After the 9/11 terrorist attacks, the U.S. feared a terrorist with a truck bomb could attack the Hoover Dam, potentially flooding vast areas and disrupting water and power supplies to several states; semi-trucks were banned from bridge, forced to take route to Las Vegas that is more than forty miles longer; new 1,900-foot-long structure will reroutes traffic off of the two-lane road atop the dam, will improve traffic in the region, and help protect the dam from terrorist threats; it is the seventh highest bridge in the world and it is held up by the longest arch in the Western Hemisphere