Well you CAN, there isn't anybody holding a gun to your head telling you no but I would NOT advise it. MAYBE you won't get arrested and MAYBE you won't get a ticket and MAYBE the cops won't harass you but I can 100fuckingpercent gurantee you that IF suspicion is aroused and someone doest call a cop and they don't arrest or ticket you they WILL be sitting down the street waiting for you to drive away then you're busted with a DUI!

I brought some banks of lights to a friends apartment and her tweaker neighbor saw me carry them in, she calls the sheriff to report suspicious activity. Sheriff comes to the door, my friend lets him in and shows him her grow room and documentation, he never asks my name or anything and wishes us a good day! Not a minute later my friend wants to go get something to eat, I was like "uhhhh, he's still sitting in his car in the parking lot, your apt. reeks like weed and you want me to drive off"??

Personally I would find a very, very discrete place to smoke if you have to do it in your car. Flaunting your semi legal rights is not very smart. JMO FWIW