When we were kids we blew off firecrackers at the police station door so they would chase us. That they did. Never caught us. Good practice for E&E training when I joined the army. Yeah, kids do stupid things, but this overpass thing never crossed out minds.
When I was a kid the thing to do was drop water bombs (balloons) down on people. Not on cars but you get the idea. Like you said dropping 10 inch rocks on cars never occurred to us.