Driver behavior and geometric elements
A roundabout brings together conflicting traffic streams, allows the streams to safely merge and traverse the roundabout, and exit the streams to their desired directions. The geometric elements of the roundabout provide guidance to drivers approaching, entering, and traveling through a roundabout.
Drivers approaching a roundabout must slow to a speed that will allow them to safely interact with other users of the roundabout, and to negotiate the roundabout. The width of the approach roadway, the curvature of the roadway, and the volume of traffic present on the approach govern this speed. As drivers approach the yield line, they must check for conflicting vehicles already on the circulating roadway and determine when it is safe and prudent to enter the circulating stream. The widths of the approach roadway and entry determine the number of vehicle streams that may form side by side at the yield line and govern the rate at which vehicles may enter the circulating roadway. The size of the inscribed circle affects the radius of the driver’s path, which in turn determines the speed at which drivers travel on the roundabout. The width of the circulatory roadway determines the number of vehicles that may travel side by side on the roundabout.
The British (2), French (3), and German (4) analytical procedures are based on empirical relationships that directly relate capacity to both traffic characteristics and roundabout geometry. The British empirical relationships reveal that small sublane changes in the geometric parameters produce significant changes in capacity.
For instance, if some approaches are flared or have additional short lanes, these provide considerably more capacity for two reasons. First, wider entries require wider circulatory roadway widths. This provides for more opportunities for the circulatory traffic to bunch together, thus increasing the number of acceptable opportunities to enter, thereby increasing capacity. Second, the typical size of groups of drivers entering into acceptable opportunities in the circulatory traffic is quite small, so short lanes can be very effective in increasing group sizes, because the short lane is frequently able to be filled.
The British (2) use the inscribed circle diameter, the entry width, the approach (road) half width, the entry radius, and the sharpness of the flare to define the performance of a roundabout. The sharpness of the flare, S, is a measure of the rate at which the extra width is developed in the entry flare. Large values of S correspond to short, severe flares, and small values of S correspond to long, gradual flares (5).
The results of the extensive empirical British research indicate that approach half width, entry width, average effective flare length and entry angle have the most significant effect on entry capacity. Roundabouts fit into two general classes: those with a small inscribed circle diameter of less than 50 m (165 ft.) and those with a diameter above 50 m. The British relationships provide a means of including both of these roundabout types. The inscribed circle diameter has a relatively small effect for inscribed diameters of 50 m (165 ft) or less. The entry radius has little effect on capacity provided that it is 20 m (65 ft) or more. The use of perpendicular entries (70 degrees or more) and small entry radii (less than 15 m [50 ft]) will reduce capacity. The presence of the geometric parameters in the British and French models allow designers to manipulate elements of their design to determine both their operational and safety effects. German research has not been able to find the same influence of geometry, although this may be due to the relatively narrow range of geometries in Germany (4).
Thus, the geometric elements of a roundabout, together with the volume of traffic desiring to use a roundabout at a given time, may determine the efficiency with which a roundabout operates.