Evaluation of design consistency methods for two-lane rural highways : executive summary

Fitzpatrick, Kay · 2000 · ROSA P / United States. Federal Highway Administration

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Summary

This study, conducted by the Texas Transportation Institute for the Federal Highway Administration, addresses the evaluation of design consistency methods for two-lane rural highways. Design consistency refers to the conformance of highway geometry with driver expectancy; inconsistencies can lead to speed errors and accidents. The research was motivated by weaknesses in the traditional design-speed concept, which relies on the most restrictive geometric element and ignores speeds on tangents or less restrictive curves. The primary objectives were to expand a previously developed speed-profile model and to investigate three alternative consistency rating methods: alignment indices, speed distribution measures, and driver workload. These efforts aimed to support the Interactive Highway Safety Design Model (IHSDM). The methodology involved extensive data collection and analysis across multiple domains. Speed data were gathered at over 200 two-lane rural highway sites to develop and validate speed prediction equations for various geometric conditions, including horizontal and vertical curves, combined curves, and tangents. Acceleration and deceleration rates were analyzed at 21 sites to refine the speed-profile model. Additionally, the study evaluated alignment indices (quantitative measures of alignment character) and speed distribution measures (variance, standard deviation, etc.) to determine their predictive capability for speed and consistency. Driver workload was assessed using vision occlusion tests, subjective ratings, and simulations across test tracks, on-road environments, and simulators to measure the visual information processing demands imposed by roadway geometry. Key findings indicate that regression equations using inverse radius ($1/R$) effectively predict 85th percentile speeds for passenger vehicles on horizontal curves, while inverse vertical curvature ($1/K$) predicts speeds on limited sight distance vertical curves. The study found that spiral transitions did not significantly affect speed, and truck/recreational vehicle speeds closely followed passenger car trends. Crucially, previous assumptions regarding constant acceleration and deceleration rates were invalidated; new models showed these rates vary significantly with curve radius. Regarding alternative methods, alignment indices and speed distribution measures failed to significantly predict desired speeds on long tangents or serve as reliable consistency indicators. Conversely, driver workload was found to increase linearly with the inverse of the curve radius, with visual demand peaking near the beginning of curves. Subjective and objective workload measures showed consistent trends, confirming that tighter curves impose higher cognitive demands on drivers. The significance of this research lies in the development of a refined speed-profile model for inclusion in IHSDM, which allows for more accurate evaluation of design consistency by accounting for variable acceleration/deceleration rates and specific geometric combinations. The study concludes that while alignment indices and speed variance are not suitable standalone consistency measures for these highways, driver workload provides a valid indicator of geometric difficulty. The findings provide engineers with validated equations and thresholds (e.g., speed change limits) to identify poor design consistency, thereby improving safety by ensuring roadway geometry aligns with driver expectations and capabilities.

Key finding

Speed prediction equations accurately modeled operating speeds, while alignment indices failed to significantly predict desired speeds on tangents and speed variance decreased on horizontal curves, making it unsuitable as a consistency measure.

Methodology

mixed_methods

Sample size: 200

Provenance

The full processing record for this entry. Every stage of this paper's journey through the pipeline is logged — what ran, with which tool and model, how many attempts it took, and when it last completed. Discovered via bulk_ingest_rosap on 2026-05-23 (6 acquisition events logged).

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

Topics

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• road geometry
• perceptual countermeasures
• speed choice