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Frequently Asked Questions

What is the Interactive Highway Safety Design Model (IHSDM)?

A product of the Federal Highway Administration's (FHWA’s) Safety Research and Development Program, IHSDM is a suite of software analysis tools for explicit, quantitative evaluation of safety and operational effects of geometric design decisions during the highway design process.

What does IHSDM do?

IHSDM evaluates existing and proposed alternative highway geometric designs and provides quantitative information on their expected safety and operational performance.

What are IHSDM's evaluation capabilities?

IHSDM can estimate the crash frequency expected for a specified geometric design, check designs against relevant design policy values, and estimate other safety and operational performance measures (e.g., 85th percentile speed and percentage of time spent following) that help diagnose factors that contribute to expected safety performance.

There are six evaluation modules in the IHSDM – HSM Predictive Method 2019 Release: crash prediction, policy review, design consistency, intersection review, traffic analysis, and driver/vehicle. IHSDM also includes an Economic Analyses Tool, which allows IHSDM users to do economic analyses within IHSDM using Crash Prediction Module evaluation results (crash frequencies and severities).

What types of highway facilities can IHSDM evaluate?

The Crash Prediction Module can evaluate rural two-lane highways, rural multilane highways, urban/suburban arterials, freeway segments, freeway ramps/interchanges (including ramps, collector-distributor (C-D) roads, and ramp terminals), and roundabouts. The Policy Review Module applies to rural two-lane and rural multilane highways, while the other evaluation modules are applicable to rural two-lane highways.

What are each module's capabilities, and how might each be used?

Policy Review Module
The Policy Review Module checks highway-segment design elements for compliance with relevant geometric design policies. It can be applied at several stages in the highway design process. For improvement projects on existing roadways, it can provide an initial assessment of how the existing geometric design compares to current design guidelines. For all projects, it can facilitate quality-assurance checks throughout detailed design and design review.

Crash Prediction Module
The Crash Prediction Module faithfully implements HSM Part C predictive methods. It estimates the frequency and severity of crashes that could be expected on a highway based on its geometric design and traffic characteristics. This module supports efforts to identify improvement projects on existing roadways, compare the relative safety performance of design alternatives, and assess the safety cost effectiveness of design decisions.

Design Consistency Module
The Design Consistency Module helps diagnose safety concerns at horizontal curves by providing estimates of the magnitude of potential speed inconsistencies. Design consistency evaluations provide valuable information for diagnosing potential safety issues on existing highways. These evaluations also provide quality-assurance checks of both preliminary and final alignment designs.

Intersection Review Module
The Intersection Review Module has diagnostic review capabilities. The diagnostic review is an expert system that systematically evaluates an existing or proposed intersection geometric design to identify potential safety concerns and suggest possible treatments to address those concerns. The Intersection Review Module can provide useful input to project scoping, preliminary engineering, and design review.

Traffic Analysis Module
The Traffic Analysis Module uses the TWOPAS traffic simulation module to estimate traffic quality-of-service measures for an existing or proposed design under current or projected future traffic flows. This module is particularly useful during project scoping and preliminary engineering to evaluate the operational performance of alternatives to two-lane cross sections, including passing lanes, climbing lanes, and short four-lane sections.

Driver/Vehicle Module
The Driver/Vehicle Module simulates driving behavior and vehicle dynamics on a two-lane highway, providing predicted time histories of speed and other response variables, along with statistical measures of safety-related performance metrics, via a simulation of single driver/vehicle combination.

Who should use IHSDM?

IHSDM is intended for individuals involved in the highway design process. That process includes project planning, preliminary engineering, final design, and design review. Prospective users include highway project managers as well as planning, design, safety, and traffic engineers in State and local highway agencies and engineering consulting firms. In addition, the transportation research and academic communities can use IHSDM in their studies and educational efforts regarding the safety and operational effects of highway geometric design.

How should IHSDM be used?

IHSDM is a decision-support tool. Decision makers use the IHSDM quantitative information to help make, justify, and defend geometric design decisions throughout the highway design process.

What need does IHSDM address?

Compliance with design policy has traditionally helped highway project developers and reviewers assure acceptable safety levels. In today's highway development environment, however, citizens want agencies to create context-sensitive designs yet demand that safety not be compromised in the process. Making, justifying, and defending geometric design decisions in this environment requires detailed, quantitative estimates of the expected safety performance of design alternatives.

Information about the safety and operational effects of highway geometric design is scattered throughout the research literature. Highway project decision makers need this information to make, justify, and defend geometric design decisions but have difficulty accessing and applying existing research, seriously limiting the research's usefulness.

How does IHSDM address this need?

IHSDM organizes available knowledge about the safety and operational effects of geometric design into a more useful and usable form for highway project developers, allowing more explicit and quantitative consideration of the safety and operational implications of geometric design decisions throughout the highway design process. It is more useful because it compiles available knowledge into a single source and applies it to project-specific analyses. It is more usable because it works in the same computer environment as and can import geometry data from highway geometric design software.

How would using IHSDM benefit a highway agency?

IHSDM expedites the evaluation of existing and proposed designs relative to design policy and with respect to safety. IHSDM results help project developers make design decisions that improve the expected safety performance of final designs, and help project planners, designers, and reviewers justify and defend geometric design decisions.

How do I acquire IHSDM software and documentation?

The current release of IHSDM software may be downloaded free of charge through the IHSDM Public Software Web site.

What technical user support is available?

Federal Highway Administration's Geometric Design Laboratory (GDL) at the Turner-Fairbank Highway Research Center provides technical support free of charge for IHSDM users. The GDL's transportation engineers researched, helped to develop, and tested IHSDM, and they have intimate knowledge of this resource. To request technical support, either telephone the IHSDM Help Line at (202) 493–3407, or send an email to

Is training available?

Yes, training related to IHSDM is offered by FHWA’s Resource Center and through the National Highway Institute (NHI). NHI offers a two-day, onsite IHSDM Training Course (Course Number FHWA-NHI-380071). The course includes classroom presentations and discussions as well as hands-on exercises in using IHSDM. It instructs highway design project managers, planners, designers, and traffic and safety reviewers in the application of the IHSDM software and provides guidance on interpretation of the output. Upon completion of the course, participants will be able to:

  1. Describe key capabilities and limitations of IHSDM.
  2. Evaluate highways using IHSDM.
  3. Recognize when and how IHSDM can be used in the project development process.

For additional information, including how to host a course and enroll in a scheduled session, refer to the National Highway Institute Web site.

"Safety Analysis of Freeway Segments and Interchanges" is a 2-day onsite workshop offered by the FHWA Resource Center. Day two is focused on using the IHSDM Crash Prediction Module (CPM) to analyze freeway/interchange safety via application of Highway Safety Manual (HSM) Part C methodologies. Refer to this document for more details.

Where can I get more information about IHSDM?

The IHSDM Web site provides an overview of IHSDM capabilities, a library of the research underlying IHSDM, and information about training and technical support. The IHSDM Public Software Web site is where you can register for and download IHSDM software and access information on IHSDM technical support.

Whom can I contact for additional information?

Clayton Chen
Office of Safety Research and Development (HRDS)
Turner-Fairbank Highway Research Center
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101

Updated: Monday, December 2, 2019