United States Department of Transportation
i
Effects of Central Island Landscape Treatments at Single-Lane Roundabouts
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2005-03-01
[PDF-3.71 MB]
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Edition:Final Report July 2002 to March 2005
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Abstract:Two schools of thought exist about the appropriate landscape treatment which should be used within the center island of roundabouts. Some believe that the central island not only provides an opportunity to create an attractive landscape but also allows the opportunity to block the drivers' view of vehicles along the portion of the roundabout opposing the entering approach. The entering drivers' attention should then be focused on 1) approaching vehicles from the left already in the roundabout's circular traffic stream. 2) making an entering right turn upon accepting a suitable gap and 3) maintaining a low speed once within the circulatory roadway. Others suggest that the more visible all surrounding elements of the roundabout, the better able the driver should be to negotiate its configuration, exit at the appropriate location, and view surrounding pedestrians and bicyclists in the process. The results obtained in a simple before-and-after central island landscaping study conducted at the first arterial roundabout constructed in the state of Nebraska indicated that a landscape treatment with significant vertical dimension (three 7-ft high by 5-ft diameter Black Hill spruce trees) and partial cross-view blockage had a positive impact on operations. A statistically significant speed reduction resulted in approach vehicles from as far as 150 ft from the pedestrian crossings. Slower vehicular speeds would inherently reduce severity of pedestrian, bicycle, and vehicular accidents (if they occurred), increasing safety. The standard deviation from the mean vehicle speeds of nearly all approaches were reduced in the after period (with trees) compared to the before period (grass only) which indicated that vehicle speeds were not only slower but more uniform. Data compared from the before and after conditions also proved that drivers entering the circulatory roadway accepted shorter gaps due to slower circulatory traffic which increased the effective capacity of the roundabout since more vehicles could occupy the circulatory roadway at once. The results of this study indicated that at least partial cross-view sight blockage had both positive operational and safety effects.
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