YOU CANNOT DRIVE ON THE HOOVER DAM!
Seriously, that's why that giant bridge that collapsed in the background is there. No more vehicle traffic(Except for official vehicles, of course). And why were the pop up bollards somehow the "safe zone" for people? Just because they were at the edge of the canyon wall? It's an EARTHquake, the EARTH moves, too.
I can already tell that this movie is going to be hard to finish.