In this geometric design u will learn about sight distance, camber, horizontal alignment, vertical alignment etc.

1. Geometric Design of Highways
Presented by –
Mr.Shashank Namdeo

2. Content
 Introduction
 Sight Distances
 Stopping Sight Distance
 Overtaking Sight Distance
 Intimidate sight Distance
 Horizontal alignment
 Super Elevation
 Extra Widening
 Transition Curve
 Vertical alignment
 Sag Curve
 Valley Curve

3. Introduction

4. Introduction
 Types of Roads-
In India, non urban roads are classified into following
functional classes –
1. Expressways
2. National highways
3. State highways
4. Major district roads
5. Other district roads
6. Village roads

5. Introduction
 The geometric design of highways deals with the
dimensions and layout of visible features of the highway.
 Certain Factors which governs the geometric designs are–
 Road user characteristic
 Safety requirements
 Topography
 Traffic volume
 Design speed
 Vehicle characteristics
 Economy in construction, maintenance, aesthetically
appearance too !!

6. Cross sectional element
Camber
Objective-
Cross slope provided to raise middle of road surface in transverse
direction to drain off rain water from road surface.
 Camber is quite undesirable but important element.
 Amount of camber slope depends on rainfall amount.
 Camber prevent the subgrade layer.

7. Sight Distances

8. Stopping Sight Distance (SSD)
Objective -
To provide sufficient length of road to driver to observe
the object and stop the vehicle before colliding with
object.

9. Stopping Sight Distance (SSD)
Concept-
SSD = lag distance + Braking distance
Lag distance = distance travelled by vehicle during reaction time.
LD = Reaction time X Velocity of vehicle
Braking distance = Distance travelled by vehicle during braking
operation.
In braking operation, it is assumed that whole kinetic energy
dissipated by application of break therefore,
Work done by vehicle = Kinetic energy of vehicle

10. Stopping Sight Distance (SSD)
Kinetic energy of running vehicle =
1
2
𝑚𝑣2 =
1
2
𝑊
𝑔
𝑣2
Work done to stop vehicle = 𝑊𝑛
100
∓ 𝑓𝑊 𝐿
𝑊𝑛
100
∓ 𝑓𝑊 𝐿 =
1
2
𝑊
𝑔
𝑣2
braking distance 𝐿 =
𝑣2
2𝑔(𝑓∓𝑛)
SSD =
𝒗 𝟐
𝟐𝒈(𝒇∓𝒏)
+ v*t

11. Overtaking Sight Distance (SSD)
Objective –
To provide the sufficient distance to the driver to overtake the
slow moving vehicle ahead safely against the traffic in
opposite direction.

12. Overtaking Sight Distance (OSD)
Phase 1 = safe distance between overtaking and overtaken vehicle before
overtaking operation.
Phase 3 = safe distance between overtaking and overtaken vehicle after
overtaking operation.
D1 = Vb*reaction time
s s

13. Overtaking Sight Distance (OSD)
 Phase 2 = overtaking operation distance, considering time
taken by overtaking vehicle and overtaken vehicle is same.
S = (0.7*Vb+6) m.
D2 = 2s+Vb *T = Vb *t + ½ aT2
T =
4𝑠
𝑎
D4 = V0*T
OSD = D1 + D2 +D4

14. Overtaking Sight Distance (OSD)

15. Intermediate Sight Distance
Intermediate Sight Distance = SSD for leg B + SSD for leg A

16. Horizontal Alignment

17. Super Elevation
Extra widening

18. Super Elevation
Objective –
To counteract the centrifugal
force acting on the vehicle at
circular curves.
Concept –
Raise the road at the edge so the
weight component vehicle
balance the centrifugal
acceleration and avoid the
slipping and overturning or
vehicle.

