Highway Engineering

measured for each section of the highway over a minimum distance of 2 km.

The design speed calculated from the resulting alignment and layout constraints

is then checked against the originally assumed design speed so the locations can

be identified where elements of the initially assumed alignment can be relaxed

in order to achieve savings in terms of either cost or the environment. This pro-

cedure allows a design speed to be finalised for each section of highway under

consideration. Equally, it may be necessary to upgrade the design if the result-

ing design speed dictates this. If any alterations to the geometry of the highway

are undertaken, it will be necessary to recalculate the design speed in order to

make sure that its value has not changed. The aim of this process is to ensure

that all sections of the highway are both geometrically consistent and cost

effective. While the design speeds for two sections running into each other need

not be the same, it is advisable that their design speeds differ by no more than

10 km/hr.

160 Highway Engineering

Example 6.1 – Design speed calculation for an existing single carriageway route

An existing 7.3 m wide single carriageway road with 1.5 m wide verges (see

Fig. 6.7) has a layout as indicated in Fig. 6.5. The length of the section of

highway under examination is 4 km (X to Y). The relevant angles are:

A =95°

B =65°

C =60°

There are a total of 24 access points and minor junctions along the length of

the highway.

Calculate the design speed.

1.5 m 7.3 m 1.5 m

Figure 6.7Cross-section of highway.

Solution

There are 24 access points along the 4 km length of roadway, therefore they

occur at a rate of 6 per kilometre, giving a grading of medium.

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