1. Interactive Highway Safety Design Model (IHSDM)
A product of the Federal Highway Administration’s Safety Research and Development program, the Interactive Highway Safety Design Model (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.
This presentation provides an overview of IHSDM.
The presentation has two main parts.
The first part describes the purpose and scope of IHSDM.
The second part highlights IHSDM’s evaluation capabilities.

2. Goal
To marshal the best available knowledge about safety effects of geometrics into a more useful and usable form for highway project planners and designers.
FHWA’s goal in developing IHSDM is to marshal the best available knowledge about the safety and operational impacts of highway geometry design alternatives into a more useful and usable form for highway project planners and designers, and traffic and safety reviewers.

3. What is the Interactive Highway Safety Design Model (IHSDM)?
A suite of software analysis tools used for evaluating the safety and operational impacts of highway geometric design.

4. Intended Usage
Throughout the highway design process (preliminary planning and engineering through detailed design to final review)
For existing and new alignments
On two-lane rural highways (2003 release)
The intent is that highway planners and designers can use IHSDM for input to decisions that impact safety throughout the project development process—from project scoping and preliminary design, through final design and design review.
IHSDM can evaluate new alignments as well as improvement projects on existing roadways.
The 2003 release of IHSDM applies only to rural two-lane highways.
Research is underway to add capabilities for evaluating multilane rural highways.

5. Partnership Software Vendors (Import data from design packages)
State DOTs (Arkansas, California, Illinois, Minnesota, North Carolina, Washington, and West Virginia)
Software Vendors (Import data from design packages)
GEOPAK CAiCE

6. Components Evaluation Modules Basic Services Policy Review
Crash Prediction
Design Consistency
Intersection Review
Traffic Analysis
Basic Services
IHSDM’s evaluation capabilities are structured in five modules:
Policy Review Module
Crash Prediction Module
Design Consistency Module
Intersection Diagnostic Review Module
Traffic Analysis Module.
Each module evaluates a design from a different vantage point and provides different indicators of safety.
IHSDM Basic Services include importing highway geometry data from civil design packages and managing data within IHSDM, as well as HTML-based Help functions and User’s Manuals.
The remainder of the presentation provides a brief description of each module.

7. Policy Review Module Scope
Checks roadway segment geometry against relevant design policy and “flags” variations
The Policy Review Module automates the process of checking designs against relevant design guidelines and “flags” (or highlights) elements that vary from values recommended in policy. This module could be used to evaluate the geometry of existing roadways in project scoping studies, to perform checks during the design process, and to automate portions of the final design review.
IHSDM provides tables of policy values from the AASHTO Green Book (1990, 1994, and 2001 editions). It also provides the capability for individual States to edit those tables to reflect their own policy values that differ from AASHTO’s.
Users may check any or all of 17 elements of cross section, horizontal alignment, vertical alignment, and sight distance.

8. Policy Review Module Cross-section checks: Traveled Way Width
Auxiliary Lane Width
Shoulder Width
Shoulder Type
Normal Cross-Slope
Normal Shoulder slope
Cross-Slope Rollover on Curves
Bridge Width

9. Policy Review Module Horizontal Alignment checks:
Radius of Curvature
Length of Horizontal Curve
Superelevation
Compound Curve Ratio
Vertical Alignment Checks:
Tangent Grade
Vertical Curvature

10. Policy Review Module Sight Distance Checks: Stopping Sight Distance
Passing Sight Distance
Decision Sight Distance

11. Crash Prediction Module Scope
Estimates expected crash frequency based upon roadway geometry and traffic volumes
The Crash Prediction Module estimates the expected crash frequency on an existing or proposed roadway segment as a function of the roadway’s horizontal and vertical alignment and cross section geometry, roadside environment and driveway density, and traffic volumes.
The crash prediction algorithm breaks the roadway into intersections and homogeneous roadway segments of uniform geometry and traffic volume and estimates the expected crash frequency on each intersection and roadway segment. The algorithm applies a base (regression) model to account for the effect of traffic volume and crash modification factors to account for the effect of intersection/roadway geometry. The algorithm includes a calibration factor that States can develop and apply to reflect their own crash experience. Additionally, when the design alternative being evaluated is an existing roadway, the algorithm provides an Empirical Bayes procedure that use historical crash data for that roadway to refine its crash estimates. Publication No. FHWA-RD “Prediction of the Expected Safety Performance of Rural Two-Lane Highways,” posted on the Web at documents the algorithm in detail.
The remaining modules are diagnostic in nature. That is, they evaluate existing roadways or proposed design alternatives for common causes of safety problems on rural two-lane highways.

