The flaw in this argument is the notion that the only determination of driving fitness would be the level of THC in the body. I contend that if you are pulled over, and are unable to demonstrate reasonable coordination or alertness, you could/should be taken off the road and investigated further. This is currently the initial process for handling DWI, right? Field sobriety exercises. You show reasonable control, you go on your way. You show impairment, you don't.
Or, you put the suspect in the back of the squad car and turn up Dark Side of the Moon. Watch for reaction...
lol, okay, that was pretty funny.
Seriously though, failing a field sobriety test is usually not enough to convict someone of DUI without a test of the blood alcohol. It is only grounds to test the blood alcohol, which then proves the person was drunk.
There is no way to prove the person was high on weed. The level of alcohol in the blood can prove if someone is drunk. But the level of THC in the blood does not prove if the person is high, only that they have THC in their system (which could be from months ago).
Also, what impairment would you test for in a field sobriety test? It's not like alcohol where you can't walk a straight line and that kind of stuff. I guess they could check for red eyes, but a little Visine would easily and quickly mask that (or a story of allergies).
True story: a friend of a friend was driving high several years ago. He got pulled over for not having a license plate on the front bumper, only the back. It's late at night and there are a bunch of us in the car, so the cop starts asking questions about what we're doing. When he found out we were coming from a party, he asks if we've been drinking. Half of us in the car say we have, the driver of course says he hasn't. He really hadn't either, but he had been smoking like a chimney. Anyway, the cop starts to give him a field sobriety test. He tells him to count backwards and say the alphabet backwards and that kind of stuff. My friend tells the cop, "Can you just give me a breath test or something? I'm dyslexic and don't want to waste your time with these tests that I could never do." The cop had another cop there give him a breath test and he passed. We were then on our way.
I just don't see how you can have the whole system based entirely on human judgement. There are way too many variables there for it to be consistently enforced.