Highway Engineering

Geometric Alignment and Design 161

Example 6.1 Contd

Given a 7.3 m wide single carriageway road with a verge width of 1.5 m,
Table 6.4 gives a layout constraint value of 25.

Lc= 25

Bendiness =(95 + 65 +60)/4

=235/4

=55°

The verge width (VW) is set at 1.5 m.
Therefore, the harmonic visibility (VISI) is calculated using Equation 6.3:
Log 10 VISI =2.46 +VW/25 -B/400
=2.46 +1.5/25 -55/400
=2.46 +0.06 -0.1375
=2.3825
VISI =241.27

The alignment constraint is then calculated using Equation 6.2:

Ac= 12 - VISI/60 +2B/45

= 12 - 241.27/60 +(2 ¥55)/45

=10.4

From Fig. 6.6, a design speed of 100 km/hr is selected (the mandatory speed
limit for this class of highway is 96 km/hr).

Example 6.2 – Comparison of observed speeds with calculated design speed
Taking the existing stretch of highway referred to in Example 6.1, Table 6.5
shows the results from a speed survey taken along the route.

Speed range (km/hr) Observed cars

Less than 60 15

60–64 10

65–69 16

70–74 101

75–79 140

80–84 196

85–89 62

90–94 15

95–99 6

Greater than 100 1

Table 6.5Speed survey

Contd