12. Crash Prediction Module
Crash prediction algorithms for:
Roadway segments
Intersections:
3-way STOP-controlled
4-way STOP-controlled
4-way signalized

13. Crash Prediction Module
Predicts the number of crashes for a design
based on the cross-section and alignment (vertical and horizontal)
Calibration procedure

14. Crash Prediction Module
Roadway segment algorithm considers:
AADT
Lane and shoulder width
Horizontal curve radius, length, spiral transition, superelevation
Grade
Driveway density
Two-way left turn lanes,
passing lanes,
short four-lane segments
Roadside hazard rating

15. Crash Prediction Module
Intersection algorithm considers:
AADT
Skew angle
Traffic control
Exclusive left-turn and right-turn lanes
Intersection sight distance

16. Design Consistency Module Scope
Estimates 85th percentile speed profile along alignment to evaluate operating speed consistency
Approximately 35 percent of all fatal crashes on two-lane rural highways occur on horizontal curves. Speed inconsistencies are a common contributor to crashes on curves, particularly on older, lower-design speed highways.
The Design Consistency Module estimates the speeds of free-flowing vehicles along an alignment. Its output is an 85th percentile speed profile which can be used to check expected operating speeds against the roadway’s assumed design speed and to estimate changes in 85th percentile speeds between successive alignment elements. The speed reduction from an approach tangent to a curve is a primary indicator of an alignment’s consistency. The goal is a uniform speed profile. The larger the speed reduction entering a curve, the more likely it is that some drivers will not reduce their speed enough and the higher the probability of crashes at that curve.
The Module’s speed prediction algorithm is documented in Publication No. FHWA-RD , “Speed Prediction for Two-Lane Rural Highways,” posted on the Web at

17. Design Consistency Module Scope
Estimates 85th percentile speed profile along alignment to:
Check design speed assumption
Estimate speed reductions from an approach tangent to entering horizontal curves

18. Intersection Review Module Scope
Policy Review
Diagnostic Review
Approximately 15 percent of all fatal crashes on two-lane rural highways occur at intersections.
The Intersection Review Module includes both policy and diagnostic reviews of intersection geometry.
The Policy Review component checks intersection design elements against design policy.
The Diagnostic Review uses expert system technology in an attempt to reproduce how an experienced designer would review an intersection design, evaluate potential safety concerns, and identify possible treatments for addressing those concerns.

19. Intersection Review Module Scope
Intersection Policy Review
Corner radius
Turn lane design
Intersection angle
Intersection sight distance triangles
Intersection Diagnostic Review
On average, approximately 15 percent of all fatal crashes on two-lane rural highways occur at intersections.
The Intersection Diagnostic Review Module uses expert system technology in an attempt to reproduce how an experienced designer would review an intersection design and identify potential safety and operational issues.
The Module can also lead users through an evaluation process for identifying alternative safety countermeasures.

20. Intersection Review Module Diagnostic Review
Expert system that applies a systematic evaluation of an existing or proposed intersection geometric design to identify possible safety concerns and treatments
27 concerns (configuration and alignment)
Treatments include design improvements & mitigation measures
On average, approximately 15 percent of all fatal crashes on two-lane rural highways occur at intersections.
The Intersection Diagnostic Review Module uses expert system technology in an attempt to reproduce how an experienced designer would review an intersection design and identify potential safety and operational issues.
The Module can also lead users through an evaluation process for identifying alternative safety countermeasures.

21. Traffic Analysis Module Scope
Uses TWOPAS to estimate traffic quality of service measures (average speed and % time following other vehicles)
Evaluates the operational performance of alternatives to passing and climbing lanes, and short four-lane sections
In the Traffic Analysis Module, IHSDM uses TWOPAS, a traffic simulation model for rural two-lane highways, to estimate traffic quality of service measures.
TWOPAS is the model used to calibrate the two-lane rural highway capacity analysis procedures in the Highway Capacity Manual.
In contrast to the Design Consistency Module that considers free-flowing vehicles, the Traffic Analysis Module evaluates traffic operations under current or future traffic flows. It can provide estimates including mean speed and percentage of time spent following, the two principle quality of service measures used for two-lane highways in the Highway Capacity Manual.

22. Driver/Vehicle Module Scope
Driver performance model linked to a vehicle dynamics model that estimates driver speed and path, skidding and rollover potential
Alpha version available
Expected release in 2004
The Driver/Vehicle Module has two components:
A human factors-based driver-performance model evaluates a roadway’s geometric design from an unimpeded driver’s perspective and estimates their likely choice of speed and path.
These speed and path predictions are input to a vehicle dynamics model that estimates the vehicle’s friction demand and, for trucks, rollover potential along the roadway.
These measures are additional indicators of the likely safety performance of a design.

23. 2003 Release 1st public release of IHSDM
May be downloaded free-of-charge from IHSDM Web site
Technical support is available (see IHSDM Web site for details)
The 2003 release of IHSDM is the first public release for testing and evaluation purposes. (There were limited releases during 2001 and 2002 to volunteer beta testers.)
The 2003 release is available for downloading free of charge from the IHSDM Web site (see next slide for Web address).
Although the 2003 release is fully functioning, FHWA qualifies the release “for testing and evaluation purposes” to emphasize that software development is ongoing. FHWA encourages feedback from users, which it will use in prioritizing enhancements for future planned releases.

24. IHSDM Web Site www.tfhrc.gov/safety/ihsdm/ihsdm.htm
The IHSDM Web site is
This site provides access to the software registration and download site.
The site provides an overview of IHSDM, an electronic library of the research underlying IHSDM, and information on how to obtain technical support.

25. Training Course Pilot offerings of the IHSDM training course:
July hosted by Washington State DOT in Vancouver
August hosted by Central Federal Lands in Denver
August hosted by Montana DOT in Helena
September hosted by Wisconsin DOT in Madison
September 29-October 1 hosted by Mississippi DOT in Jackson
October 7-8 hosted by Maine DOT in Augusta
Contact Ray Krammes: or (202)