be used at the discretion of the designer, having taken into consideration appro-
priate local factors and advice in relevant documentation.
Departures
In situations of exceptional difficulty where even a move to the second tier in
the hierarchy, i.e. relaxations, cannot resolve the situation, adoption of a value
within the third tier of the hierarchy – a departure – may have to be considered.
Care must be taken that safety is not significantly reduced. In order for a depar-
ture from standard to be adopted for a major road scheme, the designer must
receive formal approval from central government or its responsible agency
before it can be incorporated into the design layout.
6.4 Sight distances,
6.4.1 Introduction
Sight distance is defined as the length of carriageway that the driver can see in
both the horizontal and vertical planes. Two types of sight distance are detailed:
stopping distance and overtaking distance.
6.4.2 Stopping sight distance
This is defined as the minimum sight distance required by the driver in order to
be able to stop the car before it hits an object on the highway. It is of primary
importance to the safe working of a highway.
Table 6.7 indicates the stopping sight distances for the different design speeds.
Both desirable minimum and absolute minimum values are given. As seen, the
latter category constitutes in each case a relaxation equal to one design speed
step below the desirable minimum.
The standard TD 9/93 requires stopping sight distance to be measured from
a driver’s eye height of between 1.05 m and 2 m above the surface of the highway
to an object height of between 0.26 m and 2 m above it. These values ensure that
drivers of low-level cars can see small objects on the carriageway ahead. The
Geometric Alignment and Design 163
Design speed (km/hr)
Stopping sight distance (m) 120 100 85 70 60 50
Desirable minimum 295 215 160 120 90 70
Absolute minimum 215 160 120 90 70 50
Table 6.7Stopping sight distances for different design speeds (source: TD 9/93 (DoT, 1993))
