I'm still on the fence about actually todding stuff at the car, but I do know a rock on my windshield gets my attention every time. Of course I pay attention when I drive so I usually see it coming and try to duck out of the way as if it were gonna hit me like I was on my bike...
I usually try some sort of .... hand signal:) Usually not very polite, but the WTF look is a good starting point.
Only time I can see it being difficult is on the highway where you might not really have room to get outa the way, or it may not be safe to take your eyes off the road for even the second it takes to give them the WTF. Some sort of "back off" hand menuver would be good.
