 ### Design procedure of Superelevation Banking of road

Design procedure includes following steps:

Step 1:-
Calculate the Superelevation (e) necessary for 75% design speed[0.75V] and assume no lateral friction is developed

As per practical conditions, it is suggested that superelevation should be provided to fully counteract the centrifugal force due to 75% of the Design Speed(V) by neglecting lateral friction (f=0) developed.

Superelevation formula = e + f = [V2]/127R

Consider lateral friction f = 0;

75% of design speed which means V= 0.75V

As per formula,

e + f = [V2]/127R

e + 0 = [0.75V]2/127R

e = V2 / 225R

Note: e value should not be more than 0.07

If the calculated value (ecal) is less than (emax) then consider the value of (ecal) if not proceed to next step,

if  (ecal<emax) then e = ecal

If the calculated value (ecal) is greater than (emax) then consider the value of (emax)

if  (ecal<emax) then e = emax

Step 2:-

Let's assume e = emax = 0.07 and proceed to next step.

Step 3:-

Calculate the value of friction for ‘e’ Value

Superelevation formula = e + f = [V2]/127R

= 0.07 +f = [V2]/127R

f = [V2]/127R – 0.07

Step 4:-
From the above step we found the value of e. Now lets find the value of the lateral friction (f) for the known value of emax Value

Superelevation = emax + f = [V2]/127R
= 0.07 + f = [V2]/127R
fcal   = [V2/127R] – 0.07
If fcal < fmax (0.15)
Then fcal = f;

If fcal  > fmax (0.15)

Then e = 0.07 is safe.

But if fcal > 0.15

Then fcal =fmax (0.15)

Step 4:-

Lets calculate the value of Restricted Speed (Va)

Therefore V = Va

Super Elevation = e + f = [V2]/127R

= 0.07 + 0.15 = (Va)2/127R

V = [127R(0.22)]½

If Va > V then e = 0.07, f = 0.15

If Va < V still e = 0.07 and f = 0.15 but the speed restriction board is provided along with consists the value of Va

NOTE:

1. It may be seen that the contribution of super e;evation “e” (.007) is much less than that of lateral friction (0.15) in counteracting centrifugal force.
2. In some situations eg at intersection or some obstruction it is not possible superelevation in such cases friction counteracts the centrifugal force & if friction is also sufficient then radius of curve is increased , which if it is not possible speed of vehicle negotiation. ​​​​​​​

From drainage consideration it is necessary to have a min cross slope to drain of surface water.

If calculated e is less than cross slope of the road, then the crown and negative superelevation on outeredge.

In very flat curve with large radius, having less value of Pc & negative superelevation may be permitted on outer edge

As per IRC radius of curve  beyond which "e" is required is as follows;