Walkable Streets II: The Documenting

This time last week, I was considering common issues associated with walkable streets and mentioned that 35-40kph (25mph) moves the most traffic. I didn’t even think about it as I wrote it. As something long-embedded in my brain, I just said it. Matter-of-factly.

Readers took me to task, wanting to know the source.

I had to dig back into my urban design thesis to find what actually shows a deduction derived from multiple sources, keeping in mind that predicting and studying human behaviour is a social science. The exact reference for 27mph has not turned up yet, but here is the rationale and some supporting material that gets you in the neighborhood.

1. The first source is a graph that was widely published in the book “Traffic Engineering” for Arterial Average Capacity that plots Average Travel Speed against Total Traffic (Vehicles Per Hour Per Lane VPHPL). The findings show that, at 40mph, you get about 500VPHPL, at 30mph around 820VPHPL, and at 10mph 1050VPHPL. The observable reason for this is that as cars speed up, they spread out. This data, as presented, suggests a free-flow condition.


2. A second graph in this Transport Policy post creates a great rationale for 20mph speed limits being the future of urban transport. The graph, Figure 4, also plots predicted traffic flow against speed. What’s interesting here is that after 20mph, there is really very little difference in additional capacity per lane, levelling out at 50mph. 25mph fits in nicely here as well.


3. Once this post went live, I received still another graph — this one from the Transportation Research Board’sHighway Capacity Manual” via Community Builders San Bernardino — demonstrating similar phenomena and settling in on a similar range of speed.

Highway Capacity

4. “The Boulevard Book” by Jacobs et al. is must read… a MUST OWN. One of their key precedent studies is a street called “The Esplanade” in Chico, California. This is the major route leading from the city’s Downtown, a multiway boulevard built in the 1920s. Not only does it move lots of traffic elegantly and efficiently at 25mph, it has desirable real estate — commercial, single family homes, multi-family homes, and schools — directly facing it. Now, when you go down a mile from the downtown, the street turns into an “arterial,” presumably carrying the same traffic at 40 mph. The first thing you do is sit at a light with a bunch of other drivers amidst a horrible, nightmarish suburban shopping center scene. A more expensive street than its older brother (the Esplanade), it behaves so badly that it diminishes property values and human happiness at every intersection. Now, I don’t need a book to tell me this, as I’ve have worked there on a number of occasions and can speak from experience.

The Esplanade, Chico, California.

The Esplanade, Chico, California.

5. Intersections: So, we have determined that lower speeds can support comparable, if not more, vehicles per lane. Any traffic engineer will tell you that actual capacity is really constrained at intersections. So here is the last piece of data. You can only fully realize the advantages of lower speeds in free-flow conditions. This only happens in two ways: 1. On high speed thoroughfares, which are not conducive to a walkable street environment; and 2. Where you can time your lights. This is where 40mph more than fails… in that after your 40mph free flow, the light turns red and you sit through a light cycle. Once it turns green, the stacked cars slowly move forward, spreading out one by one to reach 40mph+ just in time to hit the next light. In urban situations, with lots of intersections and on major thoroughfares, and at 25mph, you can effectively time the lights for a periodic free-flow condition for most hours of the day. This is how the Esplanade does it, and actually so do many downtowns throughout North America. And during peak hours, yes, they get congested. Just like arterials.

But at least the pedestrians, bikes, and transit can still do their thing. That alone tips the scales.

Geoff Dyer

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  1. 3. Roundabout, Blackson twist or other unsignalised, narrowed, direction-change-forcing intersection (see Poynton video). Rather than risking the start-stop of signals, just get rid of them, and design intersections as places where traffic slows. Intersections are the hub of urbanism: unimpeded free-flow might even be as bad as idling bunches. Instead, make it clear that the pedestrian is king at intersections, cyclists princes and drivers invited at their whim. If no-one’s around, the driver can slow and carry on; if people are crossing, she can also slow, farther back, and carry on behind them.

