Many tunnels in Sydney, Australia aren't tall enough to permit tractor trailers to move through safely. This animated gif shows a warning system that informs truck drivers when they're about to crash into a tunnel entrance. When sensors detect a vehicle that is too tall, the system pours water across the entrance to the tunnel and projects a stop sign onto that water curtain.
--Leslie <;)))>< Fish
http://www.neatorama.com/2012/11/01/One-Tough-Bridge/