Reducing Crash Severity with Speed Limit Setting and Speed Management

Photo courtesy of WSDOT.

Despite bold leadership across all levels of government on promoting roadway safety for all users, the rates of crashes resulting in fatal and serious injuries are still headed in the wrong direction. Speed is a contributing factor to the fatal and serious injury crashes on our roads. The 2024 Washington Strategic Highway Safety Plan identified injury minimization speed limit setting as a strategy to support the goal of zero fatalities on state roads.

Recent policy changes are in place to allow Washington’s local governments to address speed, and there are new tools in place to support this effort. This blog will cover background and the policy foundation for safe system speeds, how to get to safe system speeds, and examples from Washington State.

Background

The following terms allow us to discuss the new policy changes and tools in the 2024 Washington Strategic Highway Safety Plan:

• Design speed: the speed on which the geometry or physical elements of the roadway are based.
• Operating speed: the speed at which drivers are traveling along a roadway, as determined by various analyses of measured speeds.
• Posted speed limit: the maximum lawful speed as displayed on a regulatory sign.
• Statutory speed limit: speed limit ranges established under law that apply in the absence of a posted speed limit.
• Category speed limit: the assigning of speed limits to specific categories of streets, such as major arterials or local streets.
• Target (desired) speed: the highest speed at which vehicles should operate on a roadway based on the road and land use context, potential for different crash types, the impact forces that result, and the human body’s tolerance to withstand those forces.
• Speed management: includes the use of engineering, traffic control, and road design to induce drivers to travel at target speeds.
• Self-enforcing or self-explaining road: one that is planned/designed to encourage drivers to select operating speeds consistent with the posted speed limit.
• Safe system speeds (i.e., injury minimization speeds): speed-setting practices that aim to minimize injury severity should a crash occur.
• Safe system approach: a strategy of designing and operating roads that addresses multiple factors affecting safety.

Existing Research

There are many reasons jurisdictions may set speed limits on roadways, including improving the safety of all travelers. The faster a driver travels, the narrower their field of vision, the further they travel before they can react and before their brakes bring them to a stop.

However, not all speed-related fatalities are the result of drivers speeding. In some circumstances, drivers may be operating within the posted speed limit, but it was not set to reduce the severity and incidence of crashes appropriately for that street and context.

Historically, conventional road design parameters encouraged higher design speeds than the target speed for roadways. However, this led to higher operating speeds and, ultimately, increases in the posted speeds. Instead, when redesigning a roadway, if the designer selects a desired target speed in line with safe system speeds, in addition to any speed management treatments to supplement the design, the engineer can aim to ensure the resulting posted speed can meet this target speed.

Beyond the selection of design speeds, national guidance also directs engineers to set and reevaluate posted speeds based on the 85th percentile speed. However, in Washington State, the 85th percentile speed is not the only tool considered in the setting of speed limits per WAC 468-95-045. Section 2B.21 of the 11th Edition of the Manual on Uniform Traffic Control Devices (11th Ed. MUTCD) also outlines various factors to consider and discourages the use of the 85th percentile speed for certain contexts.

Policy Foundation

Washington State has released the following reports in support of a safe system speed limit setting:

• The 2020 Injury Minimization and Speed Management Policy Elements and Implementation Recommendations (Injury Minimization Report) identified tools to implement injury minimization.
• The 2021 Active Transportation Plan 2020 and Beyond considered the relationship between driver speeds to active transportation safety, finding that most pedestrian and bicyclist fatalities (83%) occurred on roads with a posted speed over 25 miles per hour (mph) in 2019. For 2010-2019 the figure is 86%.
• The 2024 Target Zero Update recommended local jurisdictions and the state work towards achieving injury minimization speeds.

In addition, in 2022, SB 5687 amended RCW 46.61.415, which addresses when local governments may establish or alter maximum speed limits. The law now states:

(3)(a) Local authorities in their respective jurisdictions may establish a maximum speed limit of 20 miles per hour on a non-arterial highway or part of a non-arterial highway.

(b) A speed limit established under this subsection by a local authority does not need to be determined on the basis of an engineering and traffic investigation if the local authority has developed procedures regarding establishing a maximum speed limit under this subsection.

