Here’s an insane looking chart from streets.mn alumnus David Levinson’s Transportationist blog, showing a “Typical Signal Schedule and Traffic Flow Diagram, North-South across Market Street, San Francisco (1929) [with a] “green wave” set to 10.5 MPH.”
Peep your eyes on this!
The complex trade-offs that go into something as seemingly simple as how to program a stop light show you how, when it comes to street design, you can’t take anything for granted.
In his post, a primer on traffic signal timing, Levinson explains a bit about what’s going on here. Here’s the main trend that began in the 1920s:
Since the early twentieth century dawn of what Peter Norton calls ‘Motordom’ in his book ‘Fighting Traffic‘, street space has steadily been regulated and enclosed, limiting the rights and privileges of pedestrians while promoting those of drivers as a class, in the name of safety and efficiency. But we should ask safety and efficiency for whom? Prior to traffic signals, pedestrians could and did cross the street whenever and wherever they wanted, before the term ‘jaywalking’ was invented and street crossing was regulated. The introduction of signals prioritised the movement of motor vehicles at the expense of pedestrians, whose effective walking speed through the city necessarily slowed. The consequences of making it easier to drive and harder to walk on people’s choice of mode is pretty straight-forward, and consistent with the rise of the automobile in the 20th century.
The rest of Levinson’s post goes dives into the weeds, detailing stuff like coordination and actuation. He ends with a list of recommendations for pedestrian advocates:
- Pedestrians, like vehicles, should be counted automatically at controlled intersections.
- Pedestrian time must be considered (and prioritised) in the traffic signal timing algorithms so that their weight is equal to or higher than the weight of a passenger car.
- Pedestrians should get the maximum feasible amount of green time on a phase, rather than the minimum, so that pedestrians arriving on the phase have a chance to take advantage of it, and slower moving pedestrians are not intimidated by cars.
- Pedestrians should get a ‘leading interval’ so they can step into the street on a ‘walk’ signal before cars start to move on a green light, increasing their visibility to drivers.
- Pedestrian phases should be automatic, even if no actuator is pushed. Instead, the actuator should make the pedestrian phase come sooner.
- Many more intersections should have an all-pedestrian phase (what is referred to as a ‘Barnes Dance’) in addition to existing phases so pedestrians can make diagonal intersection crossings without having to wait twice.
This is great stuff. For example, Saint Paul has been quietly rolling out LPIs at key intersections in town over the last two years. Next time you’re watching for a WALK signal, I hope you remember this chart!
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