Highway Engineering

176 Highway Engineering

Example 6.5

A transition curve is required for a single carriageway road with a design

speed of 85 km/hr. The bearings of the two straights in question are 17° and

59° (see Fig. 6.16). Assume a value of 0.3 m/s^3 for C.

Calculate the following:

(1) The transition length,L

(2) The shift,S

(3) The length along the tangent required from the intersection point to the

start of the transition, IT

(4) The form of the cubic parabola and the co-ordinates of the point at

which the transition becomes the circular arc of radius R.

Contd

q

Straight No.1

Straight No.2

Bearing = 17∞

Bearing = 59∞

Figure 6.16

Intersection angle q

between straights.

Solution

The design speed is 85 km/hr, therefore the desirable minimum radius is

510 m, assuming superelevation of 5%.

Length of transition:

Using Equation 6.25:

L=(85)^3 /(3.6^3 ¥0.3 ¥510)

=86.03 m

Note: Equation 6.26 dictates that the transition be no longer than (24R)0.5.

In this case:

Therefore the derived length is less than the maximum permissible value.

LRmax==¥= 24 24 510 110 6 m > 86.03 m.