The Memorial Bridge carries US-93 over the Colorado River immediately downstream of Hoover Dam. The structure itself is striking — a long composite-concrete arch that was, when built, the longest of its kind in the western hemisphere — but the real reason to come is the pedestrian walkway along the upstream side, which gives the best free, head-on view of the dam.
You park at the Hoover Dam access road and walk across; the walkway is fully separated from the traffic by a high concrete wall. The view down to the dam, the lake behind it, and the canyon below is genuinely vertiginous. Allow about 20 minutes for the walk; bring water in summer.