Not only do they spring down when run over but when they go down an eyelid wipes the little reflective eyes so evey time you run over one you clean it.

I saw a real cool thing in south england little reflectors with a small circuit in it so that a solar panal charges a battery and then at night when a headlight sticks the cats eye a small intense led switchs on lighting up the cats eye really well but the cool thing is it stays on for a few seconds though so you have a trail of lights following you.