The National Academies

NCHRP 07-36 [Pending]

Guide for Self-Explaining Roads in the Context of the Safe System Approach

  Project Data
Funds: $700,000
Contract Time: 36 Months
Staff Responsibility: Dr. Yi Zhao


Self-explaining roads (SERs), also known as self-enforcing roads, use the design of the roadway itself to achieve the goals of a proactive and equitable Safe System Approach. SERs, designed to be recognizable, distinguishable, interpretable, and safe, can be effective in terms of managing speed, thereby reducing the occurrence of fatal and serious injury crashes. Two fundamental criteria a SER must meet are homogeneity (sameness) within and heterogeneity (differences) between roadway classifications and contexts to increase predictability within the road environment.

Roadway characteristics and competing stakeholder demands often undermine the application of design and operational characteristics and features, partially because the impacts of those features are not well understood, communicated, or implemented in the context of improving safety for all road users. A SER reinforces the desired functions of a facility by emphasizing visual differences between roadway categories that are often categorized by multimodal considerations and speed [e.g., 0-25 miles per hour (mph), 25-40 mph, 40+ mph].

Defining safe speeds and setting credible speed limits have a long history, with many studies exploring the impact of specific features on driver speed profiles, including the most recent ones such as NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide (2021) and FHWA-HRT-17-098 Self-Enforcing Roadway, A Guidance Report (2018). The vast but unconsolidated literature on speed lacks a format easily applicable by professionals in safety, planning, design, and operations.

This research is necessary to bridge the gap between the SER concept and its implementation in different functional classes and contexts, and identify the critical SER features so that practitioners can be more intentional and cognizant of the impacts of their decisions on all road users. Specifically, the final product of the project is expected to include recommendations that support active transportation and can be applied during the various phases of project development. These phases include geometric design, active transportation facilities, roadside design, land use, and traffic control devices, resulting in operating speeds that closely match target speeds.



The objective of this research is to develop a guide for applying the SER concept to transportation planning, design, and operations to improve multimodal user safety, especially for vulnerable road users (VRUs) on nonfreeway arterial and collector systems in urban and suburban areas.

Accomplishment of the project objective will require at least the following tasks.



PHASE I - Preliminary Research

Task 1a. Conduct a comprehensive literature review on SER features and designs (e.g. distinctiveness of appearance) as well as the effectiveness of speed management treatments in the United States and other countries on nonfreeway arterials and collectors in urban and suburban areas. Apply appropriate methodology, such as meta-analysis to identify key factors that influence driver decision-making and understanding, including their choice of speed. The review shall include published and unpublished research conducted through the NCHRP; the Federal Highway Administration; other national, state, and local agencies; and international organizations. 

Task 1b. Synthesize the results to identify the knowledge gaps related to the project objective. These gaps shall be addressed in the final product or the recommended future research as budget permits.

Task 2. Propose a data collection plan, which shall include the data elements to be collected, such as land use, geometric, roadside, pavement, operational (crashes, speeds, and motorized and non-motorized volumes), type and level of enforcement, and socioeconomic demographic data at various locations, facilities, and from state and local agencies. Refer to Special Note A for additional information on data collection.

Task 3. Propose a comprehensive research methodology to meet the project objective. The research team shall consider the following:

  • Context classification, functional classification, and speed category groups for this project;
  • The differences between operational speed data and the posted/target speed or design speed if available;
  • The relationship between excess crash frequency, severity, and operational speed under various contexts;
  • Design characteristics that do not match the context classification;
  • Design features and trade-offs that influence operational speeds in transition zones and along the length of the segment;
  • The relationship between operational speed and the built environment;
  • Homogeneity (sameness) within and heterogeneity (differences) between roadway functional classifications and contexts to increase predictability for road users;
  • Visually distinct characteristics of a self-explaining roadway;
  • Visual cues that impact decision making;
  • Formulas for projecting the operational speed of a facility based on its visual and design characteristics;
  • A method for modeling speed choice and multimodal impact; and
  • A method for validating model results.

Task 4. Develop an annotated outline for the guide. The annotated outline shall consider specific features that result in operational speeds matching the target speed, both at transition zones and along the length of the segment, which is intended to provide the foundation, context, and framework for the draft guide to be developed in Task 10.

Task 5. Prepare Interim Report No. 1 that documents Tasks 1 through 4, including the data archiving plan (see Special Note A). The Interim Report shall also provide an updated and refined work plan for the remainder of the research. The updated plan must describe the process and rationale for the work proposed for the following phases. 


PHASE II - Research Execution 

Task 6. Perform data collection as per the approved plan documented in Interim Report No. 1.

Task 7. Analyze the collected data based on the methodology documented in the approved Interim Report No.1.

Task 8. Revise the proposed annotated outline for the guide and provide a sample chapter based on the results from Task 7. The sample chapter will be selected by the NCHRP and shall be publication-ready.

Task 9. Prepare Interim Report No. 2 that documents Tasks 6 through 8 and provides an updated work plan for the remainder of the research, no later than 12 months after approval of Phase I. The updated plan must describe the process and rationale for the work proposed for Phase III. 


PHASE III - Guide Development and Final Products 

Task 10. Develop the guide detailing features that result in operational speeds matching target speeds, including examples and graphics for each SER feature across different roadway classifications and contexts. The draft guide shall be submitted to the NCHRP 6 months prior to the contract end date.

Task 11. Develop a practitioner-focused pamphlet that summarizes the SER features and their distinguishing characteristics. 

Task 12. Develop outreach materials with case studies, participate in a virtual workshop hosted by TRB to share findings with a large group of stakeholders; and develop outreach materials to support practitioners’ understanding and implementation of the research findings.

Task 13. Prepare the final deliverables, which shall include the following:

  • A research report that documents the entire research effort, the guide, and findings from the field implementations along with an executive summary;
  • A practitioner-focused pamphlet to summarize the SER features and their distinguishing characteristics;
  • Public outreach strategies for general audiences, including the dissemination of public safety announcements, to communicate the need for behavior change;
  • Media and communication materials (presentation, graphics, press releases);
  • Prioritized recommendations for future research; and
  • A stand-alone technical memorandum titled “Implementation of Research Findings and Products.” 

STATUS: A research contractor has been selected for the project.  The contracting process is underway.


To create a link to this page, use this URL: http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=5493