I travel close to 200 miles and carry propane for the grill, 20 pound cylider to be exact. I would think with our modern cars that flow through ventilation would take care of any small amount of leakage will moving. But if you are going to park your car and go shopping on a hot day I'd have to say that that person is a candidate for the Darwin award.

Unfroutnatley these idots always seem to hurt the inocent people.

I always use my truck and have made tank carriers for my 20#, 30# and 40# cylinders and they are tied down so they don't roll or "bounce" around the back of the truck.
I can see it now, a law that says you can't carry propane tanks in the car. Sheeeeeesh
Bernd