Developed and implemented back in 2011, Laservision's Softstop system projects a familiar bright red stop sign, seemingly in mid-air, at the tunnel's entrance if a vehicle too large to enter is detected. In fact, the stop sign does not float in mid-air, but rather is projected onto a water screen that, should a vehicle not heed its command, would cause no damage from being driven through.
The Softstop water stop sign is a last resort, mind you. If a driver's truck is tall enough that it would crash into the tunnel were it to enter, a number of brightly flashing warning signs are activated that signal the driver to stop earlier. Only if those flashing signs are ignored is the Softstop system activated at the tunnel's entrance. Likewise, the Softstop system can be activated for other reasons, including keeping drivers out for their own protection if there's a fire or other disaster inside the tunnel.
Though we admit to chuckling at the occasional truck-crashes-into-low-overpass video ( 11foot8.com is a whole website devoted to vehicles crashing into a low railroad trestle in Durham, NC), this clever solution for preventing such accidents is even more interesting to watch in action.