The National Academies

NCHRP 17-129 [Pending]

Assessing Safety Performance of Atypical Service Interchanges

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


Many service interchanges include custom designs that do not fit into the traditional interchange definitions contained within the American Association of State Highway and Transportation Officials (AASHTO) Green Book, which makes it challenging to forecast and compare the relative predicted crash frequency of various atypical service interchanges. While the Highway Safety Manual (HSM) provides crash prediction for interchange elements such as basic ramps and ramp termini configurations, it does not offer a comprehensive crash prediction methodology applicable to such atypical service interchanges.

Atypical service interchanges lack a one-size-fits-all safety assessment method due to their uniqueness and complexities. To address this, engineers envision a "building-block" approach to deconstruct these interchanges into fundamental elements, e.g., ramp geometries, configurations, termini, weave segments, merge and diverge areas, access points and connections to managed lanes, active transportation, and transit facilities. This could involve assessing safety performance functions (SPFs) as well as crash modification factors (CMFs) for each element, and then aggregating these assessments to form a comprehensive analysis. However, such a generalized method is not currently available for engineers to analyze the safety performance of atypical service interchanges. 

Research is needed to apply conflict or exposure-based methods specifically tailored to these atypical service interchanges, analyzing each fundamental element, and then integrating the results from individual safety performance analyses into a comprehensive final result.


The objective of the project is to develop a method to partition interchanges into fundamental elements; analyze, quantify, and assess the safety performance of each element; and develop customized SPF models that will integrate the results from individual safety performance analyses into a comprehensive final result for atypical service interchange designs. 

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


PHASE I — Planning

Task 1a. Conduct a comprehensive literature review to identify and quantify the impact of interchange elements on crash frequency and severity for all users. The review shall include published and unpublished research conducted through the NCHRP; Federal Highway Administration (FHWA); other national, state, and local agencies; and international organizations.

Task 1b. Propose individuals from diverse geographic regions to form focus groups to identify: (1) current safety performance knowledge gaps in analyzing the atypical service interchanges; (2) preferred methodologies for safety performance assessment; and (3) interchange elements to be studied. Targeted focus group participants shall include:

  • State departments of transportation;
  • Federal agencies; and
  • Transportation industry and academia.

The roster will be reviewed and approved by the NCHRP.

Task 1c. Identify data that could help in developing exposure and safety prediction models for atypical service interchanges. Subsequently, develop a comprehensive data collection, management, and validation plan to accomplish the project objective. This plan shall include metadata for raw and processed data, details on data ownership, data restrictions (if applicable), and recommendations for data archiving.

Task 2. Analyze and synthesize the results from Task 1 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 3. Propose and prioritize the interchange elements to be included in the development of the safety performance assessment methodology. The recommendations need to be approved by the NCHRP before proceeding with Task 4.

Task 4. Develop a safety performance assessment methodology tailored for atypical service interchanges to achieve the research objective to be executed in Phase II. At the minimum, it shall include:

  • A flowchart (or decision-tree) to demonstrate the methodology, which shall be developed based on different data availability scenarios and preferences in safety performance assessment;
  • A deconstruction process to break a service interchange into fundamental elements;
  • Metrics for developing predictive safety performance models;
  • Alternative safety performance assessments and metrics where predictive models are not feasible;
  • Validation and verification approaches for the proposed models;
  • Tools to support the implementation;
  • Recommendations for research focus in the next phase, informed by overrepresentations or underrepresentations in crash severity data;
  • Recommendations for the next step in research for service interchanges that are less common or with less data to develop the predictive models; and
  • Key implementation considerations.

Task 5. Develop an annotated outline for the draft report. The annotated outline is intended to provide the foundation, context, and framework for the draft implementation plan development.

Task 6. Prepare Interim Report No.1 that documents Tasks 1 through 5 and provides an updated work plan for the remainder of the research. The updated plan must describe the process and rationale for the work proposed for Phases II and III.


 PHASE II — Execution

Task 7. Execute the data collection plan as developed in Task 1c and approved in Interim Report No. 1. Evaluate the collected data and determine the data availability for the model development. 

Task 8. Execute the methodology approved in Interim Report No. 1 based on the findings from Task 7. Predictive safety performance models or alternative models are expected to be developed to assess the safety performance of atypical service interchanges.

Task 9. Organize a virtual workshop to present the developed models to a NCHRP approved focus group (potential subset of focus groups identified in Task 1b). Then collect their feedback, refine the models, and submit the revised models for the NCHRP project panel review and approval. 

Task 10. Revise the proposed annotated outline for the draft report and provide a sample chapter, which will be selected by the NCHRP and shall be publication ready.

Task 11. Prepare Interim Report No. 2 that documents Tasks 7 through 10 and provides an updated work plan for the remainder of the research. The updated plan must describe the process and rationale for the work proposed for Phase III. 


PHASE III — Reporting

Task 12. Prepare the draft report based on the approved Interim Report No. 2 for the NCHRP panel’s review. The draft report shall be submitted to the NCHRP no later than 9 months prior to the contract end date.

Task 13. Verify the proposed crash models per the approved methodology and document the model verification efforts.

Task 14. Refine and update the draft report based on verification results collected from Task 13. Prepare draft language for consideration by AASHTO for inclusion in its next update of the HSM and develop the implementation tool. The second draft report along with other deliverables shall be submitted to the NCHRP 6 months prior to the contract end date.

Task 15. Present the research findings to appropriate AASHTO technical committees for comments and propose any revisions to the NCHRP. The research team shall anticipate making two in-person presentations during the research to the AASHTO Committee on Safety and Committee on Design. Revise the draft research report after consideration of review comments.

Task 16. Prepare the final deliverables, which at a minimum shall include:  

  • A final research report that documents the entire research effort and any lessons learned;
  • Draft language for consideration by AASHTO for inclusion in its next update of the HSM;
  • All raw and cleaned data collected and used in this research (data should be provided in as close to its raw form as possible, based on contractual or legal restrictions): input datasets, fused and integrated research datasets, data dictionaries, data models, etc.;
  • An implementation tool (including any annotated spreadsheet macro code, if used), user manual, and any other spreadsheet tool documentation;
  • Media and communication material (e.g., presentations, 2-page executive level flyer, graphics, press releases); 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.


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