Users Manual for the Highway Geometric Design Consistency Program (Metric Units)

Rao, K. S.; Oh, Hoon; Krammes, R. A. · 1995 · ROSA P / United States. Federal Highway Administration. Office of Safety and Traffic Operations

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Summary

This document serves as the user manual for the Highway Geometric Design Consistency (HGDC) program, a software tool developed to evaluate the geometric design consistency of rural two-lane highways in level or rolling terrain. The research motivation stems from the observation that drivers often select speeds on long tangents that exceed the design speeds of downstream horizontal curves, leading to significant speed reductions and operating speeds that surpass safe limits. These inconsistencies are identified as a primary cause of curve-related accidents. The HGDC program implements models previously documented in Report No. FHWA-RD-94-034 to assist engineers in plotting expected profiles of operating speed and driver workload, thereby helping to identify and mitigate these safety risks. The HGDC program is a menu-driven, interactive application designed for DOS-based microcomputers. It estimates two specific measures of horizontal alignment design consistency: the decrease in the 85th percentile operating speed and the increase in driver workload from an approach tangent to a horizontal curve. The speed model is applicable to alignments with horizontal curve radii greater than 58 meters and vertical grades up to 5 percent, while the workload model applies to radii greater than 145 meters. Users must input specific horizontal alignment data, including the stationing of the point of curvature and point of tangency for each curve, the radius of each curve, and station equations. The software automatically computes the consistency measures upon data entry, preventing manual modification of the calculated workload and speed values. The program provides both tabular and graphical outputs to visualize the results. Tabular output summarizes the speed reduction and workload increase for each horizontal curve. Graphical output displays the horizontal alignment via a bar chart representing curve radius and plots the profile of either the 85th percentile speed or driver workload along the alignment. The interface allows users to create, edit, save, and print files, as well as execute DOS commands. Users can customize graphical displays by selecting specific segments of the alignment for viewing and toggling grid displays for easier value interpretation. The software requires a minimum of 270K RAM and runs on IBM-compatible systems. This tool represents the first computer program developed specifically for evaluating geometric design consistency in rural two-lane highways. It is intended for use by geometric design professionals to assess the potential safety effects of specific design decisions. The HGDC program is part of a broader initiative to incorporate design consistency models into the Interactive Highway Safety Design Model (IHSDM). Future enhancements may include the evaluation of other geometric features such as vertical alignment, cross-section changes, and intersections, as well as direct interfacing with highway design packages. This manual covers the metric-units version of the software, with a separate manual available for the English-units version.

Key finding

The Highway Geometric Design Consistency program estimates operating speed reduction and driver workload increases at horizontal curves to evaluate geometric design consistency.

Methodology

dataset

Provenance

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Topics

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• road geometry