Citation

Todd K. Transp. Res. Rec. 1979; 737: 61-72.

Copyright

(Copyright © 1979, Transportation Research Board, National Research Council, National Academy of Sciences USA, Publisher SAGE Publishing)

DOI

unavailable

PMID

unavailable

Abstract

Although many rotaries, large and small, were constructed in the United States before World War II, they have received little attention in American literature in recent years. The present paper describes British developments in this field. In Britain, where rotaries (or "roundabouts") are a common form of intersection control, the capacity of large, conventional rotaries has been increased by 20-50 percent by reducing the size of the center island and enlarging the space available at the point of entry into the circulatory roadway. Rotaries need not be large to achieve high capacity. Small designs have been developed that generally have operating and safety characteristics superior to those of conventional alternatives, particularly when their geometry is devised to reduce approach speed and facilitate gap acceptance. At previously signalized intersections, capacity increases of 5-30 percent were obtained. Three-leg (T and Y) rotaries gave the best results for safety and capacity. Pedestrian and other fatal and injury accidents decreased substantially at sites converted from other forms of control, though an increase in property-damage-only incidents was observed at several locations. However, except at three-leg sites, fatal and injury accidents almost doubled after the center islands of conventional rotaries were reduced in size. Considerable savings in right-of-way acquisition and construction costs can be obtained on arterials, where the need for additional lanes to increase road capacity is greatly reduced by the use of rotaries. Rotaries can also reduce cost and land take when used as alternatives to, or in conjunction with, grade separation. The rotary concept can be employed in a grid of one-way streets, where blocks form rectangular center islands.