Even on a road trip one can't escape security! Traveling out of Las Vegas to Phoenix last week we decided to take the more direct route over Hoover Dam. Before being allowed to cross the dam, we had to pass through what looks like a permanent security checkpoint manned by the Nevada Highway Patrol. We were waved through but it looked like all RVs and some random cars were being directed to "secondary". Trucks and busses are not allowed over the dam at all (there are signs to this effect directing them to an alternate route well in advance of the dam itself). A similar setup exists on the Arizona side of the border.

This will all be moot however as soon as the Hoover Dam Bypass is finished. At that point nobody will be permitted to drive across the dam. http://www.hooverdambypass.org/