Highway Engineering

Ver tical alignment

The preferred location for a priority intersection is on level terrain, or where the

gradient of the approach roads does not exceed an uphill or downhill gradient

of 2%. Downhill gradients greater than this figure induce excessively high

speeds, while uphill approaches prevent the drivers from appreciating the layout

of the junction.

Visibility

Along the major and minor roads, approaching traffic should be able to

see the minor road entry from a distance equal to the desirable minimum

sight stopping distance (SSD). The required SSD depends on the chosen

design speed and varies from 70 m for a design speed of 50 km/h to 295 m

for 120 km/h. (Further details on sight stopping distances are given in Chapter

6.)

In addition, on the minor road, from a distance xmetres back along the centre

line of the road, measured from a continuation of the line of the nearside edge

of the running carriageway of the major road, the approaching driver should

be able to see clearly points on the left and right on the nearside edge of the

major road running carriageway a distance yaway. The xvalue is set at 9 m.

The yvalue varies depending on the chosen design speed along the major road

(Table 5.2).

118 Highway Engineering

Design speed along major road (km/h) ydistance (metres)

50 70

60 90

70 120

85 160

100 215

120 295

Table 5.2yvisibility

distances from the

minor road (see Fig.

5.14 for an illustration

ofxand y)

Dedicated lane on major road for right-turning vehicles

In the case of all non-simple junctions, provision of a right-turning lane, the

lane itself should not be less than 3 m. It requires the provision of a turning

length, a deceleration length and a direct taper length. Figure 5.14 illustrates

these parameters for a ghost island junction. The turning length (a) is set at

10 m; the deceleration length (b) varies from 25 m to 110 m depending on the

design speed on the major route. The taper (c) varies from 1 in 20 to 1 in 30 for

ghost island/single lane dualling intersections. (Greater values are required for

dual carriageway intersections.)