Effects of Central Island Landscape Treatments at Single-Lane Roundabouts

Schurr, Karen S.; Abos-Sanchez, Jorge · 2005 · ROSA P / United States. Dept. of Transportation. Research and Special Programs Administration

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Summary

This study investigates the impact of central island landscaping on the safety and operational performance of single-lane roundabouts. The research addresses a debate between two design philosophies: one advocating for landscaping that blocks drivers' cross-view sight lines to focus attention on immediate traffic and reduce speeds, and another favoring clear visibility to aid navigation and pedestrian detection. The study was motivated by a lack of empirical data regarding how vertical landscaping elements affect driver, pedestrian, and bicyclist behaviors. The research employed a before-and-after experimental design at the Sheridan Boulevard and 33rd Street roundabout in Lincoln, Nebraska, the first arterial roundabout in the state. The "before" condition featured a grass-only central island, while the "after" condition included three 7-foot high Black Hill spruce trees that partially blocked the cross-view sight line. Data collection involved measuring free-flow vehicle speeds using Lidar at 12 locations (approaches and circulatory roadway) and analyzing capacity metrics—critical gap and follow-up time—via video recordings. Pedestrian and bicyclist behaviors were also observed through video analysis. Speed data were collected during non-peak hours to ensure free-flow conditions, with sample sizes of approximately 250 per location to ensure statistical validity. The results demonstrated that the introduction of trees had a positive impact on operations and safety. Statistically significant reductions in mean vehicle speeds were observed at seven of the twelve measurement points, with speed reductions evident as far as 150 feet from the pedestrian crossings. Additionally, the standard deviation of speeds decreased across nearly all approaches, indicating that traffic speeds became more uniform. The landscaping also improved capacity; drivers accepted shorter gaps to enter the circulatory roadway due to the slower circulating traffic, allowing more vehicles to occupy the roundabout simultaneously. Consequently, the effective capacity of the roundabout increased. The study concludes that partial cross-view sight blockage via vertical landscaping elements yields both operational and safety benefits. By inducing lower and more uniform vehicle speeds, the landscaping reduces the potential severity of accidents involving pedestrians, bicyclists, and vehicles. The findings support the design approach that prioritizes speed reduction through visual obstruction over maximum visibility, suggesting that such landscaping treatments can enhance the overall performance of single-lane roundabouts.

Key finding

Central island landscaping with significant vertical dimension and partial cross-view blockage significantly reduced vehicle speeds and increased effective roundabout capacity by enabling drivers to accept shorter entry gaps.

Methodology

naturalistic

Sample size: 255

Provenance

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

Topics

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• intersection design
• perceptual countermeasures