Alternative design consistency rating methods for two-lane rural highways

Anderson, Ingrid; Bauer, Karin M.; Collins, Jon M.; Fitzpatrick, Kay; Green, Paul; Harwood, Douglas W.; Koppa, Roger; Krammes, Raymond A.; Parma, Kelly D.; Poggioli, Brian; Tsimhoni, Omer; Wooldridge, Mark D. · 2000 · ROSA P / United States. Federal Highway Administration

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Summary

This report addresses the problem of design consistency on rural two-lane highways, specifically investigating alternative methods to evaluate whether highway geometry conforms to driver expectancy. The traditional design-speed concept, which bases safety on the most restrictive geometric element, is identified as insufficient because it fails to account for speed differentials on tangents or the combined effects of horizontal and vertical alignments. To improve safety by reducing driver errors caused by unexpected geometric changes, the study was motivated by the need to develop quantitative rating methods for integration into the Interactive Highway Safety Design Model (IHSDM). The research evaluated three specific alternative consistency rating methods: alignment indices, speed distribution measures, and driver workload. Alignment indices were analyzed as quantitative measures of the general character of roadway alignment, assessing how changes in magnitude or rate of change between segments indicate inconsistency. Speed distribution measures, including variance, standard deviation, coefficient of variation, and skewness, were investigated to determine if spot speed variability could serve as an indicator of geometric inconsistency. Driver workload was assessed as a measure of the information processing demands imposed by roadway geometry. This evaluation utilized a multi-method experimental design involving on-road studies, closed-loop test track studies, and driving simulator experiments. Techniques included vision occlusion devices to measure visual demand, eye-mark systems to track fixation, and subjective difficulty ratings using a modified Cooper-Harper scale. The findings revealed that speed variance is inappropriate as a design consistency measure for horizontal curvature. The study determined that inconsistent features do not necessarily exhibit higher spot speed variability than consistent features, contradicting the hypothesis that speed variability correlates with guidance-level errors. In contrast, driver workload measures showed promise. Results from the vision occlusion and simulator studies indicated that workload increases linearly with the degree of curvature and peaks near the beginning of horizontal curves. The research also examined the interaction between curvature, deflection angle, and driver demographics, finding that visual demand is significantly influenced by geometric severity. Alignment indices were identified as having conceptual advantages for system-wide assessment, though specific recommended indices were detailed in the full report. The significance of this work lies in providing empirical evidence to refine design consistency evaluation tools beyond the traditional design-speed concept. By demonstrating the limitations of speed distribution measures and validating driver workload as a proxy for geometric difficulty, the study supports the development of more robust safety models. The findings contribute to the IHSDM framework, offering designers quantitative methods to identify and mitigate geometric inconsistencies that violate driver expectancy, thereby aiming to reduce speed errors and accident risks on rural highways.

Key finding

Speed variance is inappropriate as a design consistency measure for horizontal curvature, whereas alignment indices and driver workload metrics based on visual demand provide viable methods for evaluating geometric consistency.

Methodology

mixed_methods

Provenance

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

Topics

Ranked by relevance to this paper. Hover a topic for its definition.

• road geometry
• perceptual countermeasures
• lane positioning
• rail grade crossings
• mental demand
• visual occlusion

Information type

What kind of knowledge this paper contributes, grouped by family — independent of topic (what it is about) and method (how it was studied).

• Empirical Findings: behavioral performance data
• Methodological Resource: validation psychometrics, dataset resource