When engineers are designing streets, they use a concept called design speed to choose appropriate measures and elements for that expected speed of travel. A closely related concept is the “target speed”, which is the speed they hope that people will travel on that street. Design and target speeds for urban streets can be much higher than posted speed limits, creating unsafe transportation conditions throughout cities.
Various factors contribute to excessive design speeds, including width and number of lanes, the presence of pedestrians and bicyclists, street trees with minimal setbacks, and so on.
On December 15, 2016, Austin City Council adopted a comprehensive safe design speed policy that included instructions to staff to “incorporate target design speeds into plans & manuals” and “systematically evaluate arterial speed limits citywide for appropriateness.” The resolution noted that this was a logical step in the City’s commitment to Vision Zero.
The Austin Transportation Department has made tremendous progress on this effort, including a proposed $600,000 for the 2018 bond package dedicated to retrofitting streets to safe design speed. TXDOT and other cities across the state are following Austin’s lead on this issue, as national engineering leaders call for a radical shift in thinking about the design of streets and speed.
The corridors project is perhaps the single biggest opportunity currently funded to deploy safe, multi-modal streets with modern design speeds in the region. The NACTO standards for safe design speed for urban arterials – 35mph or less, would not slow down the current experience of these corridors.
Accepting this responsibility, the City of Austin has made the commitment to take design speed seriously, claiming to “fully intend to comply with NACTO standards as we design and construct the Corridor Construction Program projects,” per this April 2018 memo from Assistant City Manager Robert Goode.
This is a tremendous and laudable advance in urban planning and should be commended. It should also be the standard for all urban roadway design across Texas.
Design Speed in Corridors Construction Program
Higher design speeds and other misguided elements of transportation planning increase the volatility of systemwide speeds. Higher design speeds can contribute to increased crashes, blocking roadways, and reducing systemwide speed as a result of sudden congestion.
Using Google Maps and their travel data, we estimated the amount of time it would take to travel each of the corridors from end to end at 4pm on a weekday, and translated that into an overall travel speed.
Today, the average speed of travel on these corridors is between 13 and 31 mph, and it is not possible to travel faster than 35 mph on any of these corridors during rush hour. Austin’s new design speed standards could theoretically (and perhaps counterintuitively) increase rush hour travel times.
The stated plan for corridors funding is to focus on improving intersections, which will increase safety and throughput, with the safety improvement further decreasing delay caused by crashes. Using safe design speeds throughout the corridors will contribute to easing traffic flow by reducing crashes.
Imagine traveling in a car on a safe, urban street filled with life and people at a comfortable, dependable 25 mph rate, and not sitting at traffic lights all the time. Beyond the benefits to people in cars, safe design speed is the determining factor in our ability to build a healthy urban environment where people have the freedom of walking, biking, traveling by wheelchair, and using transit.
It is quite likely that a realistic travel demand model – that doesn’t assume induced sprawl like our current one – could show that a comprehensive safe design speed approach will actually lead to a faster – yet safer – car trip along these corridors in ten years.
Every single one of these corridors is also expected to add population, jobs, stores, schools, and other elements of a complete community. Using safe design speeds below the minimally acceptable 35 mph NACTO standard is a crucial element of optimizing this investment of half a billion dollars.
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