Chart of the Day: 1929 Signal Schedule and Traffic Flow Diagram

Here’s an insane looking chart from 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!

9 thoughts on “Chart of the Day: 1929 Signal Schedule and Traffic Flow Diagram

  1. Eric W.

    Is there any legitimate reason why an intersection would not automatically give pedestrians a “walk” signal? A lot of people don’t realize that you actually have to press the button or you may never get a chance to cross.

    1. Monte Castleman

      Because it can have pretty devastating impact to vehicle operations, and there’s no need for it if there’s no pedestrians there. This is especially true in a typical outer suburban intersection, where there might be one pedestrian every hour. If there’s just one car or a couple of cars you could have a 7-10 second green time for the side street but a pedestrian phase for a nonexistant pedestrian would require traffic on the main street be stopped for close to a full minute.

      1. Eric W.

        Sure, but I’m talking about a simple 2-phase signal in the city. If there is a regular green light (no turn arrows), why shouldn’t pedestrians moving in the same direction as the green-lighted traffic automatically get a walk signal? The only impact to vehicle traffic would be having to yield to any pedestrians present before turning.

        1. Monte Castleman

          If that’s your parameter, then there’s less to no reason why not. The newer signals on Penn Ave south the north-south the minimum green time is long enough and the crossing distance short enough that the north south pedestrian phase isn’t longer than the minimum vehicle phase, so they have the walk lights automatically go on (the push-button is for ADA requirements or if a pedestrian arrives without a car waiting to trigger it). The shorter east-west cycle you still have to push the button because even though Penn Ave isn’t very wide, the minimum green time is extremely short.

      2. Adam MillerAdam Miller

        If there’s no pedestrian there, and no one to yield to, what’s the impact on vehicle operations? Longer green phase than is neede for cars?

        1. Monte Castleman

          Yes, that’s correct. 10 seconds is normally used as the minimum green time for the side street in our area no matter how wide it is, but ped clearance time (the ped change interval plus the buffer interval) varies greatly as it’s based on a pedestrian walking at 3.5 seconds from curb to curb starting at the end of the walk interval, minimum 7 seconds.

          To use a real example, a few years ago I analyzed two intersections, Penn / 56th and American Blvd and Home Depot. Putting ped recall across Penn adds 6 seconds to the 10 second green, putting it across American adds 20 seconds. In both cases there’s no effect to putting it across the side streets. Minneapolis has the walk signal automatically light every cycle but Bloomington does not.

          1. Russell Booth

            Minneapolis may have automatic Walk signals with every green light in some locations like downtown but there is much variation across the city. Some intersections (crossing Hiawatha Ave) require pushing the button to get the Walk signal.

            I expected recently redesigned Minnehaha Ave to have improved pedestrian signal controls but was very surprised my first time crossing it. I watched the Walk signal to cross 40th Street count down the seconds to zero for crossing the side street, then change back to Walk.

            But that was not the exact sequence of events.

            Expecting the green on Minnehaha to change to red after the countdown, I started crossing Minnehaha and was surprised and alarmed that an oncoming car was not slowing.

            I had stopped monitoring the side street pedestrian signal before I started crossing Minnehaha, expecting the countdown to zero would mean the red/green cycle would change and that I would have a green. But alas, no motor vehicle was travelling in my same direction.

            It was within the crosswalk on Minnehaha that I saw that the green light for Minnehaha had not changed after the pedestrian countdown (for crossing 40th which again showed Walk). It was then I realized I had stepped into a traffic-engineered pedestrian trap.

            A rush of adrenaline followed and, fortunately, the lone car driver slowed realizing that I was crossing against a red light – that I had had no good reason to expect.

            Now that I know the pedestrian signal there is engineered to be a deceptive and potentially lethal trap, I ignore the traffic controls and cross when there is a break in traffic (if there is no motor vehicle that has requested crossing Minnehaha electromagnetically by its mere presence).

            I could – and legally should – ‘beg’ to exercise my right to cross Minnehaha while non-motorized, but I am usually driving my dog-powered urban mushing rig when I cross. I take up 17 linear feet from my dog’s nose to the back of my rear tire. I always stop so that I will not be blocking pedestrians who may be using the crossing sidewalk, so my dog is not yet on that sidewalk and I am many feet back from it. The button is located on the far side of the sidewalk from me.

            Begging to cross Minnehaha for me would mean setting down my dog scooter, walking about 25 feet to get to the button, then walking 25 feet back to my rig and picking up my rig again. Kind of an ordeal.

            Minnehaha pedestrian signals were more predictable before the redesign. There are other traffic signal timings elsewhere in Minneapolis that also do a useless and deceptive countdown for pedestrians and then change back to Walk – along East Lake Strret, for example. Beware pedestrians! It’s a trap.

            It was very surprising to me to see that modern traffic engineers would create pedestrian death traps like this. What is the purpose of the pedestrian countdown if the light will not change afterward?

            Luckily I have seen no comments ever about scoff-law urban mushers, but I also ride a bike.

            1. Monte Castleman

              Traffic signals, even with video detectors or inductive loop sensors, should be able to pick up a dog sled (assuming it has metal runners), so if they’re not doing so it should be reported so the sensors can be adjusted.

              The signals are not “pedestrian deathtraps”. It’s not a hard concept that you enter the street you want to cross only when your walk light turns white or the overhead vehicular light turns green, and don’t enter them if the lights are orange or red. You’re not supposed to be looking at anyone else’s walk or vehicular signal. Engineers would hide them from cross traffic if they could to prevent the kind of situation you described, but there’s no way of doing that which doesn’t have other drawbacks.

              The phasing you describe is standard practice for actuated signals where there’s an automatic walk light. One the a certain amount of time has passed, the signal will stay green for the main street if there’s no car or pedestrian waiting to cross on the side street. The idea is priority goes to the side street at this point, but if no one needs it you might as well keep the main street green. If the walk light also stayed constantly illuminated, if a vehicle or pedestrian arrived at the side street they’d have to wait for the full countdown for to end before crossing.

              Having the main street countdown cycle is a compromise between just keeping it on don’t walk once the minimum green time is done, or keeping it on walk and delaying the side street more. If there’s no cars or pedestrians waiting to cross the main street when the count reaches zero, resetting it allows any pedestrians arriving late another chance to cross the side street without waiting a full cycle.

  2. David MarkleDavid Markle

    At many–not all–intersections, the walk light goes on when the corresponding main traffic line goes green.

    At some intersections, the pedestrian walk light time is too short to accommodate some partially disabled pedestrians.

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