When we are designing a road alignment one of the important factors we consider is there should be less number of curves in that alignment. But it is not possible to completely avoid the curves in a pavement.
When the curves are of not large radii it is common practice to provide some extra width on the road surface at those curve regions to counteract various effects.
Some of the reasons why we provide the extra widening are given below.
Reasons For Providing the extra widening:
1. Many of the automobiles such as cars, bus, trucks etc., have a rigid wheel base. In these vehicles the distance between the wheels is fixed. Moreover it is possible to control only the front wheels.
When this type of vehicles are moving on the horizontal curve of the road, the rear wheels do not follow the path traced by the front wheels. The rear wheels will slightly offset from the actual path and will trace a different path which is present slightly towards the inner edge of the horizontal curve.
So, if the vehicle is trying to move exactly on the outer edge of the pavement, the rear wheels will move on the shoulders instead of moving on the pavement. This phenomenon where the rear wheels do not follow the path traced by the front wheels and slightly offset towards the inner edge of the curve is called as ” Off-Tracking”.
To provide the pavement surface during this off-tracking some extra width is required on the pavement at curve regions.
2. When a vehicle is moving on the horizontal curve of a road, centrifugal force acts on the vehicle which will try to destabilize the vehicle. If the centrifugal force is high and there is no sufficient lateral friction developed between the wheels and road surface, transverse skidding takes place.
Also it happens when the vehicles are not moving at design speed, hence some extra width should be provided on the horizontal curves of the roads to safeguard those vehicles as all the vehicles do not move at the design speed.
3. Also in the case of the vehicles such as trailers, the length of the wheelbase is very high and the backside units will not follow the same path traced by the front part i.e., engine or towning vehicle.
So for protecting these vehicles from offsetting the pavement we will provide some extra widening on the horizontal portion of the roads.
So all the above mentioned factors are the vehicular factors that influence the road width at the curve portion of the horizontal curves.
Apart from these vehicular factors there are some psychological factors that have to be consideration while designing the width of the pavement on the circular portion of the horizontal curves.
4. When the driver is travelling on a circular curve, if there is any obstruction present at the inside portion of the curve, to have more area under his visibility the driver will try to move on the outer edge of the pavement. Hence, more traffic on this curve portions will be confined to this outer half region. This is not good in point of view of safety. Hence, we will provide some extra width considering the safety and psychological aspects of the driver.
5. Another psychological factor is most of the drivers feel safe only when they have more clearance at the circular portions of the curves. So for making the driver feel comfortable and safe at these curve parts, keeping in mind the psychological behaviour of the vehicle driver we will provide some extra width.
Components of Extra Widening:
So in the case of extra widening generally there are 2 components.
1. Mechanical Widening
2. Psychological Widening
Mechanical widening takes into account of the various vehicular factors. And psychological widening accounts for the psychological factors.
If we calculate the amount of off-tracking, it is sufficient for facilitating the mechanical widening.
Psychological widening will be decided using an empirical formula given by IRC.