Devices and early warning methods used to reduce hits on low-clearance structures

North Dakota DOT uses the standard Advance Warning sign W12-2 and places a W12-2p sign on the structure. Some structures have been hit that have the clearance signed and some of these had the required clearances.
In work zones where structures are being repaired or constructed, canapes under the structure to catch construction debris are installed where the above signs are installed. In some cases they will direct overheight loads up ramps at interchanges if they exist.
NDDOT has used lasers and audible warning but found the cost high and the use not always effective. Some vehicles continued to the structure and were difficult to turn around or return to an exit. Some vehicles got stuck under the structure.