Speed Prediction for Two-Lane Rural Highways

Fitzpatrick, Kay; Elefteriadou, Lily; Harwood, Douglas W.; Collins, Jon M.; McFadden, John; Anderson, Ingrid B.; Krammes, Raymond A.; Irizarry, Nelson; Parma, Kelly D.; Bauer, Karin M.; Passetti, Karl · 2000 · ROSA P / United States. Federal Highway Administration

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Summary

This research report addresses the need for accurate speed prediction models to evaluate design consistency on two-lane rural highways. Design consistency refers to the conformance of highway geometry to driver expectancy, a factor critical for reducing driver errors and improving safety. The study was motivated by limitations in the traditional AASHTO design-speed concept, which relies on the most restrictive geometric element (typically a curve) to set the design speed for an entire alignment. This approach fails to account for speeds on tangent sections or the acceleration and deceleration behaviors of vehicles transitioning between features. The primary objective was to develop and validate a comprehensive speed-profile model for inclusion in the Federal Highway Administration’s Interactive Highway Safety Design Model (IHSDM). The methodology involved extensive field data collection and analysis across multiple phases. Researchers collected speed data at 176 two-lane rural highway sites to develop speed-prediction equations for horizontal and vertical curves, as well as for combined alignments. The study analyzed the effects of spiral transitions and evaluated speed behaviors for passenger cars, recreational vehicles, and trucks. To predict speeds on tangent sections, the researchers developed and tested alignment indices. Vehicle performance on grades was modeled using equations from the TWOPAS model. Additionally, specific data collection efforts were conducted to determine actual acceleration and deceleration rates as vehicles approached and departed horizontal curves. These components were integrated into a unified speed-profile model, which was subsequently validated against field observations. The findings resulted in a set of validated speed-prediction equations that account for horizontal and vertical curvature, grade effects, and vehicle type. The study established specific acceleration and deceleration rates and distances, allowing for the modeling of speed changes between geometric features. The resulting speed-profile model can generate a continuous speed profile for any given alignment, identifying locations where operating speeds deviate significantly from driver expectations. The report also analyzed the relationship between these design-consistency measures and safety, demonstrating that inconsistencies in speed profiles correlate with higher accident frequencies. The significance of this work lies in providing a robust, empirical tool for highway designers to evaluate and improve roadway safety. By moving beyond the static design-speed concept to a dynamic operating-speed model, the IHSDM can better identify geometric inconsistencies that violate driver expectancy. This allows for more cost-effective design alternatives that align with actual driver behavior, ultimately enhancing the safety of two-lane rural highways. The model serves as a critical module within the broader IHSDM framework, integrating geometric design with safety analysis.

Key finding

The developed speed-profile model accurately predicts operating speeds by integrating equations for horizontal and vertical curves, tangent alignment indices, and acceleration/deceleration behaviors, providing a robust method for evaluating design consistency on two-lane rural highways.

Methodology

field_study

Sample size: 176

Provenance

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

Topics

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• road geometry
• speed choice