Signal control sometimes gets it right, with traffic clearing the junction in the green time allowed. But more often, some traffic is caught at red, or the lights change to red just as traffic arrives. Traffic means pedestrian as well as motor traffic.

With signal control, road capacity is wasted as traffic is held at red when nothing is happening on the junction. Congestion forms at traffic lights much like iron filings cluster at the poles of a magnet.

Signal control at priority junctions produces consecutive queueing. Self-control on equal-priority junctions produces simultaneous filtering. Which is more efficient?