I think that's the biggest problem with smokers: paranoia. Every single one of my friends who smoke are always so fucking scared they're going to get caught with pot or whatever when they're driving or even if they have it on them and they're walking around (which they are almost always 100% too afraid to do).
You don't have to be scared. As long as you don't reek of pot, and have your weed out of sight (as someone already said), you'll be fine.

I tell my friends to think of it this way, just drive like any normal citizen would. Stop at stop lights, only go about 5-10 miles above the speed limit (cops here are lenient as crap, though. I was in the truancy officer's car today and the speed limit was 35, he saw them going 47, 48, 53, etc etc, IN A SCHOOL ZONE!!!) you'll do okay. I just pretend I don't even have any on me and that I'm just your normal, every day teenager that is a good, "moral" person who doesn't do drugs. ^_^