Central island
The central island of a roundabout is the raised, nontraversable area encompassed by the circulatory roadway; this area may also include a traversable apron. The island is typically landscaped for aesthetic reasons and to enhance driver recognition of the roundabout upon approach. Central islands should always be raised, not depressed, as depressed islands are difficult for approaching drivers to recognize.
In general, the central island should be circular in shape. A circular-shaped central island with a constant-radius circulatory roadway helps promote constant speeds around the central island. Oval or irregular shapes, on the other hand, are more difficult to drive and can promote higher speeds on the straight sections and reduced speeds on the arcs of the oval. This speed differential may make it harder for entering vehicles to judge the speed and acceptability of gaps in the circulatory traffic stream. It can also be deceptive to circulating drivers, leading to more loss of- control crashes. Noncircular central islands have the above disadvantages to a rapidly increasing degree as they get larger because circulating speeds are higher. Oval shapes are generally not such a problem if they are relatively small and speeds are low. Raindrop-shaped islands may be used in areas where certain movements do not exist, such as interchanges (see Chapter 8), or at locations where certain turning movements cannot be safely accommodated, such as roundabouts with one approach on a relatively steep grade.
As described in Section 6.2.1, the size of the central island plays a key role in determining the amount of deflection imposed on the through vehicle’s path. However, its diameter is entirely dependent upon the inscribed circle diameter and the required circulatory roadway width (see Sections 6.3.1 and 6.3.3, respectively). Therefore, once the inscribed diameter, circulatory roadway width, and initial entry geometry have been established, the fastest vehicle path must be drawn though the layout, as described in Section 6.2.1.3, to determine if the central island size is adequate. If the fastest path exceeds the design speed, the central island size may need to be increased, thus increasing the overall inscribed circle diameter. There may be other methods for increasing deflection without increasing the inscribed diameter, such as offsetting the approach alignment to the left, reducing the entry width, or reducing the entry radius. These treatments, however, may preclude the ability to accommodate the design vehicle.
In cases where right-of-way, topography, or other constraints preclude the ability to expand the inscribed circle diameter, a mountable apron may be added to the outer edge of the central island. This provides additional paved area to allow the over-tracking of large semi-trailer vehicles on the central island without compromising the deflection for smaller vehicles. Exhibit 6-23 shows a typical central island with a traversable apron.
Where aprons are used, they should be designed so that they are traversable by trucks, but discourage passenger vehicles from using them. They should generally be 1 to 4 m (3 to 13 ft) wide and have a cross slope of 3 to 4 percent away from the central island. To discourage use by passenger vehicles, the outer edge of the apron should be raised a minimum of 30 mm (1.2 in) above the circulatory roadway surface (6). The apron should be constructed of colored and/or textured paving materials to differentiate it from the circulatory roadway. Care must be taken to ensure that delivery trucks will not experience load shifting as their rear trailer wheels track across the apron.
Issues regarding landscaping and other treatments within the central island are discussed in Chapter 7.
In general, roundabouts in rural environments typically need larger central islands than urban roundabouts in order to enhance their visibility and to enable the design of better approach geometry (2).