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Publication Number: FHWA-RD-99-172
Date: Month Year


Alternative Design Consistency Rating Methods for Two-Lane Rural Highways

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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 built around a commercial computer-aided design package in the current vision of IHSDM. Other modules include: crash prediction, driver/vehicle, intersection diagnostic review, policy review, and traffic analysis.

The research documented in this report investigated alternattives 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.



United States Department of Transportation - Federal Highway Administration