2. CONTENTS
Average Running Speed
Sight Distance
• Stopping sight distance
• Overtaking Sight Distance
Curves
• Necessity of curves
• Horizontal curves
• Vertical Curves
Super Elevation
• Objects of providing Super Elevation
Road Geometrics:2
3. Average running Speed
The speed maintained by vehicles over a particular section of
a road is called Average running speed.
Average Running Speed = Distance covered by the vehicle
Actual Running Time
** The maximum safe speed of the vehicle assumed for geometric
design of a highway is known as design speed.
Road Geometrics:2
4. Sight Distance
Road Geometrics:2
The distance along the centre line of the road at which
the driver has visibility of an object, stationary or
moving, at a specified height above the carriageway is
known as sight distance.
It is a distance of visibility through which the driver can
see the opposite vehicle , a pedestrain or some fixed
object on the road to react timely to avoid any mis
happening and also to extend road courtesy to each
other.
6. Types of sight distance
Road Geometrics:2
The various types of sight distances considered for the
geometrical design of a road are as follows:
1. Stopping sight distance
2. Overtaking sight distance
3. Intermediate sight distance
4. Lateral sight distance
7. Stopping sight distance
Road Geometrics:2
• The clear distance ahead needed by a driver to bring
his vehicle to a stop before meeting an object on the
road is called stopping sight distance.
• Minimum Stopping distance is the distance
travelled by the vehicle during perception and brake
reaction time plus the braking distance.
** The distance covered by the vehicle between the point at
which brakes are applied and the point at which the vehicle
actually comes to stop is called braking distance.
10. Overtaking sight distance
Road Geometrics:2
The minimum sight distance needed by a driver on a two
way road to enable him to overtake another vehicle ahead
with safety against the traffic from the opposite direction.
11. Factors affecting Overtaking sight distance
Road Geometrics:2
1. Speeds of the overtaking, overtaken and vehicle coming
from opposite direction.
2. Spacing between the vehicles.
3. Rate of acceleration of the overtaking vehicle.
4. Skill and reaction time of the driver.
12. Curves
• The geometrical arcs provided at the change in the
alignment or gradient of a road are known as curves.
• Curves , being an important part of geometrical design of
a road should be so designed as to provide safety and
convenience to the traffic.
Road Geometrics:2
13. Necessity of providing curves
• To construct road according to topography of the area.
• To avoid excessive cutting and filling.
• To avoid costly land.
• To avoid religious, historical or important structures.
• To make use of existing bridges or right of way.
• To break the monotony of the continuous journey.
• To check the tendency of the driver to increase the speed
of his vehicle beyond safe limits on straight routes.
Road Geometrics:2
14. Types of curves
Road Geometrics:2
Curves on highways are divided into two main
groups:
1. Horizontal Curves :- The curves provided at the
turning points in the alignment (horizontal plane) of a
road are called horizontal curves.
2. Vertical Curves :- The curves provided at the change
of gradient ( in vertical plane) of a road are called
vertical curves.
16. Horizontal Curves
Horizontal curves used in the alignment of highways
are divided into four types:
1. Simple curves
2. Compound curves
3. Reverse curves
4. Transition curves
Road Geometrics:2
17. Simple curves
Road Geometrics:2
A circular curve consisting of a single arc of uniform
radius connecting two tangents is called simple curve.
this curve is suitable for slow moving traffic and large
radius.
18. Compound Curves
Road Geometrics:2
A circular curve consisting of series of two or more
simple curves of different radii turning in the same
direction is called a compound curve.
This curve is provided when compelled by the
topography of the area to avoid heavy cutting or filling.
19. Reverse Curves
Road Geometrics:2
A circular curve consisting of two simple curves of
same or different radii in the opposite direction is called
reverse or serpentine curve.
This curve is oftenly used in the alignment of a hill road
to go along with the topography of that area.
20. Transition Curves
Road Geometrics:2
The curves having radius varying gradually from an
infinite to a finite value equal to that of a circular curve
to be connected is called transition, spiral or casement
curve.
This curve is provided to have smooth entry of vehicle
from a straight portion to a curved portion of the road to
avoid discomfort to the passengers.
This curve permits gradual application of the super
elevation reducing shocks on the vehicle and provide
safety to the vehicular traffic.
24. Vertical Curves
Vertical curves used in
the alignment of
highways are divided into
two types:
1. Summit curves
2. Valley curves
Road Geometrics:2
25. Summit curves
Road Geometrics:2
The vertical curves having their convexity
upwards are known as summit or spur curves.
Summit curves are provided
When a rising gradient intersects falling
gradient.
When a rising gradient meets another rising
gradient.
When a rising gradient meets a horizontal path.
When a falling gradient meets another steeper
falling gradient.
27. Valley curves
Road Geometrics:2
The vertical curves having their convexity
downwards are known as valley or sag curves.
Summit curves are provided
When a falling gradient intersects rising gradient.
When a falling gradient meets another falling
gradient.
When a falling gradient meets a horizontal path.
When a falling gradient meets another steeper
falling gradient.
29. Super Elevation
Road Geometrics:2
The inward transverse inclination provided to the
cross section of the carriageway at horizontal
curved portion of a road is called superelevation,
cant or banking.
31. Objects of providing super elevation
Road Geometrics:2
1. To counteract the effect of centrifugal force
acting on the moving vehicle to pull the same
outward on a horizontal curve.
2. To help a vehicle to negotiate a curved path
without overturning and skidding.
3. To ensure safety to the fast moving traffic.
32. Summary
Average Running Speed
Sight Distance
• Stopping sight distance
• Overtaking Sight Distance
Curves
• Necessity of curves
• Horizontal curves
• Vertical Curves
Super Elevation
• Objects of providing Super Elevation
Road Geometrics:2
Er. Raghubir Singh
B.E, M.Tech (Structures)
9501547500
