Highway Engineering

1. Sanjivani Rural Education Society's
Sanjivani College of Engineering, Kopargaon 423603.
Department of Civil Engineering
Sub –Highway Engineering
B Tech Civil, Semester –VIII
Highway Geometric Design
By,
Prof. A. R. Pabale (Ph.D Pursuing)
(Assistant Professor)
Mail Id- pabaleabhijeetcivil@sanjivani.org.in

2. Unit-II Highway Geometric Design
Introduction, factors controlling design of various geometric elements,
different cross-sectional elements and road margins. Stopping Sight Distance
(SSD), Overtaking Sight Distance (OSD), and overtaking zone, problems.
Attainment of super elevation, radius of curves. Methods of introduction of
extra widening, widening of pavement on horizontal curves, Transition
Curves-shape and length of transition curves, shift of transition curves.
Design of vertical alignment, gradient and its type, IRC recommendations,
06. 2 grade compensation on horizontal curve, vertical curves: crest and sag
curves,
types of summit curves, length of summit curve for SSD and OSD.
Requirements, types of valley curves, length of valley curve for comfort and
head light sight distance criteria. Grade compensation.

3. Highway Geometric Design
• The geometric design of highways deals with the dimensions and layout of
visible features of the highway.
• The emphasis of the geometric design is to address the requirement of the
driver and the vehicle such as safety, comfort, efficiency, etc.
• The features normally considered are the cross section elements, sight
distance consideration, horizontal curvature, gradients, and intersection.
• The design of these features is to a great extend influenced by driver
behavior and psychology, vehicle characteristics, traffic characteristics such
as speed and volume.
• Proper geometric design will help in the reduction of accidents and their
severity.

4. Factors affecting the geometric design
• Highway alignment, road classification,
• Pavement surface characteristics,
• Cross-section elements including cross slope, various widths of roads and
features in the road margins.
• Sight distance elements including cross slope, various widths and features
in the road margins.
• Horizontal alignment which includes features like super elevation, transition
curve, extra widening and set back distance.
• Vertical alignment and its components like gradient, sight distance and
design of length of curves.
• Intersection features like layout, capacity, etc.

5. Factors affecting geometric design
Design speed
• Design speed is the single most important factor that affects the
geometric design.
• It directly affects the sight distance, horizontal curve, and the length
of vertical curves.
• Since the speed of vehicles vary with driver, terrain etc, a design speed
is adopted for all the geometric design.

6. Topography
• The next important factor that affects the geometric design is the
topography.
• It is easier to construct roads with required standards for a plain
terrain.
• However, for a given design speed, the construction cost increases
multiform with the gradient and the terrain. Therefore, geometric
design standards are different for different terrain to keep the cost of
construction and time of construction under control.
• This is characterized by sharper curves and steeper gradients.

7. Other factors
• Vehicle: :The dimensions, weight of the axle and operating characteristics of a
vehicle influence the design aspects such as width of the pavement, radii of the
curve, clearances, parking geometrics etc. A design vehicle which has standard
weight, dimensions and operating characteristics are used to establish highway
design controls to accommodate vehicles of a designated type.
• Human: The important human factors that influence geometric design are the
physical, mental and psychological characteristics of the driver and pedestrians
like the reaction time.
• Traffic: It will be uneconomical to design the road for peak traffic flow.
Therefore a reasonable value of traffic volume is selected as the design hourly
volume which is determined from the various traffic data collected. The
geometric design is thus based on this design volume, capacity etc.

8. • Environmental: Factors like air pollution, noise pollution etc. should
be given due consideration in the geometric design of roads.
• Economy: The design adopted should be economical as far as possible.
It should match with the funds alloted for capital cost and maintenance
cost.
• Others: Geometric design should be such that the aesthetics of the
region is not affected.

9. Cross-Sectional Elements of Road
• Camber
• Width of Carriage way
• Kerb
• Road Margin
• Width of Formation
• Right of Way (ROW)

12. Camber
• Camber or cant is the cross slope provided to raise middle of the road
surface in the transverse direction to drain off rain water from road
surface. The objectives of providing camber are:
• Surface protection especially for gravel and bituminous roads
• Sub-grade protection by proper drainage
• Quick drying of pavement which in turn increases safety.

