There is an important omission in the public debate regarding the liberalization of Canada's marijuana laws.

The missing element concerns the impact decriminalization or legalization might have on the safety of our highways given the lack of a practical, scientifically reliable, and constitutionally valid method for police to test drivers for marijuana use. In short, there is, at present, no equivalent to the roadside breathalyzer that police can use to test for stoned drivers.

The issue of "smoking and driving" is increasing in importance as marijuana laws across North America are liberalized. Colorado, in defiance of the U.S. federal government, recently allowed state-sanctioned retail outlets to sell marijuana (with Washington state about to follow suit). And New York state is joining the long list of U.S. jurisdictions permitting the use of marijuana for medicinal purposes.

Here in Canada, the Supreme Court has ruled in favour of medical marijuana. For several years, the Canadian Association of Chiefs of Police has called for the decriminalization of pot (meaning simple possession would be subject to a ticket but not criminal charges) - a proposal Stephen Harper's Conservatives have pledged to study.

And even more dramatically, Liberal Leader Justin Trudeau recently admitted to smoking pot during his tenure as an MP and suggested a Trudeau government would look at pot's legalization.

But what impact would marijuana's decriminalization - let alone legalization - have on the nation's highways? What if marijuana had the same legal status as alcohol?

Alcohol is a legal, albeit highly regulated, product in Canada. But just because alcohol is legal does not mean there are no restrictions on its use. You cannot, for example, drink to the point of intoxication and then get behind the wheel of a car without fear of consequences. The same, presumably, would be true for marijuana.

But there are currently dramatic differences in terms of the technology police can use to determine drunk driving versus stoned driving. With alcohol, police have long been able to practically and accurately measure an individual's sobriety. The technology found in a roadside breathalyzer, originally developed in the 1950s, has long been accepted by Canadian courts as a scientifically reliable and constitutionally valid means of testing for drunk drivers.

Breathalyzer technology is, in fact, so reliable that today it is used in court-mandated ignition interlock devices which repeat drunk drivers are sometimes forced to install in their cars. And the breathalyzer is discriminating enough that it can be used at the roadside by police to issue three-day administrative suspensions for drivers only somewhat intoxicated (i.e. a blood-alcohol content above 0.05 per cent) or for the laying of criminal charges against more heavily intoxicated ones (i.e. a blood-alcohol content above 0.08 per cent).

More importantly, a breathalyzer test (unlike the testing of blood samples) is non-invasive. Consequently, the Supreme Court of Canada has determined it is constitutional to require drivers to provide a breath sample to the police. The same is not true for roadside blood tests.

Blood tests cannot be taken by a police officer. Blood tests can only be administered by medical personnel and are mostly used in the case of drivers incapacitated and unable to take a breathalyzer test (e.g., an unconscious drunk driver taken to a hospital after an accident).

And what about pot?

It is true the police will be able to use non-technological sobriety tests to check for drivers high on marijuana. Non-technological sobriety tests (which vary from jurisdiction to jurisdiction across North America) include walking in a straight line, saying the alphabet backwards, standing on one leg, and the classic "touch your finger to your nose" test. Although more subjective and thus somewhat less likely to result in convictions than the breathalyzer, these tests should be effective in catching and convicting most highly intoxicated drivers - in the same way that similar tests caught some drunk drivers before the advent of the breathalyzer.

But what about the driver who is moderately intoxicated by marijuana, but not obviously so? As is the case with alcohol, some people (especially heavy users) display fewer obvious signs of intoxication than others.

Unfortunately - from a law enforcement perspective - there is currently no reliable, precise, and non-invasive equivalent to the breathalyzer to test stoned drivers. Oral swabs and urine tests can check for the presence of drugs, but if marijuana is legalized, the mere presence of pot in a driver's secretions is not a crime.

Only a blood test has the sensitivity to accurately measure levels of intoxication. But a blood test is not nearly as practical as a breathalyzer test since a blood test must be administered by qualified medical personnel and therefore cannot be routinely administered at a RIDE program checkpoint.

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In the absence of a pot breathalyzer test, the police will have a difficult time ensuring that stoned drivers stay off the highways. This represents a serious gap in the technology available to law enforcement agencies - a gap that would be further exposed should Canada's marijuana laws be liberalized and more marijuana users were to take to the roads.

Until that technological gap is filled, Canadians might want to think twice about increasing the accessibility of marijuana.