A product of the FHWA’s Safety R&D 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.

IHSDM allows users to evaluate existing and proposed alternative highway geometric designs, and provides quantitative information on their expected safety and operational performance.

IHSDM estimates the crash frequency expected for a specified geometric design, checks designs against relevant design policy values, and estimates 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 five evaluation modules in the IHSDM–HSM Predictive Method 2021 Release: 1) crash prediction, 2) policy review, 3) design consistency, 4) traffic analysis, and 5) driver/vehicle. IHSDM also includes an Economic Analysis tool, which allows IHSDM users to perform economic analyses within IHSDM using Crash Prediction Module (CPM) evaluation results (e.g., crash frequencies and severities).

The CPM evaluates 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 (PRM) applies to rural two-lane and rural multilane highways, while the other evaluation modules are applicable to rural two-lane highways.

Policy Review Module (PRM)

The PRM 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 (CPM)

The CPM includes a faithful implementation of HSM Part C predictive methods, as well as additional models developed under NCHRP projects for inclusion in the future HSM 2nd Edition (e.g., for roundabouts, six-or-more lane urban/suburban arterials, one-way urban/suburban arterials, and some intersection types not covered in the HSM 1st Edition). . 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 (DCM)

The DCM 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.

Traffic Analysis Module (TAM)

The TAM 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 (DVM)

The DVM 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.

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.

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

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, and 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 decisionmakers 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.

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. IHSDM is more useful because it compiles available knowledge into a single source and applies it to project-specific analyses. IHSDM is more usable because it works in the same computer environment as — and can import geometry data from — highway geometric design software.

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 can help project planners, designers, and reviewers justify and defend geometric design decisions.

IHSDM can import LandXML format data (Schema 1.0). Many civil engineering design software packages also support LandXML and can export data into this format. A list of such software packages is available on the LandXML website (www.landxml.org).