It really just depends on your body and system. I can't drive at all if I smoke/eat weed, I have to wait at least 6 hours. My mind just doesn't drive high. Other drugs and alcohol don't really phase my driving ability (not that I'd ever drive high or drunk).

I guess if you really want to test yourself, look to the right, look to the left and if you can remember what you saw when you were looking to the right, you might be good to go. Also, close your eyes for 30 seconds, if you're not sleeping, have your balance and wits about you when you open them again, you might be okay.

Never in any way would I ever condone anyone driving under the influence, but just in case of an emergency or something, make sure you turn on the air conditioning or open the windows if it's cold out. The cold will keep you more awake and aware. And don't turn on any music or distractions.