Green Wave

green wave by Jay Harel

A green wave is an intentionally induced phenomenon in which a series of traffic lights (usually three or more) are coordinated to allow continuous traffic flow over several intersections in one main direction. Any vehicle travelling along with the green wave (at an approximate speed decided upon by the traffic engineers) will see a progressive cascade of green lights, and not have to stop at intersections. This allows higher traffic loads, and reduces noise and energy use (because less acceleration and braking is needed).

In practical use, only a group of cars (known as a ‘platoon,’ the size of which is defined by the signal times) can use the green wave before the time band is interrupted to give way to other traffic flows. The coordination of the signals is sometimes done dynamically, according to sensor data of currently existing traffic flows – otherwise it is done statically, by the use of timers.

Under certain circumstances, green waves can be interwoven with each other, but this increases their complexity and reduces usability, so only the roads and directions with the heaviest loads get this preferential treatment. A green wave has a disadvantage; slow drivers may reach a red signal with a queue of traffic built up behind them, thus ending a wave. In general, stopping and then starting at a red light will require more time to reach the speed of the wave coming from behind when the traffic light turns to green.

Green waves are most effective with one-way traffic. A green wave in both directions may be possible with different speed recommendations for each direction, otherwise traffic coming from one direction may reach the traffic light faster than from the other direction if the distance from the previous traffic light is not mathematically a multiple of the opposite direction. Green waves are sometimes used to facilitate bicycle traffic.

Copenhagen, Amsterdam, San Francisco, and other cities, may synchronize traffic signals to provide a green light for a flow of cyclists. In Copenhagen, a green wave on the arterial street Nørrebrogade facilitates 30,000 cyclists to maintain a 12 mph (19.3 km/h) speed for 2.5 kilometers. In Amsterdam, cyclists riding at a speed of 15 to 18 km/h will be able to travel without being stopped by a red signal. Tests show that public transport can benefit as well and cars may travel slightly slower. Frederiksberg, a part of Copenhagen, has implemented a green wave for emergency vehicles to improve the public services. In the UK, it was revealed that the Department for Transport had previously discouraged green waves as they reduced fuel usage, and thus less revenue was raised from fuel taxes.

One Comment to “Green Wave”

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