Relationship between speed and lateral position on curves.

Hallmark, Shauna L. · 2012 · ROSA P / Midwest Transportation Consortium

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Summary

This study investigates the relationship between vehicle speed and lateral position on rural two-lane horizontal curves, addressing a gap in crash investigation data where post-crash speed is often difficult to determine. Since excessive speed is a primary factor in lane departure crashes, particularly on curves, the researchers used lateral position as a crash surrogate. The objective was to assess whether vehicles traveling significantly faster than the advisory speed exhibited a higher likelihood of near-lane crossings, thereby providing evidence for the link between speed and unsafe lane positioning. Data were collected at three rural two-lane curve sites in Iowa using pneumatic road tubes arranged in a Z-configuration to measure longitudinal speed and lateral displacement. The sites varied in advisory speeds (35–50 mph) and traffic volumes. Researchers manually processed data for passenger vehicles, separating observations into daytime and nighttime periods to account for varying driver behaviors. The methodology involved calculating an odds ratio to compare the likelihood of a "near lane crossing" (defined as a tire coming within 6 inches of the edge or centerline) for vehicles traveling 5 mph or more over the advisory speed versus those traveling below that threshold. The results indicated mixed statistical significance across the various locations and time periods. During the daytime, most locations showed no statistically significant difference in near-lane crossing odds between speed groups. However, at one location (221st Street at the point of curvature), vehicles exceeding the advisory speed by 5 mph or more were 4.47 times more likely to experience a near lane crossing than slower vehicles. During nighttime, three locations had insufficient data, and three showed no significant difference. At two nighttime locations, however, the odds of a near lane crossing were significantly higher for faster vehicles, with odds ratios of 2.38 and 2.37, respectively. The study concludes that while results were not statistically significant at all locations, there is evidence of a positive relationship between higher speeds and increased odds of near lane crossings on curves. When significant, vehicles traveling well above the advisory speed had 2.37 to 4.47 times greater odds of unsafe lateral positioning. These findings support the use of speed reduction metrics in evaluating roadway-based countermeasures, suggesting that interventions that lower vehicle speeds may improve lane positioning and reduce the likelihood of lane departure crashes.

Key finding

Vehicles traveling five or more mph over the advisory speed had odds of near-lane crossings that were 2.37 to 4.47 times greater than those traveling below the threshold at statistically significant locations.

Methodology

field_study

Sample size: 1000

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
• lane positioning
• speed choice
• rail grade crossings

Information type

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• Empirical Findings: crash risk outcomes