My experience is that if you drive at the same speed, you usually hit them. On the other hand, if you do anything else, you usually hit them too, but ocassionally you don't. Best to check the rearview mirror before braking!

In my lengthy driving career I have hit one chipmunk, two snakes, one cat, one bird, and one dog. The cat was merely grazed, and got away. So did the dog. The others weren't so lucky.

Oh, and then there's all those dead bugs on my conscience! ;-) I figure a couple of million of them.