For decades, police have used a breathalyzer to test whether a driver has had too much alcohol.

Now that Canada is on the verge of legalizing marijuana, is there a similar roadside test to detect pot-impaired motorists? The answer, according to the RCMP, is yes ... and no.

Q So, what are police doing to get ready to crack down on stoned drivers? Public Safety Canada and the RCMP are investigating three roadside devices that will collect saliva from drivers. It’s easier to collect, compared to blood or urine, which typically require warrants. Q How do they work? Unlike a breathalyzer, they will not tell police how impaired a person is. They will not quantify the amount of pot in the system, the RCMP says, but instead will indicate when a drug is present so further followup can be done to confirm the amount.

Q What will be the legal limit? The Canadian Society of Forensic Science says reports show the science is unclear on what amount of pot makes a person impaired. Police hope to see a low number, perhaps less than five nanograms of THC per millilitre of blood. Q What about the U.S. states where pot is legal? According to an AAA report, Colorado, Washington and Montana test for THC levels of more than five ng; Pennsylvania more than four, and Nevada and Ohio more than two; 11 states have “zero tolerance” for any THC in drivers’ blood. Q What is the downside of these roadside devices? Scientists and lawyers have questioned whether the tests can determine if a person is intoxicated by pot; some insist the legal limits used by the six U.S. states where marijuana is legal are “arbitrary and unsupported by science.” Intoxication depends on the type of pot consumed, how long before driving it was taken, and an individual’s tolerance. Using the 5 ng threshold, the AAA study concluded, could find some stoned drivers innocent and some sober drivers as over the limit. Canada’s Centre for Addiction and Mental Health is conducting a similar study.

THE TECHNOLOGY

The three devices being tested have not been approved for use in Canada, but will help officials set standards for the roadside tests that will be used here. Once the rules are finalized, manufacturers can submit their devices for testing. The ones that are green-lighted can be purchased by police agencies. The RCMP would not provide a time frame or budget for the purchase of the devices. In the meantime, police will rely on standard sobriety tests — such as touching your nose with your fingers — if they suspect someone is driving under the influence of pot.