1 - HKU Libraries

T.P.D.M. V.2.3
(Al/2001)
3.11.4 Horizontal Alignment
3.11.4.1 Whilst topography will often pose a constraint the alignment should be as straight as
possible to maintain sightlines and reduce the need for passing bays. It has been found that
one of the most efficient layouts is one where the bends are incorporated into passing bays
which should be suitably widened and/or lengthened.
3.11.4.2 For curves other than hairpin bends mentioned in 3.11.4.3 the minimum radius of curvature
measured along the inner edge of the carriageway are:
For50km/h - 44m
For30km/h - 30m
3.11.4.3 At some locations it may be necessary to provide hairpin bends. Where these are provided
on steep roads the gradient should be reduced through the bend. The inner radius maybe
zero though the outer radius should be sufficient for the largest vehicle likely to use the
road. Provision should be made at or near the bend for vehicles to pass.
3.11.4.4 Transition spirals are not necessary and any widening should be applied along the outer
edge of the carriageway.
3.11.4.5 Single track roads should have a crossfall of 2.5%. On bends the crossfall should normally
be such as to provide a superelevation of 2.5% though in some cases there may be a clear
case on safety grounds to increase the superelevation, in particular on steep roads with sharp
bends.
3.11.4.6 Superelevation should be applied over the section of road immediately prior to the
commencement of the inner horizontal curve. The rate of change of crossfall should be
between I%and2%.