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 blockage of the drivers view of vehicles along the portion of the roundabout opposing the entering approach. The entering drivers attention should then be totally focused upon 1) approaching vehicles already in the roundabouts circular traffic stream, 2) making an entering turn upon accepting a suitable gap and 3) maintaining an appropriate speed once within the circulatory roadway.ore 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 cyclists in the process. Operational and safety improvements could result from identifying if reducing the drivers sight distance across the center island improves or deteriorates operations at a roundabout. The results obtained in a before-and-after study conducted at the first arterial roundabout constructed in Lincoln, Nebraska in the Midwestern region of the United States 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 as well as vehicles within the circulatory roadway. Slower vehicular speeds reduce severity of both pedestrian and vehicular accidents (if they occurred), potentially increasing overall safety of the intersection. 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 likely due to slower circulatory traffic which increased the effective capacity of the roundabout since more vehicles could occupy the circulatory roadway at once. This before-after study was limited by the fact that data was collected at only one location due to the unusually long lag time between the opening of the roundabout (no landscaping) and the landscaping which occurred nearly four months later. With additional research, it may be possible to determine the optimal amount of landscaping that would maximize the circulatory roadway speed while minimizing the critical entry gap. This condition would provide the greatest potential capacity according to critical gap theory. The results of this study indicated that at least partial cross-view sight blockage had both positive operational and safety effects. For the covering abstract see ITRD E137145.
Abstract