How many traffic lights does London have?

How to get close to this number

Work from junctions. A city of nine million has tens of thousands of street intersections, but only the busiest need signals — most junctions are managed by priority rules and roundabouts. A few percent of intersections being signalised gives a figure in the thousands.

The central coordination detail is the modern part: all of them run from a single control centre. Signal timing across a city is a network optimisation problem, and coordinating thousands of junctions centrally allows green waves along corridors and load-shedding during incidents — which is why traffic engineering is a computing problem as much as a civil one.

The historical detail is the best fact attached to this: the world's first traffic light was installed outside a parliament building in 1868, was gas-powered, and exploded within a month. Infrastructure firsts are frequently disasters, and the technology did not return for half a century after that one.