A green wave moves out, Manhattan, 1986.
I am writing this post in response to comments by Mighk Wilson and Khalil Spencer on another post on this blog. They discussed the difficulty of cycling in a city with synchronized traffic signals (a “green wave”) set to a higher speed than cyclists can manage, and the potential of a slower green wave to make a street more attractive for cycling. I’d like to take a more general look at the green wave and how it affects traffic.
My understanding of the green wave is based mostly on experience. (And so, anyone who can provide more details based on theory, or can correct me, please do…)
In my high-school years, I lived and learned to drive in Baltimore, Maryland, USA, one of the first cities to implement traffic signal synchronization. I have also lived, driven and cycled in Manhattan, where most traffic lights are timed to create green waves.
A green wave can only work under a limited set of conditions. If these do not apply, then despite best efforts to time traffic lights for the smoothest possible traffic flow, a signal sequence can still appear random. Drivers have no clear strategy for avoiding red lights beyond speeding up when the next light is still green. On the other hand, when a green wave is working smoothly, drivers may feel as if a green-wave Tinkerbelle is darting along overhead and pointing her magic wand at every traffic light to turn it green.
Traffic engineers use clever math so a green wave, surprisingly, can be applied to streets heading in more than one direction the same time — though it words better if they are one-way. Heading north on Charles Street from church in downtown Baltimore, my family would get green lights for block after block, except at the few two-way streets, where all bets were off. Then as we headed into the more random street pattern at the north end of the city, we just had to take each traffic light as it was. On the other hand, traffic lights were less frequent in this less densely built-up area.
My experience was similar in Manhattan. The green wave worked smoothly on one-way streets and avenues, but when crossing two-way ones, and when driving on them, it didn’t. This obvious difference gives drivers a strong incentive to use one-way streets and avenues for through travel, where possible. Advocates of the sort who would view streets as a neighborhood resource often protest conversions of two-way streets to one-way, see for example this call to action. Traffic engineers who are concerned with the effect on congestion and crash rates have the opposite opinion — see, for example, this presentation. (I expect that the choice is not quite so stark as these two examples make it — as usual, such decisions must be made on a case-by-case basis.)
A green wave works smoothly only when there are no stop signs on a green wave street, though stop signs can be used on cross streets. Double-parked vehicles, vehicles that have entered the street and are waiting for a light to change, vehicles — including bicycles — that can’t keep up with the pace set by the signals — anything that slows traffic down or reduces the number of lanes available increases the likelihood of not keeping up with the pase set by the lights.
A green wave often encourages travel faster than the pace set by the signals. That’s because there is an advantage in racing to the front of a platoon — where each signal has just changed to green — when preparing a turn — then after turning, racing to the end of the block so as to catch the end of the green there. A driver may speed through another few blocks to get to the front of the platoon before turning again. The advantage of this tactic is quickly obvious: After turning the corner at the head of a green wave onto another green wave street, a driver will be facing a signal at the next intersection which is about to turn yellow, then red.
The typical 30-mile per hour speed limit in grid cities like Manhattan often leads to motorists’ speeds considerably in excess of that limit, and to more unpleasant conditions for bicyclists.
On the other hand, synchronizing signals to a speed more comfortable for bicycling will discourage use of a street for through motor-vehicle travel, making it more attractive for bicycling. I have ridden on a street in Saint Petersburg, Florida, with the signals synchronized to 15 miles per hour, and it achieved that goal quite well. It would have worked better if it had been one-way — it ran up a moderate slope from the waterfront, and for most bicyclists, 15 miles per hour was hard to maintain. Downhill, on the other hand, the speed setting could have been 20 miles per hour without creating difficulties for bicyclists.
Bear in mind, though, that comfortable level-ground travel speeds for bicyclists cover a 3 to 1 range , from about 25 miles per hour down to 8 miles per hour — not nearly as uniform as for motorists, even considering the issues with motorists’ speed already mentioned. A predictable increase in the volume of electrically-assisted bicycles and motor scooters will complicate the issue of speed setting even further. The advantage of a slow green wave, given these issues, is not so much to allow bicyclists to travel farther before facing a red light as to discourage use of the street for through travel by motorists.