Its different for different people. Back in my college days ... (
), it was safer to drive stoned than drunk since there was really no test for marijuana usage. An officer can say you look stoned, but bad allergies can give you the same "look". So long as you aren't acting and driving stupid, most people can and do get away with it.
Urine tests are not very useful because they can detect the presence of marijuana from 3 weeks ago. So if an officer forces a person to take that kind of a test (get a warrant to), the results are not very useful or conclusive on whether or not the driver was under the influence at the time.
Also 0.3 grams is 7x over the limit? I wonder how they established that. The whole driving aspect of the legalized marijuana law is concerning...