A fairly low bridge (high enough for a standard trailer, but any oversized loads on a flatbed could easily hit it) in my area had what were essentially large me

A fairly low bridge (high enough for a standard trailer, but any oversized loads on a flatbed could easily hit it) in my area had what were essentially large metal cans hanging by wires along a sign warning about the height. Anything tall enough to hit the bridge would hit the bits hanging from the sign and clang them around/together/against the vehicle which makes a pretty audible noise for the driver.