If I had to guess, the counties with the highest road-kill numbers would be deceiving, because they would be representative of both number of deer and number of people out driving around. Likely Linn, Polk, and Johnson are right up there.
My grandfather used to drive up to Elkader to play cribbage (sp?) every Wednesday night. We started calling his car the 'deermobile' because he hit 3 deer on 3 consecutive Wednesday's.