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Slow Roads Save Lives – Reframing Road Design

Slow Roads Save Lives. It’s simple physics. But to fully realize the lifesaving power of slower roads, it will take more than doing the math. We must reframe how speed limits are set and how we design roads to achieve slower speeds. 

Speed limits are the most common way that we regulate individual speed choices in order to control the significant risks drivers can impose on others. The Slow Roads Saves Lives campaign sees speed limits as a tool for controlling risk based upon known injury severity rates at different speeds. 

Join the League on November 7, 2023 at 3 pm ET as the “Slow Roads Save Lives” campaign features two experts on injury risk severity and its relationship to speed:

  • David Harkey, President · Insurance Institute for Highway Safety
    David has worked on expert systems for speed limit setting and whose organization has led on research showing the safety impact of slower speed limits.
  • Brian Tefft, Principal Researcher · AAA Foundation for Traffic Safety
    Brian published the influential 2011 study “Impact Speed and a Pedestrian’s Risk of Severe Injury or Death” and continues to publish research on speed limits.

Register for the webinar here »

Reframing Road Design

This blog is intended to explain why we need to improve how we set speed limits as part of moving bicycle and pedestrian safety from the margins to mainstream in road design.

How did the first speed limits come about in the United States?

The League of American Bicyclists was one of the first organizations to campaign for paved roads. At the time — the 1880s — it was so that bicyclists could go faster more safely. The Good Roads campaign delivered 100,000 signatures to Congress in 1889 and called for federal support for roadbuilding, before the widespread adoption of the motor vehicle. As that changed and motor vehicles became more common on the roads, traffic deaths became an increasingly common tragedy due to the size and speed of those vehicles. Speed limits began to emerge to combat the threat to public health caused by motor vehicles.

Speed limit setting policy has changed over time, but the basic ideas of the traditional car-centric approach to speed limit setting were established by the 1960s. Between 1926 and 1956, the Uniform Vehicle Code (UVC) underwent several revisions to create and consolidate various districts for speed limit setting, with slower speed limits recommended for school zones and residential areas only briefly. By 1956, the UVC recommended only two speed limits: 30 mph for urban districts and 55 mph for all other roads. That recommendation would continue unchanged until 2002 when the organization responsible for the UVC ceased to exist.

How do public agencies decide what speed limit to set?

The defining feature of traditional speed limit setting is a car-centric awareness of how the driver feels when driving on a road. Attitudes and research defined a framework where drivers, rather than engineers, elected officials, traffic safety or public health agencies, or policy professionals, were empowered to decide the safe and reasonable speed for a given road. This continues to be seen today in official policies, comments, and attitudes where public agencies say things like “we believe that the 85th-percentile speed… represents the speed of the prudent driver.” The 85th percentile speed represents the speed at which 85 percent of drivers choose to drive that speed or slower and 15 percent of drivers choose to drive that speed or higher. In the words of an old brochure from the Institute of Transportation Engineers, “The 85th percentile speed is how drivers ‘vote with their feet.’” 

According to a 2019 survey of traffic professionals by AAA, “98% of survey respondents consider the 85th percentile operating speed when raising or lowering posted speed limits.” The 85th-percentile speed limit setting method has been difficult to displace from its role because it makes a range of claims about its results.

  • The 85th-percentile method makes a claim about safety.
    • The “Solomon curve” published in 1964 is a graph based on studies of rural roads that identified lower crash rates at the 85th percentile of driver speeds. The 85th percentile speed is said to promote safety because it minimizes dangerous speed differentials between vehicles. Setting speed limits lower than the 85th percentile speed is claimed to reduce safety by increasing speed differentials between law abiding drivers and “prudent” drivers who speed.
  • The 85th-percentile method makes a claim about legitimacy.
    • The 85th percentile speed is used to provide a means for law enforcement to differentiate “bad” drivers who are speeding from “good” drivers who are simply behaving in the way the road makes them feel is appropriate. Setting speed limits lower than the 85th percentile speed is claimed to antagonize drivers against enforcement efforts and “create a poor community image.”
  • The 85th-percentile method makes a claim about objectivity.
    • The 85th percentile is a quantitative and observable metric that can be measured in a site visit, providing traffic engineers a consistent metric to reference when setting speed limits.

If the 85th-percentile method is so prevalent, how effective is it?

According to that same AAA survey, “The top reason to lower a speed is receiving requests from the public to improve safety (76%).” When the public demands safer roads, the 85th percentile method utterly fails to hear their demands.