13. Width of carriage way
• Width of the carriage way or the width of the pavement depends on the
width of the traffic lane and number of lanes.
• Width of a traffic lane depends on the width of the vehicle and the
clearance. Side clearance improves operating speed and safety.
• The maximum permissible width of a vehicle is 2.44 and the desirable
side clearance for single lane traffic is 0.68 m.
• This require minimum of lane width of 3.75 m for a single lane road.

14. Kerbs
• Kerbs indicate the boundary between the carriage way and the
shoulder or islands or footpaths.
• The purpose is to prevent traffic leaving the carriageway and are used
on dangerous curves, for pedestrian islands, and to protect footpaths or
equipment.

17. • Low or mountable kerbs : This type of kerbs are provided such that
they encourage the traffic to remain in the through traffic lanes and
also allow the driver to enter the shoulder area with little difficulty.
• Semi-barrier type kerbs : When the pedestrian traffic is high, these
kerbs are provided. Their height is 15 cm above the pavement edge.
This type of kerb prevents encroachment of parking vehicles, but at
acute emergency it is possible to drive over this kerb with some
difficulty.

18. • Barrier type kerbs : They are designed to discourage vehicles from
leaving the pavement. They are provided when there is considerable
amount of pedestrian traffic. They are placed at a height of 20 cm
above the pavement edge with a steep batter.
• Submerged kerbs : They are used in rural roads. The kerbs are
provided at pavement edges between the pavement edge and
shoulders. They provide lateral confinement and stability to the
pavement.

19. Road margins
• The portion of the road beyond the carriageway and on the roadway can be
generally called road margin.
• Various elements that form the road margins are given below.
Shoulders
• Shoulders are provided along the road edge and is intended for
accommodation of stopped vehicles, serve as an emergency lane for
vehicles and provide lateral support for base and surface courses.
• The shoulder should be strong enough to bear the weight of a fully loaded
truck even in wet conditions. The shoulder width should be adequate for
giving working space around a stopped vehicle.`

20. Parking lanes
• Parking lanes are provided in urban lanes for side parking. Parallel
parking is preferred because it is safe for the vehicles moving on the
road. The parking lane should have a minimum of 3.0 m width in the
case of parallel parking.
Bus-bays
• Bus bays are provided by recessing the kerbs for bus stops. They are
provided so that they do not obstruct the movement of vehicles in the
carriage way. They should be at least 75 meters away from the
intersection so that the traffic near the intersections is not affected by
the bus-bay.

21. Cycle track
• Cycle tracks are provided in urban areas when the volume of cycle traffic is
high Minimum width of 2 meter is required, which may be increased by 1
meter for every additional track.
Footpath
• Footpaths are exclusive right of way to pedestrians, especially in urban areas.
They are provided for the safety of the pedestrians when both the pedestrian
traffic and vehicular traffic is high. Minimum width is 1.5 meter and may be
increased based on the traffic
Guard rails
• They are provided at the edge of the shoulder usually when the road is on an
embankment. They serve to prevent the vehicles from running off the
embankment, especially when the height of the fill exceeds 3 m.

22. Width of formation
• Width of formation or roadway width is the sum of the widths of
pavements or carriage way including separators and shoulders.
• This does not include the extra land in formation/cutting.
Right of way
• Right of way (ROW) or land width is the width of land acquired for
the road, along its alignment. It should be adequate to accommodate
all the cross-sectional elements of the highway and may reasonably
provide for future development.

23. Summary
• The characteristics of cross-sectional elements are important in
highway geometric design because they influence the safety and
comfort.
• Camber provides for drainage, frictional resistance and reflectivity for
safety etc.
• The road elements such as kerb, shoulders, carriageway width etc.
should be adequate enough for smooth, safe and efficient movement of
traffic.
• IRC has recommended the minimum values for all these cross-
sectional elements.

