I did not know this, but they’re building a bridge across the Colorado River just below Hoover Dam. It will be the longest concrete-arch bridge in the U.S. and the fourth-longest in the world. It’s intended to relieve traffic atop the dam, which backs up tremendously most weekends. Also, since 9/11 the road atop the dam has been closed to truck traffic, and the nearest bridge for trucks is something like 275 miles away.
Lots of pictures, and a little discussion, here.