Horizontal
alignment
15.2 Guidelines
on superelevation:
while designing
the various
elements of the road
like
superelevation,
we design it for a particular
vehicle
called
design vehicle
which
has some standard weight and dimensions.
But in the actual
case, the road has to cater
for mixed traffic.
Different
vehicles
with different
dimensions
and varying
speeds ply
on the road. For example.
in the case of a heavily
loaded truck with high centre of
gravity
and low speed superelevation
should be less.
otherwise
chances of toppling
are
more. Taking into practical
considerations
of all such situations,
IRC has given some
guidelines
about the maximum and minimum superelevation
etc.
15.2.1 Design of super-elevation:
For fast moving vehicles
providing
higher
superelevation
without
considering
coefficient
of friction
is safe. i.e.
centrifugal
force is fully
counteracted
by the weight of the vehicle
or superelevation.
For slow
moving vehicles
providing
lower superelevation
considering
coefcient
of friction
is
safe. i.e.centrifugal
force is counteracted
by superelevation
and coefcient
of
friction.
IRC suggests following
design procedure:
Step 1 Find 6 for 75 percent of
Step 2 If el g 6.67. then
then Step 3 Find fl for the
If

fl

< 6.15.

6 = 6.67

is

Step 4 Find
If

on

then

safe
the

2 11 then

design

is

control

the

for

design
= el
design

speed. neglecting
f.
=
2. else if el
speed and max . i.e

speed.

else

i.e el =
> 6.67
fl = 57:

3
e = 57:

6.67.

maximum

the

allowable

design

speed on for

the

go to

step

4.

maximum 6 = 6.67

and f = 6.15.

on = l/6.22gR

the

adequate.

otherwise

use speed adopt

control

measures

or look

for

speed

measures.

15.2.2

Maximum and minimum super-elevation:

depends on (a) slow moving vehicle

and (b)

heavy loaded trucks
with high CG. IRC species
a maximum super-elevation
of 7 percent
for plain
and rolling
terrain.
while that of hilly
terrain
is 16 percent
and urban road
is 4 percent.
The minimum super elevation
is 2-4 percent for drainage
purpose.
especially
for large radius of the horizontal
curve.
15.2.3

Attainment

section

by:

of

super-elevation:

a) rotating

1.

the outer

slope is rotated
about the crown at
same plane as the inner half.

(b) shifting

the position

Elimination

a desired

rate

about the center

such that

(b) rotation

about the inner

as the
with

this

the

half

surface

cambered

of the cross
falls

on the

line

progressively
shifted
outwards.
section
progressively.
to attain
full
super elevation
rotating
the pavement

: The pavement is rotated

edge is depressed and the outer edge is raised
both by half
superelevation.
i.e..
by E/2 with respect
to the centre.
edge as well
superelevation

crown of

of the crown: This method is also known as diagonal

crown method. Here the position
of the crown is
increasing
the width of the inner half of cross
2. Rotation
of the pavement cross section
are two methods of attaining
superelevation
by

(a) rotation

of the

edge about the crown : The outer

the

such that
total

edge: Here the pavement is rotated

centre such that the
respect
to the inner

outer
edge.

edge is

raised

by the

thus
by:There

the inner

amount of

raising
full

the outer

amount of

15.3 Radius of Horizontal
Curve: The radius of the horizontal
curve is an important
design aspect of the geometric
design.
The maximum comfortable
speed on a horizontal
curve depends on the radius of the curve. Although
it is possible
to design the curve
with maximum superelevation
and coefcient
of friction.
it is not desirable
because realignment
would
be required
if the design speed is increased
in future.
Therefore.
a ruling
minimum
radius
Rmlmg can be derived
by assuming maximum superelevation
and coefcient
of
friction.

Ideally.
are also
addition,

the
not
it

radius of the curve should be higher than Rmlmg. However very
desirable.
Setting
out large curves in the eld
becomes difcult.
also enhances driving
strain.

large curves
In

15.4 Extra widening:
Extra widening
refers
to the additional
width of carriageway
that
is required
on a curved section
of a road over and above that required
on a straight
alignment.
This widening
is done due to two reasons:
the rst
and most important
is the
additional
width required
for a vehicle
taking
a horizontal
curve and the second is due
to the tendency of the drivers
to ply away from the edge of the carriageway
as they
drive on a curve. The rst
is referred
as the mechanical
widening
and the second is
called
the psychological
widening.
15.4.1 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
off-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 gure 15.5. 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 1 is the distance
between the front
and rear wheel, n is

the number of lanes,

then the mechanical widening

wm (refer

gure

15:1)

is derived

below:

Therefore

the

widening

needed for

a single

lane

road

If the road has n lanes,
the extra widening
should
the extra widening
of a road with n lanes is given

Please note
wm is given

that
by:

for

large

radius,

R2 % R, which

is:

be provided
by,

is

the

on each lane.

mean radius

of

the

Therefore,

curve,then

15.4.2
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

Therefore,

the

total

widening

needed at

a horizontal

curve

we is

Summary:
In our country,
the design of super-elevation
follows
IRC guidelines
wherein the initial
design is done by considering
75% of design speed and the safety
design is assessed.
Pavement is to be given extra width at curves to account for
mechanical
and psychological
aspects.
15.6

Problems

of