What Is Superelevation

• When a vehicle is moving on a curved path it is subjected to an outward force known as centrifugal force. In order to counteract the effect of centrifugal force, the outer edge of the pavement is raised with respect to the inner edge. This transverse inclination provided to the pavement surface is known as Superelevation.

• Superelevation is the transverse slope provided to counteract the effect of centrifugal force and reduce the tendency of the vehicle to overturn and to skid laterally outward by raising the pavement’s outer edge with respect to the inner edge.
• Superelevation is represented by “e”

Purpose of Providing Superelevation

1.  To counteract the effect of centrifugal force acting on the moving vehicle to pull out the same outward on a horizontal curve.
2. To help a fast-moving vehicle negotiate a curved path without overturning and skidding.
3. To ensure safety to the fast-moving traffic.
4. To Prevent damaging effects on the road surface due to improper distribution of load.

Advantages of Superelevation

1. It allows the running of the vehicle at high speed on a curved path as on a straight path without any danger of overturning and thus results in an increased volume of traffic.
2. It provides more or less even distribution of load on wheels and hence uniform stress is offered on the foundation which results in less wear on tires and springs as well as the economy in maintenance cost of the road.
3. It also helps to keep the vehicles to their proper side on the pavement and thus prevent collision of vehicles moving in opposite directions on a curved portion of the road.
4. It provides drainage of the whole width of the road towards the inner side. thus, there is no necessity of providing a side drain on the outer side of the road.

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Design of superelevation

• To design superelevation for the mixed type of traffic is a complex problem.
• The Indian Road Congress has recommended providing superelevation to counteract the centrifugal forcefully due to 75% of the design speed and limiting the maximum superelevation to 1 in 15 or 7%.
• Following are the steps adopted to design the superelevation of a road.

Step 1

Calculate the superelevation for 75% of design speed neglecting the friction i.e. f=0

V = 75%(V) = 0.75 V

We know that,  e + f = V2 / 127R

e + 0 = (0.75V2) / 127R

e = V2 / 225R

Step 2

if the calculated value ‘e’ is less than 7% or 0.07, then provide the obtained value.

if it exceeds 0.07 then provide the limiting value of superelevation emax = 0.07.

Step 3

Check the coefficient of friction developed for the maximum value of e = 0.07 at the full value of design speed,

e + f = V2 / gR  = V2 / 127 R

f = ( V2/gR – 0.07 ) = ( V2/127 R – 0.07 )

If the value of ‘f’ thus calculated is less than 0.15, the superelevation of 0.07 is safe for the design speed.

Step 4

If the calculated value of ‘f’ exceed 0.15,then the speed of the vehicle is restricted to the value Vr m/sec or Vr km/sec as calculated after allowing the limiting values of e and f in the following equation.

e + f =0.07 + 0.15 = 0.22 = (Vr)2 / gR

( Vr )2 / 127 R = 0.22

Vr = (     127 R ( 0.22 )    )1/2

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