With recreational marijuana set to become legal on Oct. 17, the Canadian government has approved the first roadside test aimed at stopping drivers who are under the influence of cannabis.

But some experts are questioning whether the Drager DrugTest 5000 will actually be effective in detecting drivers who are impaired by THC — the main psychoactive agent in cannabis.

Harrison Jordan, who specializes in cannabis law, is raising several red flags over the test.

Jordan says that while the Drager device does determine levels of THC, it doesn’t detect whether a driver is actually impaired. Jordan also points to studies that claim two nanograms of THC can remain in a person’s blood for up to seven days after marijuana usage. Another concern is that the optimal working temperate for the Drager DrugTest 5000 is between four and 40 degrees Celsius.

Jordan believes all those factors will ultimately make it difficult for test results to stand up in court.

“We’ll have to see if the tests have merit,” said Jordan. “They probably have some sort of predictive value, but are they constitutional? That’s a question that’s going to be answered by the courts, and I think the courts won’t take too kindly to the flaws that are in the test.”

Even police forces admit there are flaws in the DrugTest 5000. But an inspector with the York Regional Police Road Safety Bureau says that’s why the roadside test is only one tool officers will use.

“This particular device will measure the presence of a drug in the saliva of a person. It won’t necessarily tell you that that level of the drug is impairing for that particular person at the time,” said Inspector Ed Villamere.

“This will be just yet one more tool that we will have at our disposal to provide one more piece of evidence in addition to the observations of the officer and the opinion of the drug recognition expert that someone is impaired by a drug.”

According to Dr. Don Redelmeier who cares for patients at Sunnybrook Hospital in the aftermath of life-threatening crashes, driving under the influence of marijuana can result in a twofold or threefold increase in the risk of a fatal crash.

Redelmeier admits the roadside tests for THC aren’t completely accurate, but adds that they may be effective in applying another type of pressure to drivers.

“The test is hardly perfect, lots of false positives, lots of false negatives. I doubt it’s going to be admissible in court, but it does go some way in terms of changing public attitudes.”

Drager is standing behind the DrugTest 5000, saying it’s being used reliably in other countries.

“If we weren’t confident in the technology we wouldn’t have submitted this for testing,” said Rob Clark, Managing Director for Drager Canada.

“The concern about accuracy and false positives, false negatives, really that should be the least of people’s worries.”