Why don't cars have a place for you to put your trash?
If you have kids, or you do a lot of driving, chances are you accumulate a fair amount of trash each day from eating and drinking in the car. Unfortunately, most cars don't have anyplace to store your trash, so it ends up getting tossed around until you arrive at your destination.
I think cars should have optional trash containers to keep the trash from getting out of control. While you can't force your kids to use it, at least they won't give you the excuse that there wasn't anywhere to put it.