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If you drive on the urban street or high-speed roads, you must have noticed the road camber, which means where the road edges are slightly lower than the centre white line. Similarly, along the bends, the outer road edge is slightly higher than the inner edge.
The provision of this road cross fall is called camber and it is provided for the following purposes:
Cambers are provided in the following situations:
Normal Camber or Normal Cross Slopes are provided when the road is designed as a straight section and the centreline stays above the carriageway edges as shown in the picture below:
In this situation, the preferred cross fall of the road is 2.5% and we call it a Normal Camber situation.
The normal camber or normal cross slopes are also called Transverse Slope in some countries. When the longitudinal fall of the road is very flat, the surface water moves towards the edges but due to lack of long fall, it may not flow towards the next gully pot. That’s why it is always recommended to provide a minimum long fall of 0.5% to maintain surface water drainage flow.
Adverse Camber or Superelevation is provided when the road passes through a horizontal curve section. In this situation, the outer edge of the carriageway is designed at a super elevated level to compare to the inner edge. The level of superelevation is measured in terms of % age of cross fall or cross slopes and it is dependent on the speed of the road and radius of the horizontal curvatures.
For higher speed roads, it is recommended to provide larger radii so that the requirement of superelevation can be minimised. This concept is known as “Elimination of Adverse Camber”.
A high degree of superelevation may not provide a comfortable ride to the road users, that’s why there is a direct link of the degree of superelevation with speed, horizontal radii and departure from the standard. Similarly, a low level of superelevation would not be able to balance the centripetal and centrifugal forces, as a result, the vehicle may skid out if driving at slightly higher than the posted speed limit.
Provision of substandard superelevation is not a good practice and may not be accepted by the highway authority. The designer would need to prepare a departure report to justify the design if a substandard superelevation is proposed in the alignment design. If mitigation measures are not adequate enough, then the client can reject the design and designers need to go back to the drawing board.
The rolling crown is known as a value engineering solution generally adopted after finding flat spot issues in a newly constructed road along the carriageway. These types of flat spots are generally found when the road passes through a back to back curves in the opposite direction.
On a high-speed motorway or national road, accumulation of little water on the road surface, would create a high risk to the vehicles due to the slip or skid of the vehicle and resulting in loss of control and or collision with adjacent vehicles or road assets like road restraint system, lighting columns, etc and lead to severe fatalities.
So, the Rolling crown mitigates this risk. the rolling crown is developed within the same carriageway from the inner edge to the outer edge in a diagonal direction. See the picture below for a better understanding. Here you can compare both pictures and see how flat spots are removed and a rolling crown is introduced.
The contour diagram shown here demonstrates the location which is flat in the first picture is now transformed with a crown line.
The gcelab.com provides an online course of highway engineering. The course shares a detailed design process of the highway, junction, roundabout, etc. The course also explains the details of Camber and Superelevation and how it is applied against the design standards.
Please click here to see that in detail.
Also, there is a free view of the same course on our YouTube channel.
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