We wouldn't need them at lots of junctions; Hyde Park Corner is one that was easier, or equally easy, to negotiate before the lights were installled; but the question is more difficult than it appears. Lots of lights in London and other cities are centrally controlled. The aim is to have traffic moving well over a large area. The consequence is that individual drivers may find that they are stopped at lights for no immediately apparent reason when, in fact, more trouble is being avoided than they realise.