Circulatory roadway width
The required width of the circulatory roadway is determined from the width of the entries and the turning requirements of the design vehicle. In general, it should always be at least as wide as the maximum entry width (up to 120 percent of the maximum entry width) and should remain constant throughout the roundabout (3).
Single-lane roundabouts
At single-lane roundabouts, the circulatory roadway should just accommodate the design vehicle. Appropriate vehicle-turning templates or a CAD-based computer program should be used to determine the swept path of the design vehicle through each of the turning movements. Usually the left-turn movement is the critical path for determining circulatory roadway width. In accordance with AASHTO policy, a minimum clearance of 0.6 m (2 ft) should be provided between the outside edge of the vehicle’s tire track and the curb line. AASHTO Table III-19 (1994 edition) provides derived widths required for various radii for each standard design vehicle.
In some cases (particularly where the inscribed diameter is small or the design vehicle is large) the turning requirements of the design vehicle may dictate that the circulatory roadway be so wide that the amount of deflection necessary to slow passenger vehicles is compromised. In such cases, the circulatory roadway width can be reduced and a truck apron, placed behind a mountable curb on the central island, can be used to accommodate larger vehicles. However, truck aprons generally provide a lower level of operation than standard nonmountable islands. They are sometimes driven over by four-wheel drive automobiles, may surprise inattentive motorcyclists, and can cause load shifting on trucks. They should, therefore, be used only when there is no other means of providing adequate deflection while accommodating the design vehicle.
Double-lane roundabouts
At double-lane roundabouts, the circulatory roadway width is usually not governed by the design vehicle. The width required for one, two, or three vehicles, depending on the number of lanes at the widest entry, to travel simultaneously through the roundabout should be used to establish the circulatory roadway width. The combination of vehicle types to be accommodated side-by-side is dependent upon the specific traffic conditions at each site. If the entering traffic is predominantly passenger cars and single-unit trucks (AASHTO P and SU vehicles), where semitrailer traffic is infrequent, it may be appropriate to design the width for two passenger vehicles or a passenger car and a single-unit truck side-by-side. If semitrailer traffic is relatively frequent (greater than 10 percent), it may be necessary to provide sufficient width for the simultaneous passage of a semi-trailer in combination with a P or SU vehicle.
Exhibit 6-22 provides minimum recommended circulatory roadway widths for two lane roundabouts where semi-trailer traffic is relatively infrequent.
