The safe system approach to speed limits differs from the traditional view that drivers choose reasonable and safe speeds. In the safe system approach, speed limits are set according to the likely crash types, the resulting impact forces, and the human body’s ability to withstand these forces (Forbes and others 2012). It allows for human errors (that is, accepting humans will make mistakes) and acknowledges that humans are physically vulnerable (that is, physical tolerance to impact is limited). Therefore, in this approach, speed limits are set to minimize death and serious injury as a consequence of a crash (Jurewicz and others 2014). This approach is far more commonly applied outside of the United States, such as in Sweden (where it is called Vision Zero), the Netherlands (where it is called Sustainable Safety), and several jurisdictions in Australia (OECD 2008). However, it is now gaining acceptance in the United States, particularly in Vision Zero cities and municipalities.
By Eric Sundquist, SSTI, January 27th, 2020 NACTO, safety, speed, Vision Zero Setting speed limits based on safety, not driver behavior
The 85th percentile rule in speed limit setting—an arbitrary but longstanding convention—has begun to weaken in recent years, with new guidance now allowing for lower speeds. FHWA’s USLIMITS2, for example, allows for speeds down to the 50th percentile in certain cases.
Now there’s a growing push to take observed vehicle speed out of the speed limit equation entirely. Such is the approach that NACTO will recommend in a forthcoming practitioners’ guide. NACTO’s Jenny O’Connell previewed the guide at this month’s TRB annual meeting.
Rather than relying on the 15th-most dangerous driver (or even the 50th) out of 100 to set speeds, NACTO’s approach determines the safe speed by a street’s context across two dimensions:
- Activity, which is based on factors such as the neighborhood, types and numbers of land uses, and curbside demand.
- Conflict density, which is based on factors such as modal mixing and crossing demand.
Figure 1. NACTO will recommend a safe-speed approach to limit setting.
The desire for such a new paradigm in speed-limit setting has been growing in recent years, partly driven by the spike in fatal crashes involving bicyclists and pedestrians. Reliance on driver behavior to gauge reasonable speeds has meant that when more drivers speed, speed limits must be raised, even in the face of concern for safety. Such an outcome in Los Angeles has helped prompt the state legislature to form a Zero Fatalities Task Force, which is due to release recommendations on speed limits soon.
The NACTO recommendation has its roots in a 2017 NTSB report, which highlighted many problems with the driver-behavior approach and recommended that a “safe system” approach, like the one NACTO will propose:
The safe system approach to speed limits differs from the traditional view that drivers choose reasonable and safe speeds. In the safe system approach, speed limits are set according to the likely crash types, the resulting impact forces, and the human body’s ability to withstand these forces (Forbes and others 2012). It allows for human errors (that is, accepting humans will make mistakes) and acknowledges that humans are physically vulnerable (that is, physical tolerance to impact is limited). Therefore, in this approach, speed limits are set to minimize death and serious injury as a consequence of a crash (Jurewicz and others 2014). This approach is far more commonly applied outside of the United States, such as in Sweden (where it is called Vision Zero), the Netherlands (where it is called Sustainable Safety), and several jurisdictions in Australia (OECD 2008). However, it is now gaining acceptance in the United States, particularly in Vision Zero cities and municipalities.
NTSB addressed federal regulation in the area, recommending that “FHWA [should] revise Section 2B.13 of the MUTCD to, at a minimum, incorporate the safe system approach for urban roads to strengthen protection for vulnerable road users.” NACTO’s guidance promises to help practitioners operationalize this approach.
Eric Sundquist is Director of SSTI.
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Survey: Mayors see the problem but shy away from the solutions
Posted on February 10th, 2020 in NewsTags: cities, pedestrians, policy, safety
By Eric Sundquist
U.S. mayors recognize safety and environmental issues resulting from automobile traffic, according to a new survey from Boston University. But they are leery about implementing commonly accepted remedies like lower speeds, more enforcement, reduced parking or separated bike lanes.
“It’s a positive that mayors recognize that cities have too many cars and are too reliant on cars, and there’s promising results, particularly in regards to bike lanes; mayors were willing to give up some parking spots and driving lanes to provide more bicycle safety,” Katherine Levine Einstein, a Boston University political science professor, told Fast Company magazine. “But I was really struck across the whole series of questions about largely their unwillingness to support or implement things that we in transportation planning know are evidence-based ways to make roads safer for vulnerable road users.”
On safety, the survey found:
Nearly half of mayors believe travel for cyclists in their cities is unsafe and nearly 40 percent are concerned about pedestrians’ safety. In contrast, fewer than 10 percent believe the city is unsafe for drivers or mass transit riders. Two-thirds of mayors have implemented bicycle lanes to improve cyclist safety, while pedestrian upgrades include a more variable array of changes from improved sidewalks to traffic signalization. Mayors may not yet realize the extent to which vehicular speeds are a key lever to promote safety; 77 percent of mayors believe speed limits in their community are generally set at the right level, 56 percent believe enforcement is adequate, and 52 percent reject the idea of stronger moving traffic violations.
And on auto-centric policy, the survey found:
A significant majority of mayors (76 percent) report that their cities are too oriented towards cars and 66 percent believe vehicles are the largest source of greenhouse gas emissions in their communities. Seventy-one percent of mayors believe cities should make their roads more accessible to bicycles, even if it means sacrificing driving lanes or parking. However, mayors in the aggregate, do not perceive parking as oversupplied. Sixty percent say that their cities feature the right level of street parking; only nine percent believe that there is too much street parking. A far larger share — 27 percent — worry that there is too little parking in their cities. Additionally, half of mayors interviewed believe their parking minimums for new developments are set at the right level, while 30 percent perceive them as too high.
The survey of 119 mayors of cities with at least 75,000 residents was conducted in 2019 by Boston University’s Initiative on Cities.
Eric Sundquist is Director of SSTI
