It's a little device hooked into the odometer on the cop's cruiser. They roll the distance between two landmarks and punch the start and end points into the device. Then they watch cars going between the two landmarks and hit a start/stop button for each, and it reads out their speed. If you think you're in the middle of one of these traps (i.e., you've crossed the start but haven't crossed the finish), then the only way out of the ticket is to put flat spots on your tires.

Called a Police Pilot here in Sweden, but they generally use it while rolling in unmarked cars - nowadays also backed up by video. To give the speedee some leeway they measure the time on the car from before landmark 1 to after landmark 2, while they measure the distance "inside" the 2 landmarks, giving a slightly lower reading than if they measured everything exactly on the landmarks.

And laser is quite common, but it's been quite a while since I saw a radar used...

/Michael