The National Academies

NCHRP 17-104 [Active]

Enhancement of Roadside Design Safety Prediction Models for the Highway Safety Manual

  Project Data
Funds: 500,000
Staff Responsibility: Roberto Barcena
Research Agency: The University of North Carolina at Chapel Hill
Principal Investigator: Srinivasan, Raghavan
Effective Date: 9/6/2022
Completion Date: 3/5/2025
Comments: Research in progress


The AASHTO Highway Safety Manual (HSM) is a resource that allows safety practitioners to consider safety fully and quantitatively in project decisions. The first edition of the HSM (HSM1) provides a method for considering roadside conditions in analyses of two-lane facilities that is based on a qualitative and visual index. The user selects a factor, called a Roadside Hazard Rating (RHR), using a series of photographs and descriptions to represent the roadside on the existing or proposed facility undergoing analysis. This rating is used in a limited number of safety performance functions (SPF) and/or crash modification factors (CMF) to account for roadside features on two-lane rural roads in crash predictions. Roadside information is limited to side-slopes on undivided multi- lane roadways and is not incorporated into analyses for other multi-lane divided facility types addressed in the HSM1.

Since publication of the HSM1 in 2010 there has been research to fill the knowledge gaps in the associated safety performance models and those used in the Roadside Design Guide (RDG) for roadside features. NCHRP Project 17-54, “Consideration of Roadside Features in the Highway Safety Manual,” developed models to quantitatively consider the roadside in safety analyses. The NCHRP Project 17-54 models were of a different form than the existing HSM1 models, using exposure versus predictive models. The findings for SPFs and CMFs could not be validated by small sample testing by several states. Of particular concern was the shape (reduced road departure crash rates) of the SPFs at higher volumes, and the associated effects of roadside fixed objects and their offsets to the travel way. Review of the fixed object CMFs by NCHRP Project 17-82, “Proposed Guidance for Fixed Objects in the Roadside Design Guide” reached similar conclusions.

The research team for NCHRP Project 17-72, “Update of Crash Modification Factors for the Highway Safety Manual,” reviewed the NCHRP Project 17-54 results and the results of an AASHTO analysis to determine if data from NCHRP Project 17-54 and other sources could be used to develop an alternative roadside CMF. Rather than use the data and findings, an optional method was proposed using other data sources that correlated well with the RHR. Several states again tested the models and some states had results that over predicted crashes and some states had results that under predicted crashes. It was found that the increased level of effort to use the NCHRP 17-72 roadside method led to results that were approximately as accurate as the more qualitative but less data-intensive method already in the HSM1. This result indicated a continued need for research to improve the analysis method.

The results of prior demonstration tests, while limited in number and scope, along with the analysis of the NCHRP Project 17-54 research results, have highlighted the need to collect additional data and validate proposed models, and to determine whether improvements are needed for the roadside-related crash prediction tools proposed for future editions of the HSM. If new data are available or readily collectable, then the model could be improved. The same validation and model development would also benefit efforts to develop a performance-based RDG.


The objectives of this research are to (a) validate roadside safety performance functions (SPF) and associated crash modification factors (CMF) developed in NCHRP Project 17-54, including associated design element variables, that are appropriate for consideration by AASHTO to incorporate in the Highway Safety Manual, (b) develop or enhance roadside SPFs to supplement or replace existing models for rural, suburban, or urban arterial roadways with consideration of total versus single vehicle crashes and their frequency and severity, and (c) coordinate the research products with planned content of other performance-based design manuals such as the Green Book and Roadside Design Guide.

The research results shall be used to develop proposed draft language for consideration by AASHTO to incorporate the research findings in appropriate HSM chapters. The work will include review of available roadway and roadside design element inventory and associated crash data and the potential collection of new data. Both the roadway departure SPFs and related CMFs for design elements will be provided for the roadway facility types prioritized by AASHTO based on available or collected data.

This research shall (a) coordinate with current NCHRP Projects 17-82, “Proposed Guidance for Fixed Objects in the Roadside Design Guide,” 17-85, “Development and Application of Crash Severity Models for the Highway Safety Manual,” 17-88, “Roadside Encroachment Database Development and Analysis,” and 17-92, “Developing Safety Performance Functions for Rural Two-Lane Highways that Incorporate Speed Measures,” and (b) consider the results of completed NCHRP Project 15-65, “Development of Safety Performance Based Guidelines for the Roadside Design Guide,” NCHRP Web-Only Document 296: Guidelines for Cost-Effective Safety Treatments of Roadside Ditches, NCHRP Research Report 911: Guidelines for Traversability of Roadside Slopes, and associated projects.

STATUS: Research in progress.

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