CVE 415 Lecture 3 part 2
Highway Geometric Design
Engr. Sani Reuben .O.
Office: 36A, Civil Engineering Building
Email: reuben.sani@covenantuniversity.edu.ng
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SIGHT DISTANCE
Sight Distance is a length of road surface which a particular driver can see with an
acceptable level of clarity.
Sight distance plays an important role in geometric highway design because it establishes
an acceptable design speed, based on a driver's ability to visually identify and stop for a
particular, unforeseen roadway hazard or pass a slower vehicle without being in conflict
with opposing traffic.
Types of sight distance
• Stopping sight distance (SSD) or the absolute minimum sight distance
• Intermediate sight distance (ISD) is the taken as twice SSD
• Overtaking sight distance (OSD) for safe overtaking operation
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SIGHT DISTANCE SHOULD SATISFY THE FOLOWING STANDARDS
• Drivers travelling at the design speed has sufficient sight distance or length of road visible
ahead to stop the vehicle, in case of any obstruction on the road ahead with out collision.
• Driver travelling at the design speed should be able to safely overtake, at reasonable
interval, the slower vehicles without causing obstruction or hazard to traffic of opposite
direction.
• Driver entering an uncontrolled intersection (particularly unsignalized intersection) has
sufficient visibility to enable him to take control of his vehicle and to avoid collision with
another vehicle.
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SIGHT DISTANCE cont’d
The computation of sight distance depends on:
• Reaction time of the driver
• Speed of the vehicle
• Efficiency of brakes
• Friction resistance between the road and the tyres and
• Gradient of the road, if any
PIEV Process
• The perception-reaction time for a driver is often broken down into the four components
that are assumed to make up the perception reaction time. These are referred to as the PIEV
time or process.
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SIGHT DISTANCE cont’d
PIEV Process
Perception the time to see or discern an object or event
Intellection the time to understand the implications of the object’s presence or event
Emotion the time to decide how to react
Volition the time to initiate the action, for example, the time to engage the brakes
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SIGHT DISTANCE cont’d
Stopping sight distance
Stopping sight distance is defined as the distance needed for drivers to see an object on the
roadway ahead and bring their vehicles to safe stop before colliding with the object .
There is a term called safe stopping distance and is one of the important measures in traffic
engineering.
It is the distance a vehicle travels from the point at which a situation is first perceived to the
time the deceleration is complete.
Drivers must have adequate time if they are to suddenly respond to a situation.
Thus in highway design, sight distance at least equal to the safe stopping distance should be
provided. 6
SIGHT DISTANCE cont’d
The stopping sight distance is the sum of lag distance and the braking distance.
Lag distance is the distance the vehicle travelled during the reaction time t and is given by
vt, where v is the velocity in m∕sec2 .
Braking distance is the distance travelled by the vehicle during braking operation.
For a level road this is obtained by equating the work done in stopping the vehicle and the
kinetic energy of the vehicle.
If F is the maximum frictional force developed and the braking distance is l, then work
done against friction in stopping the vehicle is Fl = fWl where W is the total weight of the
vehicle.
The kinetic energy at the design speed is; 7
SIGHT DISTANCE cont’d
Therefore, the SSD = lag distance + braking distance and given by:
where v is the design speed in m∕sec2 , t is the reaction time in sec, g is the acceleration due
to gravity and f is the coefficient of friction.
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SIGHT DISTANCE cont’d
The distances are derived for various design speeds based on assumptions for driver
reaction time, the braking ability of most vehicles under wet pavement conditions, and the
friction provided by most pavement surfaces, assuming good tires.
A roadway designed to criteria employs a horizontal and vertical alignment and a cross
section that provides at least the minimum stopping sight distance through the entire
facility.
The stopping sight distance is comprised of the distance to perceive and react to a
condition plus the distance to stop:
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SIGHT DISTANCE cont’d
where SSD = required stopping sight distance, m or ft.
V = speed, kph or mph
t = perception-reaction time, sec., typically 2.5 sec. for design
f = coefficient of friction, typically for a poor, wet pavement
g = grade, decimal.
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Overtaking 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.
The overtaking sight distance or passing sight distance is measured along the center line of
the road over which a driver with his eye level 1.2 m above the road surface can see the top
of an object 1.2 m above the road surface. The factors that affect the OSD are:
• Velocities of the overtaking vehicle, overtaken vehicle and of the vehicle coming in the
opposite direction.
• Spacing between vehicles, which in-turn depends on the speed
• Skill and reaction time of the driver
• Rate of acceleration of overtaking vehicle
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HORIZONTAL ALIGNMENT
• The Horizontal alignment of a road is usually a series of straights (tangents) and circular
curves connected by transition (or spiral) curves.
FORCES ON A CORNERING VEHICLE
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FORCES ON A CORNERING VEHICLE
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