In scene 11, when Cameron Diaz and the "creepy thin man" are driving the motor cars a Burgundy car gets tipped over in a crash and starts skidding on the floor. There is no driver in the car or the other cars around it. The cars are waiting in a queue with traffic lights.
If you listen to the commentary track (on the DVD) during this scene, McG says the car slid way beyond where it was supposed to and nearly took out a camera. I think the cars were empty because they were either not meant to be in the shot originally, or they didn't want anyone in the path of the crash vehicle that may be injured, and figured no one would notice.