Smoking weed: When is someone too high to drive?

Roadside testing for THC is also logistically difficult.

Blood, for instance, needs to be analyzed in a lab, and collecting urine gets complicated.

In Canada, which legalized recreational pot just this year, law enforcement will test drivers with a saliva test called the Dräger DrugTest 5000, but that isn’t perfect, either.

Some private companies are trying to develop a sort of breathalyzer for marijuana. But Jonathan Caulkins, a drug policy researcher at Carnegie Mellon University, said, “There are fundamental issues with the chemistry and pharmacokinetics. It’s really hard to have an objective, easy-to-administer roadside test.”