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Publication Number:  FHWA-HRT-13-077    Date:  January 2014
Publication Number: FHWA-HRT-13-077
Date: January 2014


Safety Effects of Horizontal Curve and Grade Combinations on Rural Two-Lane Highways

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Report - FHWA-HRT-13-077 Summary Report - FHWA-HRT-13-078 Excel Calcuator Tool - HRTM 2130



The first edition of the American Association of State Highway and Transportation Officials (AASHTO) Highway Safety Manual (HSM) provides crash modification factors (CMFs) for the safety effects of horizontal curvature and percent grade on rural two-lane highways.(1) However, the HSM does not provide a method to account for the interactions between these effects. Researchers have always presumed that there are interactions between the safety effects of horizontal and vertical alignment, but this has not been demonstrated for specific alignment combinations in a form useful for safety prediction.

This document is a final report highlighting research undertaken to quantify the safety effects of five specific combinations of horizontal and vertical alignment for rural two-lane highways. The research is based on data from the Federal Highway Administration’s Highway Safety Information System (HSIS) for Washington State, including roadway characteristics, horizontal and vertical alignment, traffic volume, and crashes. The outcome is a set of safety prediction models for fatal and injury and property damage only crashes. CMFs representing safety performance relative to level tangents on rural two-lane highways were developed from the models for each of the five combinations of horizontal and vertical alignment to present the results in a form suitable for incorporation in the HSM. This report should be of interest to safety engineers and planners as well as other AASHTO HSM users.

Monique R. Evans
Director, Office of Safety
Research and Development



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