The Hoover Dam Bypass is the O'Callaghan-Tillman Memorial bridge which was built some years ago to alleviate traffic congestion on the old road atop the dam itself. The high arch of the bridge is in itself a fantastic sight from down at bridge level. But the main attraction is the pedestrian walkway along the north (dam-facing) side of the highway bridge.
First of all, note that it is *really* high. This is definitely not a place to go for anyone who has no head for heights. The bridge is solid, but the wind can be quite strong at its altitude.
The parking area for the bridge is after the dam's security checkpoint and is easy to miss. About half a mile beyond the checkpoint, look for a sign to the "bridge scenic view" pointing to the right. The parking lot is good sized, but can still be crowded. There are public restrooms there. Beyond the parking lot, a long zigzag ramp (with bypassing stairs) leads up to the bridge level. Along the way are interpretive panels about the men for the whom the bridge is named as well as information about the design and building of the bridge itself.
Your reviewer doesn't mind heights much, but the high winds and the highway traffic whizzing by just a few feet away were constant distractions. Nevertheless, the most excellent view down to Hoover Dam was absolutely worth the effort. Watch for the panels in the walkway: there is one for the center of the span, and another for the border between Nevada and California. Also, if you visit later in the afternoon, the shadow of the bridge itself will fall on the dam, adding additional visual interest.