Crew or equipment visible
1. Traffic on a bridge like that can be heavy during rush hour. During a zombie apocalypse, traffic would likely come to a halt due to a few crashes and the falling apart of towing companies/authorities, not to mention everyone frantically trying to leave.
2. Why would regular civilians be going into the zombie war zone in an orderly fashion?
Depending on the direction of traffic, it should either be totally chaotic or nonexistent. Obviously the director reserved the top of the bridge for extras without restricting access to the roadway below.