19. Super Elevation
α
Fc
W 1
e
≈
Rv

20. Super Elevation
 cossincossin
22
vv
s
gR
WV
gR
WV
WfW 






  tan1tan
2
s
v
s f
gR
V
f 
 ef
gR
V
fe s
v
s  1
2
 efg
V
R
s
v


2

21. Extra Widening
Extra widening = mechanical widening + Psychological
widening
We =
𝑛𝑙2
2𝑅
+
𝑉2
9.5 𝑅

22. Transition Curves
 Objective –
1. To avoid sense of jerking of passenger while entering
from straight road to circular curve.
2. Provide gradual introduction of super elevation.
3. For aesthetic appearance of widening of road.

23. Transition Curves
 Transition curves are designed for
3 different criteria.
1. Comfort
2. Introduction of SE
3. Min. length as per IRC
Transition curve length as per Comfort criteria-
In this criteria, centrifugal force is so acting that sense of
jerk for passenger is not causing discomfort.
Where jerk is defined as rate of change of acceleration.

24. Transition Curves
 Length of T.C =
𝑉3
𝑅∗𝐶
( all units in SI)
where, C =
80
75+𝑉
( V in kmph)
 Length of transition curve on basis of introduction of super
elevation –
L = eN(W+We) for rotation of road about inner edge or
L = e
𝑁
2
(W+We) for rotation of road about center line
Where N is rate of introduction of super elevation and its values
lie in 150 : 1 for plain terrain and 60:1 for rolling terrain.

25. Transition Curves
 As per IRC, minimum length of transition curve is
L =
2.7 𝑉2
𝑅
for plain and rolling terrain,
L =
𝑉2
𝑅
for hilly and mountainous terrain.
 Spiral, cubic parabola and
leminscate are shapes of
transition curve but spiral
curve is best shape for highway.

26. Vertical Alignment

27. Summit Curve
 Objective –
To join 2 different grades of roads with smooth vertical
curve. Four different conditions for formation summit curve
which are shown below -

28. Summit Curves
n1
n2PVTPVC
h2
h1
L
SSD
Line of Sight
Concept-
Design of summit curve on the basis of sight distance.
On summit curves, centrifugal force acting outwards hence the springs of vehicle
is not compressed and therefore passenger comfort is not a issue.
Design of summit curve as a square parabola because the rate of change of slope
is decreasing always so more sight distance available at the top of curve.

29. Summit Curves
 Equation of summit curve –
𝑦 =
𝑁𝑥2
2𝐿
For Sight distance < length of curve
𝐿 =
𝑁𝑠2
2( 𝐻 + ℎ)

30. Summit Curves
 For sight distance > length of curve
𝐿 = 2𝑆 −
2( 𝐻+ ℎ)
𝑁
H and h = 1.2 m and 1.2 m for OSD and 1.2 m and 0.15 m
for SSD
S = Sight distance
N = algebraic sum of grades

31. Valley Curve
 Objective –
To join 2 different grades of roads with smooth vertical
curve. Four different conditions for formation summit curve
which are shown below -

32. Valley Curves
n1
n2
PVI
PVTPVC
h2=0h1 L
Light Beam Distance (SSD)
headlight beam (diverging by β degrees)
Concept –
In day time, no problem of any sight distance but in night time head light sight
distance should be sufficient for vehicle to stop before colliding with object.
In valley curves, the centrifugal force will be acting downwards along with the
weight of the vehicle. This will result in jerking of the vehicle and cause
discomfort to the passengers.

33. Valley Curves
 Length of valley curve –
The valley curve is made fully transitional by providing two
similar transition curves of equal length.
Equation of transition curve –
𝑦 =
2𝑁
3𝐿2 𝑥3

34. Valley Curves
 Length of curve on the basis of comfort condition –
𝐿 =
𝑁𝑉3
𝐶
where, C =
80
75+𝑉
( V in km/h)
 Length of curve on the basis of head light sight distance –

35. Valley Curves
When length of curve > Stopping sight distance
ℎ1 + 𝑆𝑡𝑎𝑛ß =
𝑁𝑆2
2𝐿
When length of curve < stopping sight distance
ℎ1 + 𝑆𝑡𝑎𝑛ß = S −
L
2
N
ß = beam angle ( nearly 1˚)
h1= height of head light from ground ( nearly 0.75m)

36. Thank You !!