The geometric design of roads is the branch of highway engineering concerned with the positioning of the physical elements of the roadway according to standards and constraints. The basic objectives in geometric design are to optimize efficiency and safety while minimizing cost and environmental damage. Geometric design also affects an emerging fifth objective called "livability," which is defined as designing roads to foster broader community goals, including providing access to employment, schools, businesses and residences, accommodate a range of travel modes such as walking, bicycling, transit, and automobiles, and minimizing fuel use, emissions and environmental damage. Geometric roadway design can be broken into three main parts: alignment, profile, and cross-section. Combined, they provide a three-dimensional layout for a roadway. The alignment is the route of the road, defined as a series of horizontal tangents and curves. The profile is the vertical aspect of the road, including crest and sag curves, and the straight grade lines connecting them. The cross section shows the position and number of vehicle and bicycle lanes and sidewalks, along with their cross slope or banking. Cross sections also show drainage features, pavement structure and other items outside the category of geometric design. #source: 1. Highway Engineering by: Khanna & Justo 2. Wikipedia

2. » Camber or cant or cross slope is the cross slope provided
to raise middle of the road surface in the transverse
direction to drain off rain water from road surface
» Objectives of providing camber are:
˃ Quick drying of pavement.
˃ Surface protection.
˃ Subgrade protection.
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4. IRC VALUES FOR CAMBER
SURFACE HEAVY LIGHT
TYPE RAIN RAIN
CONCRETE/BITUMINOUS 2 % 1.7 %
GRAVEL/WBM 3 % 2.5 %
EARTHEN 4 % 3.0 %
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5. » The safe and efficient operation of vehicles on the road
depends very much on the visibility of the road ahead of
the driver. Thus the geometric design of the road should
be done such that any obstruction on the road length
could be visible to the driver from some distance ahead .
This distance is said to be the sight distance.
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6. Types of sight distance
Three sight distance situations are considered for design:
1. Stopping sight distance (SSD) or the absolute minimum sight
distance
2. Intermediate sight distance (ISD) is defined as twice SSD
3. Overtaking sight distance (OSD) for safe overtaking operation
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7. Stopping sight distance
Stopping sight distance (SSD) is the minimum sight
distance available on a highway at any spot having sufficient
length to enable the driver to stop a vehicle traveling at
design speed, safely without collision with any other
obstruction.
Over-taking sight distance
The overtaking sight distance is the minimum distance
open to the vision of the driver of a vehicle intending to
overtake the slow vehicle ahead safely against the traffic in
the opposite direction.
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8. » Super-elevation or cant or banking is the transverse slope
provided at horizontal curve to counteract the centrifugal
force, by raising the outer edge of the pavement with
respect to the inner edge, throughout the length of the
horizontal curve. When the outer edge is raised, a
component of the curve weight will be complimented in
counteracting the effect of centrifugal force
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9. Forces acting on a vehicle on horizontal curve of radius at a speed of are:
• the centrifugal force acting horizontally out-wards through the center of
gravity,
• the weight of the vehicle acting down-wards through the center of gravity,
and
• the friction force between the wheels and the pavement, along the surface
inward
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10. » The position or the layout of the central line of the
highway on the ground is called the alignment.
» There are two types of alignment
˃ Horizontal alignment includes straight and curved paths.
˃ Vertical alignment includes level and gradients.
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11. » Alignment decision is important because a bad alignment
will enhance the construction, maintenance and vehicle
operating costs.
» Once an alignment is fixed and constructed, it is not easy
to change it due to increase in cost of adjoining land and
construction of costly structures by the roadside.
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12. » The alignment between two terminal stations should be
short and straight.
» The alignment should be easy to construct and maintain.
It should be easy for the operation of vehicles.
» It should be safe both from the construction and
operating point of view especially at slopes,
embankments, and cutting.
» The alignment should be economical and it can be
considered so only when the initial cost, maintenance
cost, and operating cost are minimum.
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13. » Horizontal alignment is one of the most important
features influencing the efficiency and safety of a highway
» A poor design will result in lower speeds and resultant
reduction in highway performance in terms of safety and
comfort. In addition, it may increase the cost of vehicle
operations and lower the highway capacity.
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14. » Transition curve is provided to change the horizontal
alignment from straight to circular curve gradually and
has a radius which decreases from infinity at the straight
end to the desired radius of the circular curve at the other
end.
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15. » OBJECTIVE OF PROVIDING TRANSITION CURVE
» To enable the driver turn the steering gradually for his
own comfort and security
» To provide gradual introduction of super elevation
» To provide gradual introduction of extra widening.
» To enhance the aesthetic appearance of the road.
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16. » 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 first 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.
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