24. Sight Distance
• The visibility of the road ahead of the driver will help in the safe and
efficient operation of the vehicles.
• This will hence demand the geometric design to be highly efficient so
that the length of the road is highly visible to the driver even from a
distance ahead.
• This distance is hence termed as the sight distance.

25. • The actual distance that is observed along the road surface which is visible for a
driver from a specified height above the carriage way is called as the sight
distance at a point. This distance will let the driver see all the stationary and the
moving objects in front of the vehicle. Mainly in the geometric design of road
construction, mainly three sight distances are taken into consideration.
• They are:
• SSD - Stopping Sight Distance or Absolute Minimum Sight Distance
• ISD - Intermediate Sight Distance: This is twice the value of SSD
• OSD - Overtaking Sight Distance
• Other than these we have:
• Head Light Sight Distance
• Safe Sight Distance

27. SSD - Stopping Sight Distance or Absolute
Minimum Sight Distance
• The ability of the driver to see ahead a long section of road is very
important for safe and efficient operation in highway traffic. Thus obstacles
should be clearly visible to the driver some distance ahead.
• Sight distance depends upon:
• Road features and terrain
• Traffic condition
• Position of obstruction
• Height of driver from the carriageway
• Height of the object above the road surface
• Type of curve

28. OSD - Overtaking Sight Distance
• The overtaking sight distance is the minimum distance on a highway open
to the vision of the driver of a vehicle to overtake slow moving vehicles
ahead that are travelling in the same direction safely against opposing traffic
in opposite direction.
• The factors that affect OSD are:
• Speed of the overtaking vehicle, overtaken vehicle and the vehicle coming
from the opposite direction
• Spacing between the vehicles
• Skill and reaction time of the driver
• Rate of acceleration of the overtaking vehicle
• Gradient of the road

30. Types of curves
• Horizontal curve
• Vertical curve

32. Vertical Alignment design of roads
• The vertical alignment of highway generally defined as the presence of
heights and depths in vertical axis with respect to horizontal axis of
alignment.
• These heights and depths in roads may be in the form of gradients
(straight lines in a vertical plane) or vertical curves.

34. • Vertical alignment of highways consists of
• Gradients
• Grade compensation
• Vertical curves (valley curve, summit curve)

35. • Gradients
• Gradient is defined as the rise or fall corresponding to some horizontal
distance. Raise with respect to horizontal distance is called Upward gradient
(+n %) Fall with respect to Horizontal distance is called Downward
Gradient (-n %)
• Types of gradients
• Ruling gradient
• Limiting gradient
• Exceptional gradient
• Minimum gradient

37. • Limiting gradient
• This gradient is provided as shorter stretches in highways. Whenever
ruling gradients costs high for the hilly terrains then limiting gradient
is provided which will reduce the cost. This gradient is adopted
frequently in terrains with limited stretches.
• Exceptional gradient
• These are very steeper gradients given at unavoidable situations and
they are adopted for stretches not exceeding 100m of length.

38. • Minimum gradient
To drain of rain water from the road minimum gradient is needed.
Generally for lateral drainage Camber is provided. But for the
longitudinal drainage along the side drains require some slope for
smooth flow of water. For concrete drains minimum gradient of 1 in
500 and open soil drains gradient of 1 in 200 is suitable.

39. • Exceptional gradient
These are very steeper gradients given at unavoidable situations and
they are adopted for stretches not exceeding 100m of length.

40. • Vertical Curves
Generally two types of vertical curves are there to adopt for the
vertical alignment. They are Summit curve & Valley curve
Summit curve Summit curve is a vertical curve adopted mainly
when the gradient is upwards. In case of summit curve simple parabola
is considered as best curve shape. There are four different cases are
involved in adoption of summit curve as follows. Case-1: When
upward gradient meets a flat gradient.

41. • Grade Compensation
When a horizontal circular curve lies in vertical curve there will be an
increased resistance offered by the circular curve in the form of curve
resistance in addition to the component of gravity.

