I also always have cloth grocery bags in my vehicles and also carry a few plastic garbage bags in case I find trash that needs cleaned up. Carrying a pair of work gloves in your vehicle is also a good idea - you never know what kind of mess you will find!!
I even carry a few trash bags on our boat - and have my husband stop so that I can pick up the floating trash that we find in the water. (I once threatened to throw a man overboard who threw a can out of our boat - either he went in after the can voluntarily and I'd wait for him to re-board the boat, or I'd throw him in and we'd leave him there!! Of course I wouldn't have left him in the middle of that river - but I believe I would have honestly pushed him off the boat!!!