Green wave

A green wave is a run of traffic lights along a street timed so that anyone travelling at a chosen speed meets one green after another without stopping. Each signal turns green a few seconds later than the last, matching the time it takes to reach it — the classic trick of signal coordination, in use since the 1920s.

Cities use it to keep a busy corridor flowing and cut the fuel, brake wear and frustration of constant stop-start. Copenhagen made the idea famous for bikes rather than cars: its Grøn Bølge tunes signals on key cycle routes to a steady 20 km/h, and a city evaluation found average cyclist speed rose from about 15 to 20 km/h with red-light stops falling from as many as six to zero or one. Frankfurt takes a lighter touch: its trafficpilot app reads live data from 441 junctions and simply tells cyclists and drivers what speed to hold to catch the wave.

The honest limit is that a classic green wave is tuned to one speed and usually one direction; set the offsets for the morning commute and the evening flow, or a slower cyclist, falls out of step. When traffic volumes shift, fixed timings cannot respond — the gap that adaptive traffic signals try to close by retiming themselves from live sensor data, as Los Angeles does across its ATSAC network of some 4,850 signals, though at far higher cost than painting a wave onto one street.