Hoover Dam Bypass

Shown below is a picture of the bypass for Hoover Dam, now nearing completion. The idea is to allow traffic on the road that currently goes across the top of the dam to pass by without having the risk of someone driving a bomb onto the dam and destroying it.

I have a question though. How can this be considered safe? It doesn't look to me like the bridge is far enough away to do anything but require a bigger bomb... while the risk still remains.