Given there are countless substances that are hard/impossible to test for that could affect someone's ability to drive, the solution to this issue seems not to invent a new device for each substance but to come up with a more generalized test to measure someones level of impairment — reaction times are a good proxy.
Yeah, what happens when the next drug comes to legalization? A new device and a new dubious test with a 20 dollar per test charge? Maybe a cop should have a PlayStation and a driving wheel to test for impairment. Or a button that you have to press when you see something on the screen.
