Inscribed circle diameter
The inscribed circle diameter is the distance across the circle inscribed by the outer curb (or edge) of the circulatory roadway. As illustrated in Exhibit 6-1, it is the sum of the central island diameter (which includes the apron, if present) and twice the circulatory roadway. The inscribed circle diameter is determined by a number of design objectives. The designer often has to experiment with varying diameters before determining the optimal size at a given location.
At single-lane roundabouts, the size of the inscribed circle is largely dependent upon the turning requirements of the design vehicle. The diameter must be large enough to accommodate the design vehicle while maintaining adequate deflection curvature to ensure safe travel speeds for smaller vehicles. However, the circulatory roadway width, entry and exit widths, entry and exit radii, and entry and exit angles also play a significant role in accommodating the design vehicle and providing deflection. Careful selection of these geometric elements may allow a smaller inscribed circle diameter to be used in constrained locations. In general, the inscribed circle diameter should be a minimum of 30 m (100 ft) to accommodate a WB-15 (WB-50) design vehicle. Smaller roundabouts can be used for some local street or collector street intersections, where the design vehicle may be a bus or single-unit truck.
At double-lane roundabouts, accommodating the design vehicle is usually not a constraint. The size of the roundabout is usually determined either by the need to achieve deflection or by the need to fit the entries and exits around the circumference with reasonable entry and exit radii between them. Generally, the inscribed circle diameter of a double-lane roundabout should be a minimum of 45 m (150 ft).
In general, smaller inscribed diameters are better for overall safety because they help to maintain lower speeds. In high-speed environments, however, the design of the approach geometry is more critical than in low-speed environments. Larger inscribed diameters generally allow for the provision of better approach geometry, which leads to a decrease in vehicle approach speeds. Larger inscribed diameters also reduce the angle formed between entering and circulating vehicle paths, thereby reducing the relative speed between these vehicles and leading to reduced entering- circulating crash rates (2). Therefore, roundabouts in high-speed environments may require diameters that are somewhat larger than those recommended for low-speed environments. Very large diameters (greater than 60 m [200 ft]), however, should generally not be used because they will have high circulating speeds and more crashes with greater severity. Exhibit 6-19 provides recommended ranges of inscribed circle diameters for various site locations.
