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Interactive Highway Safety Design Model

The Interactive Highway Safety Design Model (IHSDM) is a suite of software analysis tools for evaluating safety and operational effects of geometric design in the highway project development process. The model was developed under contract by the FHWA.

The scope of the current release of IHSDM is two-lane rural highways.

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

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

WSDOT Implementation
WSDOT is implementing the IHSDM suite for statewide use. Work on two-lane rural highways comprises about seven percent of the overall WSDOT construction program. The software will be made available on the engineering applications menu and training will follow from project development trainers.

  • WSDOT Design Policy Adjustments for the Policy Review Module
  • IHSDM Training Project

IHSDM Modules

The 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.

This module can be applied at several stages in the highway design process. During project planning and scoping 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.

Policy Review Module User Manual (pdf, 1.17mb)
Policy Review Module Engineers Manual (pdf, 888kb)

The Crash Prediction Module
The crash prediction module estimates the frequency of crashes expected on a roadway based on its geometric design and traffic characteristics. The crash prediction algorithm considers the effect of a number of roadway variables: lane width, shoulder width and type, horizontal curve length and radius, presence of spiral transition, superelevation, grade, driveway density, passing lanes and short four-lane sections, two-way left-turn lanes, and roadside hazard rating. Intersection variables considered include skew angle, traffic control, presence of left- and right-turn lanes, and sight distance.

The algorithm for estimating crash frequency combines statistical base models and accident modification factors. FHWA derived the base models using crash data from four States. Base models are available for roadway segments and for three types of intersections: three and four-legged intersections with stop control on the minor-road approach, and four-legged signalized intersections.

The accident modification factors adjust the base model estimates for individual geometric design element dimensions and for traffic control features. The factors are the product of an expert panel review of related research findings and consensus on the best available estimates of quantitative safety effects of each design and traffic control feature. The algorithm can be calibrated by State or local agencies to reflect roadway, topographic, environmental, and crash-reporting conditions. The algorithm also provides an Empirical Bayes procedure for a weighted averaging of the algorithm estimate with project-specific crash history data.

The crash prediction module can provide input for scoping improvement projects on existing roadways, comparing the relative safety performance 

Crash Prediction User Manual (pdf, 1.55mb)
Crash Prediction Engineers Manual (pdf, 1.22mb)

The Design Consistency Module
The design consistency module helps diagnose safety concerns at horizontal curves. Crashes on two-lane rural highways are over represented at horizontal curves, and speed inconsistencies are a common contributing factor to crashes on curves. This module provides estimates of the magnitude of potential speed inconsistencies.

The design consistency module uses a speed-profile model that estimates 85th percentile, free-flow, passenger vehicle speeds at each point along a roadway. (See "Managing Speed" on page 48.) The speed-profile model combines estimated 85th percentile speeds on curves (horizontal, vertical, and horizontal-vertical combinations), desired speeds on long tangents, acceleration and deceleration rates exiting and entering curves, and an algorithm for estimating speeds on vertical grades.

The model was calibrated using speed data collected at horizontal curves and their approach tangents in six states. The module identifies two potential consistency issues: (1) large differences between the assumed design speed and estimated 85th percentile speed, and (2) large changes in 85th percentile speeds from an approach tangent to a horizontal curve.

Design consistency evaluations provide valuable information for diagnosing potential safety issues on existing highways. They also provide quality assurance checks of proposed preliminary and final alignment designs.

Design Consistency User Manual (pdf, 892kb)
Design Consistency Engineers Manual (pdf, 989kb)

The Intersection Review Module
The intersection review module includes both policy and diagnostic review capabilities. The policy review component checks the following intersection design elements for compliance with design policy: corner radius, turn lane design, intersection angle, and intersection sight distance triangles.

The diagnostic review component is an expert system that leads the user through a systematic evaluation of an existing or proposed intersection geometric design to identify potential safety concerns and possible treatments to address those concerns. The review considers design issues including:

  • Intersection configuration: multileg intersections, skewed intersections, offset-T intersections, and more than one minor-road approach on the same side of the major road
  • Horizontal alignment: intersection on horizontal curve, curve on intersection leg, and approach alignment differing between opposing approaches
  • Vertical alignment: intersection on crest vertical curve, crest or sag vertical curve on intersection approach, steep grade through intersection, and continuity of minor-road profile through intersection
  • Intersection sight distance

Intersection Review User Manual (pdf, 1.41mb)

The Traffic Analysis Module
The traffic analysis module uses the TWOPAS traffic simulation model to estimate traffic quality-of-service measures for an existing or proposed design under current or projected future traffic flows. The traffic analysis module facilitates use of TWOPAS by feeding it the roadway geometry data stored by IHSDM.

TWOPAS is the microscopic traffic simulation model used to develop the two-lane highway chapter of the Transportation Research Board's (TRB) "Highway Capacity Manual". TWOPAS produces measures including average speed and percentage of time spent following other vehicles.

The traffic analysis module is particularly useful during project scoping and preliminary engineering for evaluating the operational performance of alternatives to two-lane cross sections, including passing lanes, climbing lanes, and short, four-lane sections.

Traffic Analysis User Manual (pdf, 814kb)
Traffic Analysis Engineers Manual (pdf, 1.95mb)

Other Manuals and Resources