I'd be hesitant to leave a bike on a car rack all day long, no matter where the car might be parked. I agree with the "inside" posse for security, but there are times when bike-on-rack is the way to go and security is required. I use a Saris Bones rack on some vehicles and a fork-mount Hollywood receiver rack on others. Big padlock with a 10' cable gives me the security I'll settle for at times like these. Silver lump below rear bumper is the padlock hooking the cable (threaded through frame and wheels) to a tow loop.
