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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

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Publication Number:  FHWA-RD-99-171    Date:  August 2000
Publication Number: FHWA-RD-99-171
Date: August 2000


Speed Prediction for Two-Lane Rural Highways

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Current procedures for designing rural alinements rely on the selection and application ofdesign speeds. U.S. highway geometric design researchers and practitioners 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 the roadside in developing cost-effective highway design 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 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 alinement. 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.

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.



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