On May 23 2013, a heavy truck was travelling southbound on Interstate 5 when the oversize load it was carrying struck the top of a bridge spanning the Skagit River. Seconds later, the damage caused a span of the bridge to collapse.
The deficiencies cited by the NTSB included the failure of the driver of the pilot/escort vehicle to perform basic safety functions, inadequate route planning by the trucking company, Washington State's inadequate permitting process, and the lack of low-clearance warning signs for the bridge.
Although the trucking company had obtained a permit for the trip, it failed to check and plan accordingly for the low clearances encountered along the route. As the truck approached the bridge, it was travelling in the right lane where the clearance was lowest due to the arc design of the support brace. Investigators said that had the truck been in the left lane, where the vertical clearance was greater, the oversize load would have cleared the bridge.
The NTSB called for changes in the permitting process used by Washington State Department of Transportation to authorise movements of oversize loads on its roadways. The NTSB said that the protection of bridge infrastructure was "too vital of a state concern to leave the responsibility for assessing the risk associated with the transportation of oversize loads entirely with the motor carrier."
Since the bridge collapse, WSDOT has updated the Skagit River Bridge by replacing the support brace's arc design with a horizontal design that provides a uniform vertical clearance of 18 feet across the width of the bridge. WSDOT is also developing bridge clearance data and interactive maps to improve their permit process.
