TOOL 19
Establish a Hierarchy of Treatment Selection

Long Description.

Theme: Design and Project Procedures; Role: Planners, Engineers, Elected Officials; Complexity: Level 1; Scale: Local, Regional, State.

Overview

This tool presents a Safe System-focused design hierarchy which recognizes that, to reduce kinetic energy risk, some treatments may have more impact than others. Project constraints such as cost, timeline, operations, and public opinion can make implementing every potential safety improvement in a project infeasible, meaning that practitioners are often required to prioritize potential safety improvements or alternatives. The hierarchy offers a lens to select those tools most impactful and consistent with the Safe System Approach (SSA) principles to address exposure, likelihood, and severity risk factors.

Why Is This Important?

This tool is important for institutionalizing the SSA in planning and programming by recognizing that some treatments may have more substantial impacts than others on reducing kinetic energy risk.

Incorporating a design hierarchy provides a consistent and systematic approach to selecting effective treatments as a default rather than exception, thereby streamlining and resolving decisions for safety.

Description

A design hierarchy can help identify and prioritize infrastructure improvements or design alternatives that provide the greatest reduction in risk of serious crashes. One such hierarchy, the Safe System Design Hierarchy (SSDH), was developed by the FHWA to systematically incorporate the SSA into the Highway Safety Improvement Program. The SSDH focuses on proven safety countermeasures (PSCs) across a variety of categories including speed management, pedestrian/bicyclist, roadway departure, and intersections, as well as countermeasures that cross through all of these focus areas.

The SSDH specifies four tiers for priority consideration, ranging from Tier 1 being most in line with SSA principles to Tier 4 being least aligned (Figure 19-1). The tiers include the following:

• Tier 1: PSCs that remove or reduce the risk of severe conflict. This could include, but is not limited to:
  • Removing intersection crossing conflicts.
  • Removing fixed objects roadside in a high-speed context.
  • Providing grade-separated rail crossings.

Source: Hopwood et al. (2024).

Long Description.

The graphic begins with the heading “Safe System Design Hierarchy” and is followed by four tiers below it labeled Tier 1: Remove severe conflicts, Tier 2: Reduce Vehicle Speeds, Tier 3: Manage conflicts in time, and Tier 4: Increase attentiveness and awareness.

• Tier 2: PSCs that help reduce vehicular speeds—an essential element of the SSA and kinetic energy transfer. The SSDH tool recommends a combination of strategies such as
  • Speed management programs and strategies.
  • Context-sensitive design for achieving speed reduction.
• Tier 3: PSCs that help manage conflict points in time—meaning that, when different road users need to occupy the same space, there is a clear assignment of the right-of-way.
• Tier 4: Treatments that can be implemented to increase road usersʼ attentiveness and awareness of conflict points or changes in roadway conditions.

By using the SSDH, practitioners can prioritize countermeasures or alternatives based on their alignment with the SSA and ability to accomplish objectives of each tier. More information can be found in the Safe System Roadway Design Hierarchy guide.

Before using the SSDH, agencies should ensure context-based determinations of “movement versus place” are made and target speed is selected, to minimize misinterpretations in the hierarchy that could limit accessibility for vulnerable road users. A potential misinterpretation

could involve the decision to remove crossing opportunities in Tier 1 (in an attempt to remove severe conflicts) rather than first reducing speeds, managing conflicts in time, and increasing attentiveness to enhance crossings and access.

To address these concerns, this tool is recommended in combination with Tool 6: Consider Upstream Conditions. While the FHWA framework does not address land use, socioeconomic considerations are at the base of the Safe Systems Pyramid public health framework and provide important population-scale considerations that may be more impactful than using this tool in isolation. The Washington State Department of Transportation (WSDOT) case study in this tool illustrates how these contextual factors can be added into a hierarchy process.

Recommended in Combination with Other Tools

6. Consider Upstream Conditions

8. Advance Treatment-Specific Policy and Legislation

9. Implement Safe System Speed Management Program

16. Review Design Guidelines and Standards

17. Create Proactive Transition Plan

18. Expand Functional Classification

Case Study

Washington State Safe System Hierarchy

WSDOT introduced one of the first Safe System hierarchies in the United States. It was developed in 2019 and is a predecessor to the FHWA SSDH tool, though it differs in key ways. Similar to the SSDH, the WSDOT Safe System Hierarchy of Controls is a framework for prioritizing policies and countermeasures based on their effectiveness.