42. Vertical Curves
• Vertical Curves are the second of the two important transition elements
in geometric design for highways, the first being Horizontal Curves.
• A vertical curve provides a transition between two sloped roadways,
allowing a vehicle to negotiate the elevation rate change at a gradual
rate rather than a sharp cut

44. Crest and Sag Curves
• Sag curves are used where the change in grade is positive, such as
valleys, while crest curves are used when the change in grade is
negative, such as hills.
• Both types of curves have three defined points: PVC (Point of Vertical
Curve), PVI (Point of Vertical Intersection), and PVT (Point of
Vertical Tangency).

45. Summit Curves
• Summit curves are the vertical curves having convexity upwards,
These curves introduced for the driving comfort and aesthetic
purposes.
• In summit curves we do not need any transition curves because
centrifugal acceleration is not acting laterally outward or inward but,
its acting upward.
• The summit curve is one type of the vertical curve, It is provided on
the highway or railway when it crossed ridge or valley.

46. Types of summit Curve
• The common practice has been to use parabolic curves in summit
curves.
• This is primarily because of the ease with it can laid out as well as
allowing a comfortable transition from one gradient to another.
• Circular curve offers equal sight distance at every point on the curve,
for very small deviation angles a circular curve and parabolic curves
are almost common.

48. • Ascending and descending
• Ascending and horizontal
• If you notice you will find that the summit curve is not used when a
descending gradient meets an ascending gradient. The curve used in
such a situation is called a valley curve.

49. Head light sight distance criteria
• The headlight sight distance is the amount of highway that should be
visible at any time in night conditions in order for the vehicle to stop
before the obstacle.
• Numerically it is the same as the stopping sight distance.
• In a valley, during the daytime, there is no problem in satisfying the
sight distance.
• But during the nighttime, it becomes a problem.
• Therefore headlight sight distance is also considered a criterion.

50. • What Is Gradient of Road?
• The gradient is the rise and fall of the gradient with respect to the horizontal line.
Gradients are mainly four types those are-
• Ruling gradient: This type of gradient is the maximum gradient that is provided
on the highway. It is also termed the designer gradient. Ruling gradient depends
on terrain length, speed, power of the vehicle, etc. The ruling gradient in hilly
terrain is difficult to provide.
• Exceptional gradient: This type of gradient is a very exceptional gradient, and
it’s a very steeper gradient. The length of this gradient should not be less than
100m.
• Limiting gradient: Limiting gradient is provided in shorter stretches on the
highway. When the limiting gradient is costlier, then we provide a limiting gradient.
• Minimum gradient: For drainage purposes of rainwater, the minimum gradient is
required. For concrete drain 1 in 500 slopes is provided, and soil drain 1 in 200
slopes is required.

51. Types of Gradient of Road
Gradients are mainly categorized into six categories, those are-
1. Limiting Gradient of Road

52. • Limiting gradient is that type of gradient which is steeper than
the ruling gradient. This limiting gradient is mainly used in rolling
terrain and hilly terrain. This gradient is also known as a
maximum gradient.

53. 2. Ruling Gradient of Road
The ruling gradient is that type of gradient which is used to make the alignment of the road. This is adopting widely because this gives maximum safety at minimum
cost.

54. Exceptional Gradient of Road
Exceptional Gradient is that type of gradient which is steeper than limiting gradient,
this is used only for special situations. This gradient is adopted in an unavoidable
situation like mountainous terrain that’s why the gradient is steeper ad it causes
more fuel requirement and frictional losses.

55. • Purpose of Providing Gradient of Road
• This is mainly provided to clear out the drainage purpose of rainwater.
• It mainly connects the two points at different levels.
• It is mainly provided to construct the side drains properly.
• By cutting filling operation the earthwork for road construction is economical.
• Importance of Gradient in Roads
• The most important part of roads is the gradient.
• The movements of motor vehicles smoothly due to the gradient.
• The gradient is mainly provided to construct the side drains properly.
• At first the vehicular cost, construction cost is measured then we choose what kind of
gradient is to be used.