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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:**

- 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.
- 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;

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