The shutdown will cut off the primary link over the Colorado River between Nevada and Arizona during evening and overnight hours. Daytime travel will be unaffected.

"They're moving some large equipment and need to close off the road," Bob McKenzie, a spokesman with the Nevada Department of Transportation, said Friday of the Federal Highway Administration bypass project.

Typically, around 13,000 cars traverse the dam each day, according to state statistics.

"Since they're not going to start this until 8 o'clock at night, it shouldn't be too bad," McKenzie said. "You have much less traffic in the evening hours."

During the overnight shutdowns, traffic will be detoured from U.S. 93 onto southbound U.S. Highway 95 at Railroad Pass to state Route 163 and cross the Colorado River at Laughlin, a detour that adds 23 miles to the trip from Las Vegas to Kingman, Ariz.

Electronic message boards on U.S. 93/95 ahead of the dam will warn drivers of the closures, according to McKenzie.

The $240 million, four-lane bypass bridge, being built about a quarter-mile south of the dam in Black Canyon, is intended to carry U.S. 93 after the bridge's scheduled opening in late 2010.

At that time, traffic will be barred from the two-lane dam, which has been a frequent bottleneck for drivers crossing the river, and from where trucks have been barred since 2001.

Work has fully resumed after a months-long construction slowdown that followed the collapse of a multi-tower crane system amid high winds last fall. No one was hurt in that incident.

When complete, the 1,905-foot span, formally named the Mike O'Callaghan-Pat Tillman Memorial Bridge, after the former Nevada governor and the football star-turned-soldier who was killed in Afghanistan, will stand 890 feet over the river, and will actually look down at the dam.