How will over-the-limit, DUI driving infractions be determined

[Spike89]

92. They've been doing it for decades

It is pretty much the same thing they do with prescription drugs, other recreational drugs, and other impairments (too sleepy). They observe unsafe driving, pull you over, try and determine why you were driving poorly (spill coffee, talking on a cellphone, etc). If they suspect you're under the influence, they have you do a field test.

None of that is any different than alcohol. By the time they have you blow during a DUI stop, they've pretty much already made a case. The actual test pretty much only strengthens the case, blowing below .08 isn't proof you are innocent of impaired driving, and isn't even an automatic non-drunk driving event.

Without doubt, there will be some overzealous cops trying to pin DUI on pot smokers who aren't stoned while driving. There will also be plenty of pot smokers behind the wheel when they shouldn't be. Pretty much just like it has always been. I don't think blood tests for THC will hold up, the test currently isn't meaningfully connected to the subject's level of impairment. Chronic/regular smokers will have a high level long after the effects have worn off and casual smokers may not register much even though they could be totally baked.