Horizontal alignment design consistency for rural two-lane highways.

Krammes, R. A.; Brackett, R. Q.; Shafer, M. A.; Donnell, Eric T.; Anderson, I. B.; Fink, K. L.; Collins, K. M.; Pendleton, O. J.; Messer, C. J. · 1995 · ROSA P / United States. Federal Highway Administration

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Summary

This 1995 Federal Highway Administration report addresses the safety risks associated with inconsistent horizontal alignments on rural two-lane highways. The research was motivated by the failure of the traditional U.S. design-speed policy to ensure alignment consistency. Under current policy, designers select a design speed for individual curves, but drivers often travel at higher speeds on long approach tangents. When these drivers encounter curves designed for lower speeds, they must make abrupt speed reductions, creating a disparity between operating speed and design speed. This inconsistency is identified as a primary cause of curve-related accidents, with accident rates on such curves reported to be 1.5 to 4.0 times higher than on tangents. To address this, the researchers developed models to evaluate alignment consistency based on operating speed and driver workload. The operating-speed model was calibrated using speed and geometry data from 138 horizontal curves and 78 approach tangents across five states. The driver-workload model was calibrated using data from two occluded vision test studies involving 55 subjects, which measured the mental effort required to navigate curves. Additionally, the study reviewed geometric design policies in Europe, Australia, and Canada to identify alternative consistency evaluation procedures. The findings confirmed that accident rates increase significantly as the required speed reduction from an approach tangent to a horizontal curve increases. Analysis of a database containing 1,126 curve sites across three states demonstrated that model-estimated operating-speed reductions were better predictors of accident experience than the degree of curvature alone. The study also found that the 85th percentile operating speeds generally exceed the design speeds of horizontal curves when the curve’s design speed is lower than the drivers’ desired speed on the preceding tangent. The developed speed-profile and workload-profile models allow engineers to plot expected operating speeds and driver workload along an alignment. The significance of this work lies in its shift from isolated geometric design to a holistic assessment of driver-vehicle-roadway interactions. The report concludes that design consistency should be evaluated based on the uniformity of operating speeds and driver workload rather than just adherence to design speed standards. The models developed in this study were intended for incorporation into the Interactive Highway Safety Design Model (IHSDM), providing designers with tools to assess the safety effects of geometric decisions and mitigate the risks associated with speed inconsistencies.

Key finding

Accident experience increases as the required speed reduction from an approach tangent to a horizontal curve increases.

Methodology

mixed_methods

Sample size: 1126

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified_with_issues.

Topics

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• road geometry
• perceptual countermeasures
• rail grade crossings
• speed choice