How to Achieve Safe System Speeds

State regulations allow local agencies flexibility to set safe system speed limits. For more information, check out MRSC’s webpage, Speed Limits and Traffic Calming.

Guidance for setting speed limits is found in the WAC modifications to the MUTCD under WAC 468-95-045. This section replaces all of MUTCD Section 2B.13 for R2-1 signs to direct authorized agencies to consider:

• The 85th percentile speed of vehicles traveling on the road;
• Road characteristics, shoulder condition, grade, alignment, and sight distance;
• The pace speed;
• Roadside development and environment;
• Parking practices and pedestrian activity;
• Reported crash experience for at least a 12-month period; and
• Other factors such as route development or comprehensive plans.

WAC 468-95-045 may change once the state adopts the 11th Ed. MUTCD — updates will be on the Washington State Department of Transportation’s (WSDOT) MUTCD webpage.

There are existing practitioner tools, specifically NACTO’s City Limits and the Injury Minimization Report, which provide methodologies for the injury minimization speed setting approaches summarized in the sections below.

Establish safe system target speeds for all roads

Based on the human body’s tolerance to withstand impact forces, safe system target speeds for all roads may include:

• A 20-mph target for non-arterials, especially in residential or business districts;
• A 25-mph or less target for arterials that are not limited access in town centers where origins and destinations are within a walking (one mile) or biking (three mile) distance;
• A 30-45 mph target on rural roads where there are no median barriers and head-on collisions are possible.

Set category speed limits

RCW 46.61.415 requires cities and counties set category speed limits, especially for 20-mph target speed corridors on local streets.

Evaluate operating speeds on corridors

Corridors that require an engineering and traffic investigation for speed limit setting — where the posted speed limit is 25 mph or greater — should be evaluated for safety and potentially updated.

Determine if speed management is necessary

Based on operating speeds and target speeds, determine if speed management is necessary to achieve the target speed along any of the evaluated roadways (The Injury Minimization Report and City Limits discuss when speed management may be needed).

Develop speed management project list

Localities should develop a list of projects for speed management to achieve the target speeds on roadways that require it and implement these projects as funding becomes available. These may include iterative projects to lower speeds over time.

Recommend speed limit changes to your governing body

City or county staff should provide any recommended speed limit changes to the local legislative body, which can then adopt an ordinance or resolution to effect these changes. For details on how one city did this, see Reducing Local Street Speed Limits in Bellevue: Safety Starts on Your Street – Part 1 and Part 2.

Throughout all steps, communicate to the public why speed is critical, how your agency plans to reduce speeds, and where speed limit reductions and management will be effective for improving safety. Bellevue’s example highlighted how it could be beneficial to begin with a smaller area or a single corridor to collect data for further communication as speed is managed throughout a community.

Washington Cities and Counties Implementing Safe System Speeds

Like Bellevue, other cities and counties have begun to implement safe system speed limit procedures in their communities. Below are a few examples.

• Seattle reduced speed limits throughout the city through an iterative process beginning in 2015. As a result, most arterial street speed limits are set to 25 mph, and non-arterial streets are 20 mph.
• Tacoma conducted a speed limit study, and, as a result, in 2022, it lowered the default speed limit on residential streets to 20 mph, and on some arterial corridors in neighborhood business districts to 25 mph as part of its Vision Zero plan.
• Bainbridge Island conducted an island-wide speed limit study, and as a result, in 2023, it lowered the speed limit on many local access roads to 20 mph and other arterials throughout the city.
• Lake Forest Park conducted a safe speed study in 2024, including recommendations to set programmatic speed limits based on injury minimization throughout the city.

Conclusion

For local agencies interested in setting and achieving injury minimization or safe system speeds on their roads, refer to MRSC’s Speed Limits and Traffic Calming webpage and the Injury Minimization Report. For state routes, WSDOT is also considering ways to achieve safe system speeds, especially where Complete Streets applies per RCW 47.04.035.

Opportunities to fund safe system speed studies or implementation may be available through the following WSDOT programs: Highway Safety Improvement, Safe Routes to School, or Pedestrian and Bicycle. For further questions, feel free to reach out to me.