When a vehicle navigates a curved section of a road, it experiences centrifugal force– an outward force that is influenced by a vehicle’s speed and the sharpness of the curve, which may cause skidding or overturning. As the speed increases or the curve radius decreases, the risk to the driver increases.

Therefore, it is crucial to implement proper superelevation, also known as road banking, in horizontal curves designed. When a road is said to be superelevated, the surface is tilted towards the inner side of the curve, utilizing gravity to counteract the centrifugal forces- basically helping the vehicle to embrace the curve rather than resisting it.

There are three primary purposes of implementing superelevation:

(1) To enhance stability by reducing the chances of vehicles skidding off the road or overturning at high speeds

(2) For passengers’ comfort as the banking will reduce the effect of sideways “pull” and provide a more natural feel while traversing the curve, and

(3) For surface drainage, preventing waterlogging on the pavement as a tilted surface easily channels the runoff into the proper roadside drainage system.

Components of a superelevation #

A horizontal Curve is made up of three main sections, which are the Entry (Into the curve), the Main Curve, and the Exit (Out of the curve).

To ensure a smooth entrance to the curve or exiting out of the curve, engineers will be required to design the transition lengths (this is the distance required to transition from standard Normal Crown to the Full Superelevation), which consist of the runout length and the runoff length.

Key Phases in Superelevation #

In our road module, the road transitions from a standard Normal Crown (NC) to a Full Superelevation (FS) or Partial Superelevation (PS) through a series of defined phases, which can be observed in the provided Superelevation Report.

Carriageway #

Normal Crown #

Sections of the road that follow the typical crossfall of 2.5%, sloping downward from the centerline.

Level Crown #

Section of the road where the higher (outer) side of the carriageway is rotated to 0%, while the lower (inner) side of the carriageway is maintained at the typical crossfall of 2.5%.

Reverse Crown #

Section of the road where the higher (outer) side of the carriageway is rotated upward, opposite to the normal crown (which is the lower side of the carriageway sloping down from the centerline).

Start/End Superelevation #

The entire section of the road is rotated to the maximum design rate, e_{max}, based on the design radius and design speed.

Partial Superelevation #

The partial superelevation section occurs during the Runoff Length, where the crossfall is always greater than the Reverse Crown (+2.5%), but has not yet reached the designed Maximum Superelevation ( e_{max}).

This design scenario typically occurs on wide, gentle curves where the full superelevation is not needed for stability, and the transition only requires partial superelevation to maintain the comfort and stability of the vehicles.

As seen from the cross-section above, the crossfall at the curve only reaches 4.76% instead of the maximum superelevation rate of 6.00% before transitioning back to Reverse Crown.

Shoulder #

Normal Shoulder #

Sections of the road where the shoulders are sloping down from the adjacent carriageways, following the default crossfall of 4.00%.

Begin/End Shoulder Rollover #

Points where the slope shoulder is adjusted (raised to a value just enough) to maintain the Maximum Shoulder Rollover, reducing the dip between shoulders and adjacent carriageway due to differences in the slopes.

Low Shoulder Match #

Section where the crossfall of the shoulder on the lower side of the carriageway is adjusted to match the crossfall of the adjacent carriageway.

Benchmarking of the superelevation phases #

Detailed computation of the superelevation phases for both carriageway and shoulders can be referred to the related articles below.

• What is Transition Length, Road Runoff, and Road Runout on Tangent

• MiTS VS Civil 3D: Shoulder Slope Treatment in Road Superelevation