The arches will eventually measure more than 1,000ft across. At the moment, the structure looks like a traditional suspension bridge. But once the arches are complete, the suspending cables on each side will be removed.

Extra vertical columns will then be installed on the arches to carry the road. The bridge has become known as the Hoover Dam bypass, although it is officially called the Mike O'Callaghan-Pat Tillman Memorial Bridge, after a former governor of Nevada and an American Football player from Arizona who joined the US Army and was killed in Afghanistan.
Work on the bridge started in 2005 and should finish next year. An estimated 17,000 cars and trucks will cross it every day.

The dam was started in 1931 and used enough concrete to build a road from New York to San Francisco. The stretch of water it created, Lake Mead, is 110 miles long and took six years to fill. The original road was opened at the same time as the famous dam in 1936.