Washington state's top police officer says cracking down on marijuana users who get behind the wheel to drive is proving tricky for the state.

"We're still learning, it's ongoing," said Washington Patrol Chief John Batiste.

Canada's prime minister-designate Justin Trudeau has promised to legalize pot for recreational use in Canada once he takes the reins from Stephen Harper, and that's raising questions about how to catch impaired drivers here at home.

Canadian courts have found drug impairment tests untrustworthy and a poor indicator of impairment. That's why out of 50,000 charges laid each year for drunk driving in Canada, fewer than 1,000 are for drug impairments.

Batiste says while Washington state legalized marijuana nearly a year and a half ago, they have recently seen an increase in the number of people getting behind the wheel while high.

"We are addressing that through a variety of ways: through information sharing and teaching our troopers on how to better detect [it]," said Batiste.

The key to the state's enforcement is a 2013 law that limits the amount of active THC — the element of pot that makes you high — in a driver's blood. The state has set a maximum of five nanograms per millilitre of blood, which state officials believe is the equivalent of a blood alcohol level of .08.

A similar law is also in place in Colorado, which also legalized marijuana use in 2013. To enforce it officers need to order a blood test, which can be very controversial.

The states have trained officers to look for signs of marijuana use on the road; distracted driving, light body tremors, different sized pupils, impaired motor skills and the smell of marijuana in the vehicle.

Impairment difficult to measure

But research has shown measuring impairment based on THC levels is not clear cut.

That's because unlike alcohol, people metabolize THC at different rates, so impairment can vary widely from person to person making it hard to determine if a person is impaired solely based on THC levels.

In addition, these tests have been challenged in courts, where people have claimed to have smoked days before their blood test registered the presence of THC.


Prime minister-designate Justin Trudeau has promised to legalize marijuana use in Canada. (Getty Images)

One researcher has found most heavy marijuana users would be below the five-nanogram level within hours of last consuming the drug, and virtually all users would be below the mark after 24 hours. But the research also found signs of impairment in heavy, chronic, daily users were still observable after three weeks of abstinence.

Batiste says Washington is also looking for technology like breathalyzers that could detect if someone is high, but so far, there's no hand-held device that police can use to measure the amount a suspected driver has consumed or determine impairment.

Laws on the books

Penalties for driving under the influence in Washington state range from fines of $350 and at least 24 hours in jail and a 90-day driving suspension for a first offence, to a $5,000 fine, a year in jail and a three-year driving suspension for a third or subsequent offence.   

In Canada there is no legal limit specified for any drug other than alcohol, but the current Criminal Code states that: "Every one commits an offence who operates a motor vehicle or vessel or operates or assists in the operation of an aircraft or of railway equipment or has the care or control of a motor vehicle, vessel, aircraft or railway equipment, whether it is in motion or not, while the person's ability to operate the vehicle, vessel, aircraft or railway equipment is impaired by alcohol or a drug.''

 In 2008 the law was amended to authorize police to demand field sobriety testing if they have reason to suspect drug use and to establish procedures for gathering evidence of drug use.

In Canada some police officers have also been trained to recognize the effects of several types of drugs, including marijuana, and order blood or urine tests.

With files from Richard Zussman and Mike Laanela, and the CBC's Firsthand