I actually don't recommend rocks or concrete behind chicken cut-outs because in todays world, they can sue you. In PA a guy lived where the road curved and he always had cars fly into his yard. So he put a big rock there, they made him take it out because it was hazardous to drivers. Guess it doesn't matter if the drivers are hazardous to people that live there... How about some nice thorny plants (Black Raspberries or Harrison's Rose) planted as close to the road as you can get away with? If they swerve they'll scrape along the vehicle, if they have a window down maybe they'll scrape along their face? Either one of those have a lot of thorns and can arch out 10 feet or so.
I can only hope that folks that run over critters get a bone stuck in a tire and it goes flat. The speed limit on our road is 20 mph, kids seem to think that doesn't apply to 4 wheelers, go-carts, and snow mobiles. So when I see them riding by, I smile big (ok, maybe it actually looks like I am laughing at them) and they tend to not come back. See if you can legally install speed bumps, I am sure they would also have to be marked. I like the pothole idea. Or a rotten egg launcher...
I don't know why people would do something like that. But Karma will get them.
I've seen people serve to hit crows, seagulls, cats and a tourist aimed and ran over a duck and its ducklings.