Gee, that sounds familiar.
I don't see much problem with establishing acceptable limits. If you read the NHTSA reports I linked to previously yo will see that there are well established protocols for driver testing. In fact they routinely test new prescription drugs for effects on driving.
Here's how I see it being done. Get a group of drivers of varying skill levels. Benchmark them individually sober. Then feed them alcohol and test at various levels of intoxication to just over the legal limit. Again, individually benchmark the drivers. Then, long after they have sobered up, repeat with marijuana and keep feeding them marijuana until they drive as poorly as they did at the legal limit for alcohol. I would think that they should test about 500 or more drivers this way.
In my personal experience this wouldn't be reliable. I have noticed that if I smoke pot after a lengthy absence, I get seriously stoned and wouldn't be fit to drive at all. If I smoke regularly for a couple months I could drive without much difficulty or risk to anyone.
In other words a tolerance develops quite rapidly with regular use.
The primary difference I have noticed with myself between alcohol and pot is that when drunk I sometimes think I can drive OK when I cannot whereas with pot I know if I can or not. Pot doesn't seem to impair judgement the way alcohol does.
The biggest risk I see with stoned drivers isn't that they can't operate a vehicle effectively, but that time can seem distorted and disorienting. Sort of like a split second lapse in attention that feels like it was much, much longer. There may be some potential for problems from this while driving.
In any event I don't see stoned driving being anywhere near the public safety issue that alcohol is, even if the number of people smoking pot was equal to the number drinking alcohol.
Still, I would think a blood test would be just fine. *IF* I am driving poorly and *IF* I fail the field sobriety test, then go ahead and draw some blood. If the test reveals I am currently under the influence, then arrest me.