This may just be the way California does things but the reason that most of our clients get pulled over are for not using their turn indicators, speeding, driving patterns (swerving, going over the line 1-2 ft etc), having a tail light out, headlight out shit like that. Oh and there have been the few who have "fallen asleep" in the middle of an intersection with their foot on the brake. ( I dunno) So, yes, there has to be reasonable suspicion and probable cause for an officer to pull a person over in the first place. And of course, if the subject, has red watery eyes, an alcohol smell emitting from their breath and slurred speech, ( we all know law enforcement is trained and they all are experts at this type of thing -eye roll) thats when they start in with their DUI crap. And you are right, people are all different when they have alcohol in their system. I dont drink, so one beer will usually give me a killer buzz, but for someone who drinks more often, they of course would not have the same effect from that beer that I did. But, all the same, alcohol would still be in our system which it is against the law to drive under the influence.