How can you tell if your client is driving while stoned?

As Canada moves closer to legalized pot, a sure-fire way of testing drivers for cannabis impairment remains elusive.

Bill C-45, currently before the Senate, would make possession of up to 30 grams of recreational marijuana legal. Bill C-46, a companion bill, would give police the power to demand an oral fluid sample from drivers at the road side. It also proposes three different limits for tetrahydrocannabinol (THC) concentration. Drivers with two and five nanograms of THC per millilitre of blood could face fines of up to $1,000. Penalties for higher levels would be more severe.

But setting “per se” cannabis limits — analogous to the .08% blood alcohol limit under the Criminal Code — is of limited value, University of Massachusetts Boston psychology professor Michael Milburn told Canadian Underwriter Friday.

That’s because cannabis is fat soluble, unlike alcohol. Therefore, the concentration of THC in someone’s blood “is not a good measure at all” of impairment, Milburn added.

A better legal standard for determining drug impairment might be an “accumulation of evidence,” he said. That means a combination of factors, such as the smell of marijuana coming from the car, failing a drug test, failing a field sobriety test and failing a test using a mobile app.

Milburn created an app, dubbed DRUID, for Apple and Android devices. Among other things, it tests how quickly a user can touch the screen and follow instructions generated by the app, such as following a moving circle with one’s fingers.

“If I were running an insurance company, I would want to give a break to any company to any driver who uses DRUID on a regular basis,” Milburn said in an interview.

The House of Commons has already passed Bills C-45 and C-46 (both of which also need to be passed by the Senate before becoming law).

But Canada still does not have a roadside THC screening device approved for use by police, a Public Safety Canada spokesperson told Canadian Underwriter Friday.

“Devices will need to be tested against Canadian standards before they can be approved for the Canadian market and procured or used in Canada,” the Public Safety spokesperson added.

Several Canadian police departments used drug screening devices on a trial basis between December 2016 and March 2017.

With alcohol, driver impairment “increases with rising alcohol concentration and declines with dropping alcohol concentration,” the U.S. National Highway Traffic Safety Administration stated in Marijuana Impaired Driving – A Report to Congress. By contrast, “the level of THC in the blood and the degree of impairment do not appear to be closely related,” NHTSA added. “Peak THC level can occur when low impairment is measured, and high impairment can be measured when THC level is low.”

Last July, two lobby groups, DUID Victim Voices and Smart Approaches to Marijuana Canada, argued that a driver should be guilty of operation while impaired under a sequence of conditions. That sequence would be if, first “the driver was arrested by an officer who had probable cause, based on the driver’s demeanor, behavior and observable impairment to believe that the driver was impaired,” and second there was “proof that the driver had any amount of an impairing substance in his/her blood, oral fluid, or breath.”