  2. As an engineer, I love the graphs and numbers even if I’m not a “traffic” engineer. As a frequent biker and walker, I am more concerned with the observed results. To put it differently, if it doesn’t work it really doesn’t matter how you reached your technical conclusion. While I would love to see many roads reduced to a posted speed limit of around 25 m.p.h., this is probably not going to happen as most of them where I walk and live were recently increased to 45 m.p.h, arguably to increase the speed before Michigan’s Complete Streets legislation took full effect. Many of these are two lane roads (or stroads for those that follow Chuck Marohn) in residential areas consisting exclusively of homes, schools and churches, but they do act as recognized connectors between other areas. They are generally controlled and maintained by the County Road Commissson and/or the Michigan DOT. There are sidewalks or a convention we have called “safety paths” sometimes better known as shared use paths, but they are seldom used. So my question to the more learned people that probably read these posts is why 45+ m.p.h.? As noted in this article, it does little to move more cars, Lights and intersecting traffic conditions further reduce the actual average speed and traffic count, it certainly diminishes perceived comfort and safety for pedestrians but I highly doubt this is a goal especially in a school zone, at least I hope it isn’t. Just curious if there has been any other study that shows people in cars want to drive over 45 m.p.h. regardless of the location or conditions as that does seem be the the new normal where I am. For what it’s worth, I can often get from my house to our small downtown faster by bicycle as the cars often back up at the traffic lights. I can often do better or equal than a car by walking during high traffic times. This is without any bike lane or special provisions and 4 foot wide sidewalks in reasonably good conditon. It seems to me that this is a problem for both the motorist and pedestrian.

  3. I’ve seen all these graphs for most of my career as well, and the issue is the assumption that cars space themselves farther apart at higher speeds. Freeway throughputs of 22-2400 vphpl have been measured reliably in general purpose lanes, and throughputs of 1800-1900 vphpl basically operate at free-flow conditions. This is because vehicle spacing has dramatically decreased over the decades since the original analysis was done and driver behavior on a freeway resembles that of a race-car driver rather than a crash avoidant driver–Rather than trying to avoid a crash by being able to come to a full stop before reaching an incident, drivers avoid crashes by looking for and reserving alternative lane options. It simply isn’t true that a local street can compare to the throughput of a freeway.

    Having said that, I don’t want anyone walking on a freeway. Knowing that when you throw signals, intersections and pedestrians in the mix, the maximum throughput is generally achieved at a 25-35 mph speed does help argue that we all don’t need to move that fast but we all want to move that fast, particularly when uncongested conditions prevail for 90% of the day. There is a place for high speed facilities. There is a place for congested, speed-regulated facilities. There are also places for what my Landscape Architect friends called “tubes”–the non-place that provides vehicular or bicycle connections between “places.” Knowing that from a pedestrian standpoint, an area larger than a few square miles is outside the reasonable walking range of a person, the question becomes what do we do with all that other area? I’m no fan of Stroads, but their existence in a land of property rights preeminence is no surprise.

    • The problem with Stroads is when any old business has access rights to the main drag — at which point you no longer have simplified freeway-style movements, but instead you get many stop lights and conflict points.

  4. Hi, I live in Saronno (Lombardy, Italy), a little town in the middle of Milan, Varese and Como (distant 25-35 Kilometers from them). It is the 36th if we classify all the 8’000 italian municipalities by population density (population/km^2).. 3’650 habitants every square kilometer (total 40’000/12km^2) . The town has two highway exits and two train stations. For those motivations has an high level of traffic congestion and the level of CO2 and pullution is over the legal limits for months during the year. Two years ago the Mayor introduce a 30Km/h limit in the entire city without differences between areas, hours, citizens.. that limit has existed for about 6 months but then was substantialy eliminated. Why? It was really ridicolous when there was no traffic to go at that speed and when there was traffic the speed was already slow, slower than 30km/h. The penalties were seen impopular and many ilarious songs and posters about the policy. No effects on air pollution because the major problem were home’s emissions and traffic conjestion.. I have read UK reports saying that this policy was used for people security, to reduce crashes, not to reduce smog because emissions are lower at 80km/h. However thi story reveal that people are able to do collectively something really great and strange, even if impopular, if imposed. This is not to say to impose a 30km/h speed limit but to say that something more serious and useful, even if difficult to implement, can be made.

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