The National Academies

NCHRP 17-29 [Completed]

Methodology to Predict the Safety Performance of Rural Multilane Highways

  Project Data
Funds: $700,218
Research Agency: Texas A&M Research Foundation
Principal Investigator: Dr. Dominique Lord
Effective Date: 7/1/2004
Completion Date: 4/30/2008


There is a significant need to improve the explicit consideration of highway safety in making decisions on roadway planning, design, and operations. To receive appropriate consideration, safety needs to be dealt with quantitatively in the design process; many other design factors are already estimated in quantitative terms, but safety is often dealt with only in a qualitative sense.

A group of Transportation Research Board (TRB) committees identified the need for more explicit and quantitative consideration of safety and proposed the development of a Highway Safety Manual (HSM). Portions of the Manual are being developed under several NCHRP projects, and the HSM will be recommended for adoption by AASHTO. The HSM will serve as a tool to help practitioners make planning, design, and operations decisions based on safety. It will serve the same role for safety analysis that the Highway Capacity Manual (HCM) serves for traffic-operations analyses. The purpose of the HSM will be to provide the best factual information and tools in a useful and widely accepted form to facilitate explicit consideration of safety in decision making.

The TRB committees interested in the HSM have formed a Task Force to guide its development. Two current NCHRP studies are under way to develop key aspects of the HSM. NCHRP Project 17-18(4), Highway Safety Manual, which is nearly complete, is a scoping study charged with recommending the content and outline of the first edition of the HSM and developing a plan for a research program needed to develop that first edition within five years. Recommendations developed in NCHRP Project 17-18(4) and concurred with by the Task Force indicate that the first edition of the HSM should include models for making quantitative estimates of the expected safety performance of rural multilane highways, rural two-lane highways, and urban and suburban arterials. Research to address the latter two of these facility types has already been initiated. Predictive models for rural two-lane highways have been developed by FHWA, and NCHRP Project 17-18(4) has developed a prototype HSM chapter incorporating those models. NCHRP Project 17-26 is developing models for predicting the safety performance of urban and suburban arterials; therefore, the only facility type to be included in the first edition HSM for which further modeling work is needed is rural multilane highways.

Other ongoing initiatives that are relevant include: NCHRP Project 17-25, "Crash Reduction Factors for ITS and Traffic Operations"; FHWA's Interactive Highway Safety Design Model (IHSDM); FHWA's SafetyAnalyst; NCHRP Project 17-18(8), which will outline development of the Human Factors Guide; AASHTO's Transportation Safety Information Management System (TSIMS); and AASHTO's efforts to implement its Strategic Highway Safety Plan.


The objectives of this research are to develop a methodology to predict the safety performance of rural multilane highways and to prepare a chapter on rural multilane highways for inclusion in the Highway Safety Manual. The methodology shall apply to both highway segments and at-grade intersections but does not include full access-control highways.

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

TASKS (1.) Conduct a review of completed and ongoing research (domestic and international), including the draft outline and chapter(s) of the HSM, to identify existing accident modification factors and models (See Special Note B). Review policies of the TRB Task Force for the Development of a Highway Safety Manual, regarding measures of safety, research protocols, and other potentially applicable topics. (2.) Identify candidate model inputs, including the design inputs and elements currently used in the AASHTO Green Book and relevant human factors issues (e.g., age and experience of road users). Compile existing accident modification factors, models, and sources of available data. Also identify high-priority planning, design, and operational safety issues. The contractor shall use the results of previous surveys conducted as part of NCHRP Projects 17-25 and 17-26 and FHWA's "SafetyAnalyst." If necessary, design a supplemental survey instrument and determine the sample size for this project. (3.) Survey transportation agencies to collect supplemental data and conduct follow-up interviews, as necessary. (4.) Based on the findings of Tasks 1 through 3, propose the transportation elements to be analyzed and the methodology to predict the safety performance for rural multilane highways. Separate model sets will likely be required for analysis of highway segments and intersections. The methodology should identify the different modeling approaches to be taken for each set. (5.) Develop a draft chapter outline and a revised work plan (including a data collection plan) addressing model development and validation, accident modification factor validation, and application of the models to practical examples. (6.) Submit an interim report, summarizing the findings of Tasks 1 through 5, for review by the NCHRP. (7.) Meet with the NCHRP panel to review the Task 6 interim report approximately 2 months after its submittal. Submit a revised interim report addressing the panel's review comments. (8.) Execute the approved work plan. (9.) Submit a final report documenting the entire research effort. The final report shall describe how the project was conducted and identify areas for further research. The report shall include a stand-alone chapter outline and a chapter for inclusion in the HSM with examples and worksheets for use by the practitioner. The prototype chapter developed under NCHRP Project 17-18(4) should be used as a model for the HSM materials developed.

Status: The contractor has submitted the draft final report and revised Chapter 9 for the Highway Safety Manual (HSM).  The draft final report has been posted on the TRB website as a Web Only Document 126.  The draft Chapter 9 has been provided to the HSM production contractor (NCHRP Project 17-36).

Product Availability: Draft final report.

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