I saw something like this the other day at a stop sign. Normally there is a lot of traffic going through this stop sign at rush hour, but it doesn't get backed up. The other day there was a police car sitting near the stop sign. It caused a huge backup that stretched for several blocks. Turns out people were being extra careful to come to a complete stop for a few seconds because of the police car, and that caused the line to move more slowly than normal.