This document discusses the widening of pavement on horizontal curves due to mechanical and psychological factors. Mechanical widening accounts for off-tracking of vehicles' rear wheels on curves and is calculated based on traffic lanes, wheel base length, and curve radius. Psychological widening considers the design speed and curve radius to address driver comfort. The total widening is the sum of mechanical and psychological widths. Numerical examples are provided to demonstrate calculating off-tracking and extra widening for given curve parameters and vehicle/design specifications.

1. Radius of Horizontal Curve &
Widening of Pavement on Horizontal Curves
“Horizontal Alignment”
Lecture - II

2. 1. Radius of Horizontal Curve

6. 2. Widening of Pavement on Horizontal Curves
a. To prevent “off tracking”
of rear wheel base on
horizontal curve.
On horizontal curves, when they are not of very large radii, it is a common
practice to widen the pavement slightly more than the normal width. The
object of providing extra widening of pavements on horizontal curves are
due to the following reasons:
Fig. Off – tracking and
mechanical widening
on horizontal curve

8. Thus, extra widening of pavement on horizontal curves is divided into two parts:
(i) Mechanical and (ii) Psychological widening.
Mechanical Widening
The widening required to account for the off – tracking due to the rigidity of
wheel base is called ‘mechanical widening’ (Wm) and given as:
𝑊𝑚 =
𝑛𝑙2
2𝑅
Here, n = traffic lanes,
l = length of wheel base (6 m),
R = radius of horizontal curve, m

9. Psychological Widening
𝑊𝑝𝑠 =
𝑉
9.5 𝑅
Where, V = design speed, kmph

10. Note: It has been a practice to provide extra width of pavement on horizontal
curves when the radius is less than about 300 m as per IRC.
Hence the total widening We required on horizontal curve is given by:
We = 𝑾 𝒎 + 𝑾 𝒑𝒔 =
𝒏𝒍 𝟐
𝟐𝑹
+
𝑽
𝟗.𝟓 𝑹
(𝒎)

12. Numerical Problem

13. Try to solve:
1. Determine off-tracking of vehicle with wheel base 7.0 m while negotiating a
horizontal curve of radius 100 m.
2. In a mountainous terrain, a circular curve of radius 50 m and length 40 m has
transition of 20 m on both ends. Calculate extra widening if the design speed is 30
kmph. Suggest method of providing the widening on a two lane pavement.