You must have driven on steep uphill or downhill roads, but do you know what is road gradient and why it is provided?
Please read this interesting blog for details.
When the vertical levels of a road are not the same between two points, it is termed as the longitudinal slope of the road, and the rate of change of road level is termed as Road Gradient. In Simple terms, Road Gradient means Steepness along the road. In hilly terrain, road gradients are steeper than flat terrain due to the local topography of the area. so that’s raised a question why Road Gradients are provided?
Why Road Gradients are Provided?
Roads are built to connect two or more destinations and it is preferred to develop road gradient by following more or less existing topography to minimise significant cutting or filling.
For example, in hilly terrain, the road gradient is designed and constructed to align with the existing ground profile, but efforts are made to comply with the desirable maximum gradient. As a result, road gradients are generally steeper (5% to 8%).
In flat terrain (Urban streets or highways), the road gradients are designed and constructed to first align with the existing ground profile and second to drain out surface water through adequate drainage system like Pipe /Gully or Kerb Drains. It is important to note that a minimum road gradient of 0.5% is maintained for completely flat terrain to provide an adequate drainage system.
Why it is Important to provide Road Gradient
- The primary importance of road gradient is to minimise substantial cutting or embankment by developing road gradient similar to the existing ground (A compliant design).
- Road gradients are provided to accumulate and drain out road surface water through the acceptable drainage system.
- Vertical gradients are used to reprofile two or more vertical gradients of varying range with the help of crest or sag curves.
The outcome of the Road Gradient
- Road Gradient plays a very important role to govern the operating speed of the highway. For example, in the case of a high-speed road, overtaking lanes are provided (along uphill direction) to separate heavy vehicles from the high-speed vehicle (Cars, Motorbikes, etc).
- Two consecutive vertical gradients (one uphill and one downhill) join together with a crest curve, creates a high point along the alignment, as a result, it reduces the stopping sight distances (SSD) and/or overtaking sight distances (OSD). Therefore, it is very important for designers to provide an adequate crest curve (larger crest curve) to provide desirable minimum SSD and FOSD.
- Steep downhill gradients are high-risk elements for the skidding of vehicles. Therefore, it is very important to high friction surface or high PSV (Polished Stone Value) surface course to enhance the skid resistance of the road surface and minimise the skidding of vehicles.
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The factor that influences the selection of Gradient:
Existing Ground Topography
Existing Ground Topography directly influence the proposed road gradient. This means if existing ground topography represents a steep hilly terrain, then the proposed road profile needs to be within the desirable maximum gradient range.
Existing and Proposed Drainage System
Existing and Proposed Drainage System significantly influence the requirement of the proposed gradient. If the proposed road is designed in a place where drainage attenuation is essential, then it would be recommended that the proposed road is designed on the embankment to accommodate a large size attenuation pond.
Type of Road
Type of Road directly influences the road gradient. For high-speed motorways, the desirable maximum gradient is 3% and for all-purpose single carriageway road, the desirable maximum gradient is 6%.
Approach to the bridge
The approach to the bridge at both ends directly influences the gradient of the approach road at both ends. In most cases, the surface water drainage of the bridges treated separately from the associated road drainage system, but to provide a smooth transition from the road to the bridge, the gradient of the road and the bridge must be aligned.
Criteria and Limitation of Road Gradient:
Desirable Maximum and Permitted Relaxations to Gradient
Desirable maximum gradient for Motorways and the all-purpose road is recommended to provide as shown in the table below. Relaxations are the permitted gradient provided when the desirable maximum gradient is not feasible due to some site constraints.
When the road is very flat, it is recommended to provide a desirable minimum vertical gradient of 0.5% to accumulate and transfer road surface water to a robust drainage system like Kerb Drainage.
Cycle Track Gradient:
Cycle Tracks are used by a cyclist of different skillset and fitness levels. So, it is very important to design cycle tracks in such a way that it is comfortably used by cyclists of all skill and fitness levels.
For that reason, it is recommended to provide a vertical gradient of the cycle track in the range of 2% to 10%. An average 2% vertical cycle track gradient is preferable and an occasional cycle track gradient of 6% for a short section is acceptable as a relaxation gradient.
Hope this blog helps you to understand the importance of Road Gradient and how it is influenced by some key parameters. Please feel free to like and share your comments.
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