The reasons for the mechanical widening are: When a vehicle negotiates a horizontal curve, the rear wheels follow a path of shorter radius than the front wheels

**Mechanical
widening**

The reasons for the mechanical widening are: When a vehicle
negotiates a horizontal curve, the rear wheels follow a path of shorter radius
than the front wheels. This phenomenon is called o -
tracking, and has the effect of increasing the effective width of a road space
required by the vehicle. Therefore, to provide the same clearance between
vehicles traveling in opposite direction on curved roads as is provided on
straight sections, there must be extra width of carriageway available. This is
an important factor when high proportion of vehicles are using the road.
Trailor trucks also need extra carriageway, depending on the type of joint. In
addition speeds higher than the design speed causes transverse skidding which
requires additional width for safety purpose. The expression for extra width
can be derived from the simple geometry of a vehicle at a horizontal curve as
shown in figure. Let R1 is the radius of the outer track line of the rear
wheel, R2 is the radius of the outer track line of the front wheel l is the
distance between the front and rear wheel, n is the number of lanes, then the
mechanical widening Wm is derived below:

**Psychological
widening**

`Widening of pavements has to be done for some
psychological reasons also. There is a tendency for the drivers to drive close
to the edges of the pavement on curves. Some extra space is to be provided for
more clearance for the crossing and overtaking operations on curves. IRC
proposed an empirical relation for the psychological

Widening
at horizontal curves Wps:

**Length of
transition curve**

The length of the transition curve should be
determined as the maximum of the following three criteria: rate of change of
centrifugal acceleration, rate of change of superelevation, and an empirical
formula given by IRC.

Rate of
change of centrifugal acceleration

At the tangent point, radius is infinity and hence
centrifugal acceleration is zero. At the end of the transition, the radius R
has minimum value R. The rate of change of centrifugal acceleration should be
adopted such that the design should not cause discomfort to the drivers. If c
is the rate of change of centrifugal acceleration, it is given by an empirical
formula suggested by by IRC

**Vertical
alignment**

The
vertical alignment of a road consists of gradients(straight lines in a vertical
plane) and vertical curves. The vertical alignment is usually drawn as a pro
le, which is a graph with elevation as vertical axis and the horizontal
distance along the centre line of the road as the the horizontal axis. Just as
a circular curve is used to connect horizontal straight stretches of road,
vertical curves connect two gradients. When these two curves meet, they form
either convex or concave. The former is called a summit curve, while the latter
is called a valley curve.

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