Table 6.8 should be adhered to. Where overtaking is not permitted, sight dis-
tances should not greatly exceed those required for safe stopping.
6.5 Horizontal alignment,
6.5.1 General
Horizontal alignment deals with the design of the directional transition of the
highway in a horizontal plane. A horizontal alignment consists, in its most basic
form,ofa horizontal arc and two transition curves forming a curve which joins
two straights. In certain situations the transition curve may have zero length.
The design procedure itself must commence with fixing the position of the two
straight lines which the curve will join together. The basic parameter relating
these two lines is the intersection angle. Figure 6.10 indicates a typical horizon-
tal alignment.
Minimum permitted horizontal radii depend on the design speed and the
superelevation of the carriageway, which has a maximum allowable value of 7%
in the UK, with designs in most cases using a value of 5%. The relationship
between superelevation, design speed and horizontal curvature is detailed in the
subsection below.
Geometric Alignment and Design 167
Straight Straight
Transition Transition
Arc
Intersection angle
Figure 6.10Typical
horizontal alignment.
