This topic is specific but not limited to the US & Canada.
This is not the equivalent of an alcohol breathalyzer, it is a tool designed to enable police to target literally any person they want.
Driving High: Is the test for weed reliable? - The Fifth Estate
The status quo for impairment testing is an absolute abomination of our rights and totally unsubstantiated by quality empirical evidence. Just wait until all they have to do is pop something into your mouth, they wait for a beep, and off to jail you go.
This is not about removing impaired drivers from the roads, these tests do not test for impairment, they merely test for the presence of THC.