The cop car obeying the speed limit and every one behind it doing the same is not an example of congestion. As long as the cars are flowing at the posted speed limit then I don't see that as congestion, the traffic is just not speeding along as quickly as one wishes it would.

By that logic, any street that has two or more cars following the speed limit in close proximity to each other could be classified as congestion.

Once you have a buildup of cars that are traveling slower than the intended speed of the road that causes further slowing up the line that eventually creates bumper-to-bumper traffic with little movement, then you have congestion.