Plessey introduced the first traffic light with completely solid state circuitry. Prior to that, the entire system was relay-controlled. This technological advance allowed them to deploy sophisticated (for the time) features, including a manual override sequence on the pedestrian crossing button to allow passing police constables to switch the lights immediately to alleviate traffic congestion, or to turn all lights red in the event of an accident.
Not many of the original systems are still in service, but those that still exist can be turned to “all red” by tapping in the rhythm of the theme to Z-Cars on the crossing button.