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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Public Roads - Spring 2018

Spring 2018
Issue No:
Vol. 82 No. 1
Publication Number:
Table of Contents

Internet Watch

Improving IHSDM to Increase Roadway Safety

by Abdul Zineddin

Safety is at the forefront of all stages of project development, from planning, analysis of alternatives, and design to construction and operations. To help practitioners improve safety in their projects, the Federal Highway Administration has developed a variety of advanced analysis methods and tools. One of these is the Interactive Highway Safety Design Model (IHSDM), a suite of software analysis tools for evaluating the safety and operational effects of decisions related to geometric design.

IHSDM supports data-driven safety analysis, one of the innovations FHWA is championing under round four of its Every Day Counts initiative. Typical applications of the model include evaluating the safety impact of highway improvements, comparing the relative safety performance of design alternatives, and assessing the cost-effectiveness of design decisions in relation to the safety results. IHSDM helps users implement the American Association of State Highway and Transportation Officials’ Highway Safety Manual Part C: Predictive Method. The tool also can be an integral part of road safety audits.

The software includes six evaluation modules, which provide quantitative information on the expected safety and operational performance of a highway design: Crash Prediction, Policy Review, Design Consistency, Traffic Analysis, Intersection Review, and Driver/Vehicle. In fall 2017, FHWA released version 13.0.0 of IHSDM. The latest version adds the Economic Analyses Tool, enabling users to conduct benefit-cost assessments within IHSDM using evaluation results (crash frequencies and severities) from the Crash Prediction module.

Adding New Analytic Capability

The initial version of the Economic Analyses Tool applied to freeways and was expanded in March 2018 to include all facility types covered by Part C of the Highway Safety Manual. These include rural two-lane and multilane highways, urban and suburban arterials, and freeway segments and interchange components (such as ramps).

The Economic Analyses Tool supports benefit-cost analyses, including comparisons of crash cost reductions of alternative designs compared to construction and maintenance costs. The tool provides default values for crash unit costs—the costs per crash for different severities—as well as discount rate and crash cost index, used to determine the “present value” of benefits and costs. Users also may enter their own values by using the IHSDM Administration Tool.

In November 2017, FHWA hosted a webinar titled “IHSDM 2017 and the New IHSDM Economic Analyses Tool.” A recording of the webinar, as well as presentation slides are available The developers also added a tutorial and user guide for the Economic Analyses Tool into the software itself, which includes a sample project when downloaded.


These screenshots show alternative 2A for Pennsylvania’s interchange project on I–70 as illustrated in the IHSDM Highway Viewer(above) and the interface of the IHSDM Economic Analyses Tool (below) with the calculated present value of crash costs for 2018–2038.


Applying the Economic Analyses Tool

To illustrate the capabilities of the tool, FHWA performed an economic analysis on a project that had previously used the IHSDM for a safety evaluation.

In 2015, the Pennsylvania Department of Transportation (PennDOT) identified a need to modify an existing set of closely spaced interchanges on I–70 in western Pennsylvania. A request submitted by PennDOT to the FHWA Pennsylvania Division Office focused primarily on two alternatives. The first, alternative 2A, retained the two closely spaced interchanges with some mainline improvements. The second, alternative 3, removed one of the interchanges and added a new connector road and local roadway network improvements.

FHWA’s Geometric Design Lab used the IHSDM Crash Prediction Module to perform a safety evaluation of the two alternatives for the period of 2018–2038. The results showed that alternative 2A was superior in terms of expected safety performance (587 expected total crashes compared to 652 for alternative 3).

In 2017, to provide an example for using the new IHSDM Economic Analyses Tool, the Geometric Design Lab performed an economic analysis using the results of the I–70 Crash Prediction Module evaluation for the freeway components. As part of a benefit-cost analysis, the researchers estimated the present value of crash costs for 2018–2038 at $104 million for alternative 2A.

Monique Evans, director of FHWA’s Office of Safety Research and Development, says, “The IHSDM Economic Analyses Tool will assist State and local highway agencies in making sound investment decisions by quantifying safety-related benefits and costs of project alternatives.”

Download the IHSDM software for free at

For more information, contact Abdul Zineddin at 202–493–3288 or

Abdul Zineddin is a transportation specialist in FHWA’s Office of Safety Research and Development.