Have you ever been driving (or riding along in) a car and had something like an apple core or banana peel that you needed to get rid of? Think about throwing it out the window? Decide it's OK because the litter is biodegradable? I used to think so. Fortunately, I learned that animals come to scavenge your scraps and thus put themselves in harm's way and get used to going to the roadway to look for food. This behavior is bad for animals and motorists.
On another note, I also learned that throwing gum out the car window is also a bad idea. Birds can eat it and it can mess up their insides or make them unable to chew...it kinda gets stuck in their mouth sometimes (if what I hear is true, that is).
The best thing to do is hold on to your trash until you stop the car and find a proper trash can.