Increase the width of pavement by increasing the number of lanes, this is done just to resolve the problem of increase in traffic volume is called widening. Additional width of carriageway that is required on horizontal curve is referred as extra widening. The rear wheels follow the inner path on the curve as compared with the front wheels, this phenomena is known as off tracking.
Basic reasons to provide extra widening are,
- To avoid off-tracking due to rigidity of wheel base
- To counteract transverse skidding
- To increase the visibility at curve
- To encounter psychological tendency of driver negotiating the curve
Extra-widening is dividing into two parts,
- Mechanical Widening
- Psychological Widening
1. Mechanical Widening: It is provided due to the rigidity of wheel base,when a vehicle travels on a horizontal curve, the path followed by front wheel is not same as the path followed by rear wheel on the curve this phenomena is known as off-tracking. Mechanical widening is provided to avoid off-tracking due to rigidity of wheel base.
calculation of mechanical widening,
where,
n = Number of lanes
R = Radius of curve
l = Length of wheel base
2. Psychology Widening: There is tendency for the drivers to drive closer to the edges of the pavements on curves and to maintain side gap between another vehicles, so psychological widening is also required to provide . IRC proposed an empirical formula for the psychological widening,
Calculation of psychological widening,
where,
V is in kmph
therefore, the total extra-widening (We) needed to be provided on horizontal curve will be given as,
IRC Recommendation,
IRC recommended values of extra widening for single and two lane pavement are given below,
- If R < 50 m, then extra widening is provided at inner edge
- If R > 300 m, then extra widening is not provided
- If 50 < R < 300 m, then extra widening is provided at both the edge
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