Where I live there's something called 'block driving' (I haven't found a better translation for the Dutch word 'blokrijden'). On busy summer days, when the highway to the coast is jammed, a police car will block the highway, then drive at a constant speed (and slower than the maximum speed), with no one being allowed to pass the police car. Apparently this helps to resolve the traffic waves.
The California Highway Patrol appears to do this in Southern California as well. I've seen a patrol car start weaving between two lanes, then three, weaving more and more until it's blocking all lanes of traffic like a weavy pace car.
It's called "running a break", and I've only seen it used to clear debris or disabled cars out of lanes, or so that a construction crew can close a lane. It'll freak you out the first time a cop car swerves in front of you across 5 lanes of traffic.
