Iterating to Safety (Excerpt from Confessions of a Recovering Engineer)

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In the work that Urban3 has done modeling the financial productivity of different places, there is one correlation that stands out above all others. Wherever development patterns are most productive, wherever the highest value per acre is measured, those are the places where people will be found outside of a motor vehicle. Where humans are found in their habitat, those are the places building the greatest amount of community wealth. The more people that are consistently found, the more productive a place is likely to be. To borrow a phrase from ecology: People are the indicator species of success.

Since wealth is created by having humans naturally in their habitat, a prerequisite for building wealth is that a street be safe. The stroad-to-street conversion approach from Chapter Two is a good place to start: (1) slow traffic, (2) prioritize people over throughput, (3) build a productive place, and (4) embrace complexity. These are not steps to be done in series but four things to work on simultaneously.

As an example, many people—including myself, members of the Springfield city council, and the city’s engineering staff—have observed that people routinely struggle to cross State Street. Traffic moves too fast, and the gaps that people must cross while exposed to traffic are too great. The distance to signalized intersections is too great, and the time spent waiting and crossing is a higher threshold than people have shown they are willing to spend. This has been observed and well documented.

This is where we pull out the book Tactical Urbanism: Short-term Action for Long-term Change (Island Press, 2015) by Mike Lydon and Anthony Garcia and start trying things. On my own without collaboration and input from a Street Design Team, I am not absolutely positive where I would start, but my experience suggests that I would begin with paint and traffic cones at least 500 feet on each side of the entrance to the Springfield Central Library. In those places, I would start temporarily narrowing the lanes, changing the geometry of the street to figure out the best way to communicate to drivers that more caution, and slower speeds, are warranted.

In the last chapter, I explained the 85th percentile speed and how it relates to the comfort or tension people experience when driving. I wrote the following:

The only way to have meaningful reductions in speed, and a significant impact on the number of crashes, is through “geometric change.” The design of the street needs to prompt people to drive slower.

For streets, where we need complexity in order to build a productive place, traffic needs to flow at a neighborhood speed (15 mph or less is optimum) to make human habitat that is safe and productive. To achieve this on a street, the street design needs to shift drivers from the passive awareness of System 1 to the mental state of heightened engagement found in System 2.

Where traffic engineers have done a brilliant job recognizing the limits of human cognition and adjusting speed limits on roads to reflect the 85th percentile speed, they now need to apply the same insights to streets. The only difference is that, instead of using the 85th percentile speed to adjust the speed limit, on streets we need to use the measured speed to adjust the design.

Where drivers are traveling too fast, the geometry of the street needs to change, and keep changing, until a safe 85th percentile speed is achieved.

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