1 - HKU Libraries

T.P.D.M.V.2.4
4*5.15 Inscribed Circle Diameter
4,5.15.1 The inscribed circle diameter, as shown in Appendix 2 Pig. 1,
should be large enough to accommodate the swept turning path of
the design vehicle which may vary in accordance with the
location and expected classes of traffic. Taking the worst
case, a 16.0m long articulated vehicle with a single axle at the
rear of the trailer, the smallest inscribed circle diameter for
a normal roundabout is 28m. If this cannot be accommodated, a
mini roundabout should be used. It should be noted however that
the requirements of adequate vehicle deflection at normal
roundabouts will generally dictate the minimum inscribed circle
diameter.
4*5.15*2 Based on the design vehicle above, as a guide, the turning space
requirements for normal roundabouts from 28m to 36m inscribed
circle diameter are shown in Drg, No. 4.5.15.1. For diameters
greater than 36m the circulatory carriageway widths should be
checked against Table 4.3*15.1 in the section on Priority
Junctions.