So it's not really practical as a field test. To pull this off, they'd need to know what your reactions were like sober. Maybe once they've already got you for something they could get a certified tester in there to administer a test, get a base line for your reaction times, THEN administor the test. But then it would cost too much and you'd be detained for longer than need be, and the ACLU would get involved.
It's not like how your eye twitches when you're drunk or how your speech is slurred and you stink like a distillery.
I'm all for a realistic way to test being on substance. People shouldn't drive on any drug, don't care what it is. That's a good step towards at LEAST decriminalization if there were an effective and reliable field test. That and there is no "legal limit" to how high on coke/e/whatever you can be like alcohol. So they'd have to study and figure out what amount in your system constitues being "too high to drive".