Design of Horizontal Alignment Super Elevation

In order to resist the centrifugal force, it is usual practice to super elevated the roadway cross section. The horizontal alignment design elements include,

• Super Elevation 
• Transition Curve
• Extra-widening 
• Set Back Distance

Super Elevation : 

When a vehicle moves on a horizontal curve then it is subjected to an outward force, commonly known as centrifugal force acting in outward direction which causes discomfort to the passenger and increases the possibility of accidents. So, design of horizontal alignment is an important feature which influence the efficiency and safety of highway. The design speed is the single most important factor which governs the design of horizontal alignment. 

We know,

Presence of horizontal curve in horizontal alignment introduces centrifugal force in outward direction of curve,

So the effect of centrifugal force on the vehicle are:

1. Overturning of Vehicle
2. Lateral Skidding
3. Combination of Overturning and Lateral Skidding 

From the above figure,

Overturning Moment is, Mo = P x h

and

Resisting Moment is, Mr = W x (b/2)

Conditions :

1.  When the moment of resistant (Mr) is greeter than moment of overturning  (Mo) then it is safe or stable condition for overturning, or if we put the values in condition we can say that in safe condition impact factor (I) is less than (b/2h). 
2.  When the impact factor (I) is less than lateral friction (F) the it is safe condition for lateral skidding.

Rising of outer edge wit respect to inner edge to counter balance the centrifugal factor and to reduce tendency of vehicle to overturn is called Super elevation.

where, 

W = wight of vehicle

F  = Friction force between wheels and pavements

θ = Transverse slope due to super-elevation 

Super-elevation by IRC recommendation,

Sr No	Terrain	Max Super elevation
1	Urban Area	4 % or 0.04
2	Plain & Rolling Terrain	7 % or 0.07
3	Steep & Mountainous Terrain	10 % or 0.10