If you get high one night, the next day you're not high and fine to drive, just like alcohol (if you don't overdo it).
So the most fair thing would be to increase the cutoff level of THC allowable in the system on the drug tests. It's stupid to get in trouble for the joint you smoked two weeks ago.
The feds are essentially prohibiting illegal drug use in drivers by the way the drug test works, which is sensible seeing as they are deemed illegal presently. But since in CA marijuana may be legal, it would seem to make sense to up the allowable THC level in the drug test to reflect that someone may actually be still "impaired." (By the way, I have heard a few arguments that people actually drive better when high on pot.)
As an aside, one can still do magic mushrooms 'till the cows come home, as they are not tested for—so hey it's not all doom and gloom! Happy happy, joy joy!