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Publication Number: FHWA-RD-99-173
Date: Month Year
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Current procedures for designing rural alignments rely on the selection and application of design speeds. U.S. highway geometric design researchers and practioners generally recognize the need to supplement current design procedures for two-lane rural highways with reliable, quantitative, safety-evaluation methods. To address this need, the Federal Highway Administration is developing the Interactive Highway Safety Design Model (IHSDM) as a framework for an integrated design process that systematically considers both the roadway and roadside in developing cost-effective highway designs alternatives. The focus of IHSDM is on the safety effects of design alternatives. Design consistency is one of several modules which are to be integrated with commercial CAD/roadway design software. Other IHSDM modules include: crash prediction, driver/vehicle, intersection diagnostic review, policy review and traffic analysis.
The research documented in this report provided a speed profile model that can be incorporated into the design consistency module of IHSDM. The model can be used to evaluate the design consistency of the roadway or can be used to develop a speed profile for an alignment. The model considers both horizontal and vertical curvature and the acceleration or deceleration behavior as a vehicle moves from one feature to another. The research also demonstrated that predicted speed reduction on a horizontal curve relative to the preceding curve or tangent has a strong relationship to accident frequency. In addition, the research investigated alternatives that could be used in the design consistency module of IHSDM. The three methods studied included alignment indices, spot speed variability measures, and driver workload. Based upon the findings, alignment indices and speed variability measures were not recommended for use in the design consistency module. Driver workload, however, has a good potential as a design consistency rating measure.
Michael F. Trentacoste, Director
Office of Safety Research & Development
This document is disseminated under the sponsorship ofthe Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof. This report does not constitute a standard, specification, or regulation.
The United States Government does not endorse products or manufactures. Trade and manufactures's name appear in this report only because they are considered essential to the objective of this document.
Topics: research, safety
Keywords: Two-lane rural highway, design consistency, IHSDM, speed prediction, speed profile, acceleration/deceleration, alignment indices, speed distribution, driver workload, visual demand
TRT Terms: research, Safety and security, Safety, Transportation safety