Depends on what kind of weed you get and how much you smoke. A debilitating indica will be harder to drive on than a nice uplifting sativa. Or if you're so high you're hearing and seeing shit... not a good idea to drive. Being drunk+high is not a good idea either.

I personally drive a lot and don't mind lighting up while on long stretches of interstate at night. I smoke a strain that gives me a really alert, clear headed high. As long as you don't overdo it you'll be ok, and you can't be conspicuous about it.