Compared to the SSDH tool, the WSDOT framework offers more holistic controls based on public health best practices, seeking upstream interventions to the need and role of transportation trips, routes, choices, and modes (elimination and substitution) before turning to engineering design considerations (those covered by the SSDH).

As lower tiers, the WSDOT framework also includes administrative controls such as behavioral strategies to provide education and enforcement, as well as policy or programmatic changes for system use. The final tier added to the hierarchy includes considerations for personal protective equipment for road users and in work zones.

The five tiers of the Safe System Hierarchy of Controls framework are shown in Figure 19-2. Washington Stateʼs 2024 Strategic Highway Safety Plan specifies that this hierarchy is used to guide WSDOT in design and operational decision making. More information is available through WSDOT.

Additional Resources

• International Road Assessment Programme—iRAP Road Safety Toolkit: The iRAP Road Safety Toolkit is a comprehensive, online resource that provides evidence-based guidance for assessing and addressing crash risk across all road user groups and environments. iRAPʼs approach reflects a Safe System-aligned intervention hierarchy, emphasizing system-level design and risk mitigation over user behavior change. Their model loosely follows this order of priority:
  1. Eliminate or Separate Conflicts

     Examples: Grade separation, median barriers, protected bike/pedestrian paths.

     Goal: Physically prevent crashes or separate users to reduce conflict points.

Source: WSDOT (2024).

Long Description.

The graphic has five boxes. The first box reads: Elimination: Eliminate exposure to the crash before it can occur. The second box reads: Substitution: Replace with strategy that lowers injury severity in the event of a crash

labelled Elimination

; ; Engineering Controls, make changes to how we design and operate the road system, vehicles, and programs; Administrative Controls, change the way people use the system through, for example, education, legislation, and policies; and Personal Protective Equipment, motorcycle helmets and protective gear, work zone safety vests. To the left is a bar stretching the height of the graphic with Most Effective at the top and Least Effective at the bottom.

• 2. Reduce Speeds at Conflict Points

     Examples: Roundabouts, raised pedestrian crossings, traffic calming.

     Goal: Lower kinetic energy to survivable levels in inevitable crashes.

  3. Improve Roadside and Road Design

     Examples: Clear zones, forgiving roadsides, shoulder widening, tactile paving.

     Goal: Minimize crash severity and guide safer use.

  4. Provide Safe Facilities for All Users

     Examples: Sidewalks, crossing refuges, protected intersections.

     Goal: Ensure safe access and mobility for pedestrians, cyclists, motorcyclists.

  5. Support Safe Use through Delineation and Warning

     Examples: Road markings, signs, lighting.

     Goal: Enhance visibility and attention but considered less effective on its own.

  6. Enforce Compliance and Educate Users

     Examples: Speed cameras, public safety campaigns.

     Goal: Support safe behavior but only as a supplement to safe design.

• FHWA—Safe System Project-Based Alignment Framework: The FHWAʼs Safe System Project-Based Alignment Framework guides practitioners in selecting the most impactful safety treatments by prioritizing those that eliminate hazards, reduce kinetic energy through speed management, and spatially or temporally separate different users. Using a structured, tiered hierarchy of controls, the framework supports decision-making under constraints by quantitatively rating alternatives via a matrix that evaluates kinetic energy risk exposure, likelihood, and severity for both vulnerable users and motorists, alongside qualitative prompts addressing safe vehicles, road users, and post-crash care.

References

FHWA. Safe System Project-Based Alignment Framework. Accessed July 3, 2025. https://highways.dot.gov/safety/zero-deaths/safe-system-project-based-alignment-framework

Hopwood, C., K. Little, and D. Gaines. Safe System Roadway Design Hierarchy: Engineering and Infrastructure-related Countermeasures to Effectively Reduce Roadway Fatalities and Serious Injuries. FHWA-SA-22-069. 2024. https://highways.dot.gov/sites/fhwa.dot.gov/files/2024-01/Safe_System_Roadway_Design_Hierarchy.pdf

International Road Assessment Programme. iRAP Road Safety Toolkit. Accessed July 3, 2025. https://toolkit.irap.org/

Washington State Department of Transportation. Strategic Highway Safety Plan: Target Zero. Washington Traffic Safety Commission. November 4, 2024. https://targetzero.com/wp-content/uploads/2024/11/Washington_Target_Zero_Plan_FINAL_11-04-2024_Accessible.pdf