Livin the Dream?
Disillusioned UPSer
Please tell them that you are concerned and suggest that they don't smoke & drive. If that does not work then go to the police. I'm sure there is a hot line for this in your area.
Sober, I think Matt has a good idea. Go to them directly, and tell them about your concerns. I would probably be even more direct and tell them that they are risking losing their jobs (probably don't care anyway or they wouldn't be smoking on property) and they need to take it somewhere else. Tell them if they don't you will report it. JMHO
Talking to the stoners in the car is just such a bad idea, I don't know where to start. It won't work, for one, and makes you a target as well.
Go to management in the morning; tell them you are concerned that your car is gonna get dinged by one of the pots smokers in the parking lot when they drive off stoned. Do it with a Union Rep present. Ask them if they would like you to call the cops or should they? Then go to work. Problem will be gone tomorrow. You have a right to a safe work environment - that includes the parking lot.