Making two-lane roads safer: IHSDM is an invaluable safety evaluation software package for highway designers and planners

Public Roads, Jan-Feb, 2003 by Raymond A. Krammes, Carl Hayden

The social, environmental, and economic context in which today's highways are designed demands trade-off assessments that require more explicit and quantitative consideration of safety issues than is possible with available tools. The Federal Highway Administration's (FHWA) Interactive Highway Safety Design Model (IHSDM) is a suite of software analysis tools for evaluating safety and operational effects of geometric design decisions on two-lane rural highways. IHSDM will provide highway project planners, designers, and reviewers in State and local departments of transportation (DOTs) and engineering consulting firms with a suite of safety evaluation tools to support these assessments.

The 2003 release of IHSDM culminates a multiyear research and development effort. Highway project decisionmakers how can use IHSDM to check designs for conformance with design policy, estimate their expected safety performance, and diagnose potential safety and operational issues throughout the highway design process.

A Need in Today's Highway Design Environment

Traditionally, designers have relied on compliance with design policy to assure an acceptable level of safety. In today's highway development environment, citizens are asking designers for more context-sensitive designs with broader application of the flexibility afforded by design policy without compromising safety. Making decisions in this environment calls for more detailed, quantitative estimates of a design alternative's expected safety performance.

As Timothy R. Neuman, vice president and chief highway engineer for CH2M HILL, puts it, "We now operate in an era of increasingly challenging project choices and decisions involving issues of competing values and priorities, citizen involvement, and limited resources for projects. Designers need to understand the explicit safety consequences of their design plans, and they need to be able to communicate this information to project stakeholders."

Ronald Erickson, geometrics engineer for the Minnesota DOT, adds, "Especially with the difficult challenge of replacing technical staff, the application of IHSDM will be a valuable tool for project engineers to evaluate and check their designs."

IHSDM 2003

Through IHSDM's quantitative estimates of the expected safety performance of geometric design, IHSDM will help project planners, designers, and reviewers make more cost-effective decisions about safety measures within cost constraints, context, and other considerations.

IHSDM is intended for use throughout the highway design process--from preliminary planning and engineering through detailed design to final review. It may be used both for projects to improve existing roadways and projects to construct new roadways.

The 2003 release of IHSDM for two-lane rural highways has five evaluation modules: (1) policy review, (2) crash prediction, (3) design consistency, (4) intersection review, and (5) traffic analysis. Each module provides different measures of the expected safety performance of an existing or proposed highway geometric design.

The policy review module automates the current process of checking a design against applicable, quantitative design guidelines. The crash prediction module provides quantitative safety performance measures, including expected crash frequency and severity.

The remaining modules diagnose factors contributing to safety performance. The design consistency module assesses operating speed consistency. The intersection review module evaluates design elements that influence the safety performance of at-grade intersections. The traffic analysis module evaluates traffic operations on the roadway under current or projected traffic loads.

Additional capabilities are planned for future releases. Research is underway to develop capabilities for IHSDM to perform similar evaluations of multilane rural highways; A sixth evaluation module for two-lane rural highways, driver/vehicle, is also under development and will provide measures of vehicle dynamics, including lateral acceleration as well as rollover and skidding potential.

Policy Review Module

The policy review module checks roadway-segment design elements for compliance with relevant highway geometric design policies. The module provides electronic files replicating quantitative policy values specified by the American Association of State Highway and Transportation Officials in the 1990, 1994, and 2001 editions of A Policy on Geometric Design of Highways and Streets and automates checks of design values against those policy values. IHSDM also provides a tool for inputting policy tables from other agencies' design policies.

The module organizes checks into four categories: cross section, horizontal alignment, vertical alignment, and sight distance. Cross-section checks include through-traveled way width and cross slope, auxiliary lane width and cross slope, shoulder width and cross slope, cross slope rollover on curves, clear zone and roadside slope, normal ditch design, and bridge width. Horizontal alignment checks include radius of curvature, superelevation rate and transition design, length of horizontal curve, and compound curve ratio. Vertical alignment checks include tangent grade and vertical curve length: The policy review module also can check stopping; passing, and decision sight distance.