that's partially why i bought a Honda fit. we've driven into a drive in food joint and injured a bike (only a mirror, how lucky we were!)... i like my bike inside. Even on the back of a car, damage can happen. I watched two cars with bikes on back back into each other in a parking lot!!!

we've also had friends incur THOUSANDS of dollars of damage on expensive bikes which were sitting on top of their cars. That little sign is a good idea. hopefully you can hang it at EYE LEVEL!