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|Federal Highway Administration > Publications > Public Roads > Vol. 66 · No. 4 > Making Two-Lane Roads Safer|
Making Two-Lane Roads Safer
by Raymond A. Krammes and Carl Hayden
IHSDM is an invaluable safety evaluation software package for highway designers and planners.
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 now 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."
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.
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.
Charles Clements, roadway design engineer with the Arkansas State Highway and Transportation Department, says, "The policy review module [can] be used to not only check our current designs, but we [can] also use it to check existing roadways to see what revisions could be made in their design to bring them up to today's standards."
David Olson of the Washington State DOT adds, "Considering the number of new engineers working in design, the policy review module will be invaluable in quickly analyzing how well a design complies with design policy. Not only is it a great tool for tracking compliance with policy, it also has an educational element for new designers."
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-legged intersections with stop control on the minor-road approach, four-legged intersections with stop control on the minor-road approaches, 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 of design alternatives, and assessing the safety cost-effectiveness of design decisions.
Describing how he envisions using this module, Charles Clements, of the Arkansas State Highway and Transportation Department, says, "The crash prediction module [can] be used to predict the number of crashes along a particular design. By using it in this manner, we could avoid hazardous accidents in design before allowing the project to be constructed."
Dewayne Sykes, assistant State roadway design engineer with the North Carolina DOT, observes, "The number of crashes is an indicator of safety easily understood by the nontransportation engineer. A tool that can provide this information will be invaluable when evaluating proposed highway designs and when explaining the relative safety of these designs to the public as well as others."
Design Consistency Module
The design consistency module helps diagnose safety concerns at horizontal curves. Crashes on two-lane rural highways are overrepresented 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.
David Olson of the Washington State DOT explains, "The ability to evaluate design consistency from a perspective of driver expectancy is a tremendous addition to our project development toolset. Inattentive application of design guidelines presents opportunities for designs that conform to design policies and yet surprise the driver. The design consistency module is specifically developed to identify surprises and bring them to the attention of the designer. Once these locations have been identified, the designer can make modifications and quickly reevaluate their impacts."
Ron Erickson of the Minnesota DOT adds, "This is my favorite module because, as a designer, evaluating a project for design consistency is really difficult and for that reason rarely done. Now the design consistency module will add another quality check into our design process."
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:
The intersection review module can provide useful input to project scoping, preliminary engineering, and design review. "Most accidents occur at intersections, making them key safety concerns," says Dewayne Sykes of the North Carolina DOT. "The intersection review module with its diagnostic review component will help designers produce a better, safer intersection design."
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.
Integrating IHSDM with Other Software
To facilitate safety evaluations in the highway design process, IHSDM will be able to import horizontal and vertical alignment and cross-section geometric data from commercial roadway design software packages. To achieve this integration, FHWA entered into cooperative research and development agreements with commercial vendors of roadway design software, including Bentley Systems' GEOPAK Corporation and CAiCE Software Corporation. FHWA also is supporting standard design data exchange formats, including LandXML.
Regarding the impact of integration with IHSDM, Alan Akman, CEO of CAiCE, notes, "We expect many of our State DOT and municipal clients to make this a standard part of their engineering design process as we achieve full integration."
Finally, FHWA Administrator Mary E. Peters, concludes, "These agreements are ideal examples of how public-private partnerships help bridge the gap between research and real-world applications. The integration of these products provides the highway industry with critical access to a tool that will significantly improve highway safety."
Raymond A. Krammes is a highway research engineer and roadway team leader in FHWA's Office of Safety Research and Development. Krammes manages development of IHSDM. He is a registered professional engineer (Texas) and received his B.S., M.S., and Ph.D. in civil engineering from The Pennsylvania State University.
Carl Hayden is a highway engineer and has served in FHWA's safety group, now the Office of Safety Design, for 26 years in various positions related to wet weather safety, highway design, safety data analysis, and traffic records. He is a registered professional engineer (DC), received his B.S. in civil engineering from the Virginia Polytechnic Institute and State University, and completed FHWA's Highway Engineering Training Program.
The 2003 release of IHSDM for two-lane rural highways is distributed through the IHSDM Web site, www.fhwa.dot.gov/research/tfhrc/projects/safety/comprehensive/ihsdm/index.cfm. See the site for details on registering, downloading the software, and contacting FHWA for technical support, software maintenance, and training.
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