What can the police do?

Imagine a scenario where the speed limit on the road is 40. And you have a police cruiser with the occupant holding a laser gun about a couple of hundred feet ahead of you hidden in an alley or small street, or under a bridge. Now lets say that everybody around you with their own vehicles were driving about 55 to 60mph (including you). What can the police officer do when everybody passes by him driving that fast?

He can step out and flag over the lead car that he got a laser reading on.

Seeing another motorist being pulled over is good incentive for the others to slow down.