  • The 85th-percentile method is not used when safety is a priority.
    • When communities decide to prioritize safety for places like schools and parks, they often use statutory speed limit authority to lower speed limits for those places and do not use the 85th-percentile method. In these circumstances, the speed choice of drivers is already understood as being outweighed by the safety of others.
  • The 85th-percentile method has not de-legitimized speeding or legitimized agencies building roads for safer speeds.
    • Decades of research shows that speed limit compliance is low, so the 85th-percentile method has habituated turning “bad” speed limit violators into “good” drivers by raising speed limits based on the 85th-percentile speed chosen by drivers. The last national survey of speed limit compliance by NHTSA showed that more than 18 percent of drivers violated the post speed limit by 10 mph or more for all roadway types reviewed.
    • The 85th-percentile method of differentiating “good” and “bad” drivers struggles to make sense of the idea that a “good” driver can still be going too fast for the safety of others and assumes a solution of enforcement rather than redesigning the road. Agencies “responsible for building and maintaining the state transportation system [with] no authority to cite vehicle violations of any kind” will nevertheless use the legitimacy of enforcement as a justification for not lowering speed limits instead of addressing how their planning and construction responsibilities can effectively manage speeds. 
  • The 85th-percentile method promotes a limited scope of objectivity.
    • The 85th-percentile method identifies driver speeds, but struggles to account for people outside of vehicles, who rarely walk at a rate of more than 3 mph or bike at a rate of more than 20 mph. These speeds are always on the low-end of the speed distribution and do not meaningfully impact the 85th-percentile speed chosen by drivers.
    • The 85th-percentile method can be, and is, often supplemented by the consideration of other metrics but those metrics are rarely as quantitative, observable, and seemingly objective. Crash frequency is the second-most used consideration, but crash reporting is often incomplete, crash frequency is not necessarily observable in a site visit, and crashes may be attributed to driver behaviors other than speed choice. In response, some public agencies dismiss speed limits lower than the 85th-percentile as “unreasonably low speed limits, also called irrational speed limits” without seeming to consider that requests for slower safer speeds are reasonable and rational even if the slower speed cannot be achieved without redesigning the road.

A Reformed Approach

Reformed approaches to speed limit setting and road design tend to start with the existing practice of car-centric speed choice and driver preferences, and look to add safety measures on the margin. This can be seen in the diversity of considerations that supplement the dominant use of the 85th-percentile method, in designs like sharrows that can be applied without otherwise changing the roadway, and in the structure of guidance documents which identify operating speed as a primary design consideration without recognizing or attempting to correct its historic predominance. A reformed approach is a start towards safety, but it still leaves bicyclists, pedestrians, and other non-drivers on the margins. 

A Reframed Approach

The reframed approach proposed by the Slow Roads Save Lives campaign is to put people and safety first. The AAA survey in 2019 found only eight percent of respondents considered injury severity statistics, but injury minimization is a signature of the speed limit setting approach used by other countries with better traffic safety records than the United States. According to NHTSA, “countries with a zero deaths vision and framework…no longer rely on the 85th percentile or other operating speed distributions, but set limits according to injury minimization principles.” The Slow Roads Save Lives campaign is necessary to encourage shifting this currently fringe consideration of people’s lives to being a central component of how speed limits are set and understood.

Injury minimization principles, like the 85th-percentile method, also make claims about why they are proper.

  • Injury minimization principles make a claim about safety.
    • Injury minimization principles are based upon decades of research showing that injury severity is linked to the speed at which a crash occurs. By setting speed limits for slower speeds, the force involved in a crash is lower and the risk of serious injury or death is reduced. Physics dictate the potential harm in the crash and once a speed limit is chosen it is up to the public agencies that build and design roads to ensure that the targeted speed limit is one most drivers choose based on the road design..
  • Injury minimization principles make a claim about legitimacy.
    • According to injury minimization principles, a speed limit is legitimate because it represents the amount of risk acceptable to the community. Once the level of risk is chosen by policy makers, then it is up to public agencies to design roads, implement enforcement systems, and create a culture of driving that achieves natural compliance by most drivers with the chosen speed limits. Rather than legitimacy being an outcome of driver choice or experienced solely through the lens of enforcement, legitimacy derives from a direct discussion about the amount of risk imposed by drivers and whether that risk is acceptable.
  • Injury minimization principles make a claim about objectivity.
    • Crash severity is a quantitative and observable metric that can be measured and has been found to be directly related to the speed of a crash for decades. Crash severity is not measured through a site visit, but can be objectively considered for any site and systemwide because it is a function of physics.

What can we do to slow down?

The 85th-percentile speed may be a measure of speed limit compliance, but it should not be an independent justification for a speed limit. It is reasonable, rational, and justifiable for a community to want a slower speed limit to improve traffic safety and provide a better experience for people outside of vehicles on a street. It is up to roadway designers, planners, and engineers to design streets so that they meet those demands and support people who want to comply with the speed limit rather than people who want to speed. As FHWA said in a recent report to Congress, “changes to roadway design features and the context of the roadway have direct impacts on operating speeds.” We can and should make physical roadway changes to impact operating speeds so that they are slower and safer for people.

Speed limits matter and they matter because people will be injured or killed if they are hit by a vehicle moving at high speed. At 30 mph, the urban district speed limit recommended by the UVC from 1956-2002, nearly one-quarter of pedestrians hit by a vehicle traveling the legal speed limit are expected to die. According to the NTSB, “lowering speed limits can lead to sustained traveling speed reductions and crash reductions in urban areas.” 

The Slow Roads Save Lives campaign says that we can do better than lethal legal speed limits with an expected 25 percent risk of death when a person is hit. Community demands for slower speeds are worthy of action, regardless of the speeds drivers currently choose. Reframing road design means that people living in communities choose whether or not a speed is rational, and public agencies must be prepared to achieve those target speeds through better design that helps people drive at those community-chosen speeds.

Help the League show the broad support across the country for Slow Roads. Take the “Slow Roads Save Lives” pledge to help us show how many people across the country support slow speeds and slow roads that save lives.

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