to determine marijuana use. Cannabix Technologies, a Vancouver-based company, will soon begin trials on one that, if everything goes well, could be ready for use in 2017. But the machine can read only whether marijuana has been ingested in the past two hours. It can't determine levels of concentration.

For now, police use the old-fashioned Standardized Field Sobriety Test – stand on one leg, walk a straight line, follow the pencil – to determine whether a driver is drug-impaired, which could eventually lead to a blood test. But drivers who are only mildly impaired may escape detection or conviction.

That doesn't mean that legalizing marijuana must result in an orgy of impaired driving and carnage on the roads. As Ms. Holmes points out, all of the lessons learned about deterring drunk driving are available to deter drugged driving. "We don't have to invent the wheel, we're not starting from scratch," she observes.

Mr. Murie believes that, among other measures, the new law should authorize police to collect saliva samples, because there are machines that can more accurately test THC levels using that method.

"I am confident that we can have the safeguards in place if we do things right," Dr. Mann says. He maintains that legalization might actually make it easier to have a testing and enforcement regime in place, and to devote the necessary resources to change the behaviour of marijuana users who drive.

The government said in a statement that a task force will study the issue and make recommendations before any legislation is passed.

Story continues below advertisement

The risk of increased fatalities because of marijuana-influenced driving is real. But legalization advocates insist that the risk can be managed. To do so, we must convince each other that friends don't let friends toke and drive.

Editor's note: an earlier version of this story incorrectly identified the company Cannabix Technologies as Cannibex. This story has been corrected.