07/01/1437
Sight Distance Sight Distance: Definition
CE 416 Lecture 3
The safe and efficient operation of vehicles on the
road depends very much on the visibility of the road Sight distance: Length of the roadway ahead
ahead of the driver. visible to drivers, must allow driver to perceive,
reaction, stop, change speed, and swerve etc.
Thus the geometric design of the road should be done when necessary
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.
Types of Sight Distance Calculation of Sight Distance
The computation of sight distance depends on:
Reaction time of the driver
Speed of the vehicle
Efficiency of brakes
Stopping Sight Distance (SSD) Frictional resistance between the tire and the road
Decision Sight Distance (DSD) Gradient of the road.
Passing Sight Distance (PSD)
Intersection Sight Distance (ISD)
Headlight Sight Distance (HSD)
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Reaction time of the driver Speed of the vehicle
Reaction time of a driver is the time taken from
the instant the object is visible to the driver to The speed of the vehicle very much affects the
the instant when the brakes are applied. sight distance.
Many of the studies shows that drivers require Higher the speed, more time will be required to
about 1.5 to 2 secs under normal conditions. stop the vehicle.
However, taking into consideration the variability Hence it is evident that, as the speed increases,
of driver characteristics, a higher value is sight distance also increases.
normally used in design. AASHTO suggests a
reaction time of 2.5 secs.
Frictional resistance between
Efficiency of brakes the tire and the road
The frictional resistance between the tire and
The efficiency of the brakes depends upon the road plays an important role to bring the
age of the vehicle, vehicle characteristics etc. vehicle to stop.
If the brake efficiency is 100%, the vehicle will When the frictional resistance is more, the
stop the moment the brakes are applied. But vehicles stop immediately. Thus sight required
practically, it is not possible to achieve 100% will be less.
brake efficiency. No separate provision for brake efficiency is
Therefore the sight distance required will be provided while computing the sight distance.
more when the efficiency of brakes are less. This is taken into account along with the
factor of longitudinal friction.
Also for safe geometric design, we assume that
the vehicles have only 50% brake efficiency. The value of longitudinal friction in between
0.35 to 0.4.
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Stopping Sight
Criteria for Sight Distance Distance (SSD)
Required for every point
along alignment (horizontal
Driver eye height: for passenger vehicle’s = 1080 mm above surface
and vertical) – Design for it,
Height of object in roadway = 600 mm (SSD) or sign for lower, safe speed
Height of opposing vehicle = 1080 mm (PSD)
Deceleration rate: AASHTO: 3.4 m/s2
Deceleration is within capability of drivers to stay within their lane Available SSD = f(roadway alignment, objects off the
and control the vehicle when braking on wet surfaces and is alignment, object on road height)
comfortable for most drivers
AASHTO represents friction as a/g which is a function of the roadway,
tires, etc
Stopping Sight Distance SSD Stopping Sight Distance SSD
Stopping sight distance (SSD) is the minimum sight distance available Perception reaction distance (dp.r) : is the distance the vehicle
on a highway at any spot having sufficient length to enable the traveled during the reaction time t and is given by:
driver to stop a vehicle traveling at design speed, safely without
collision with any other obstruction. dp.r = vt
There is a term called safe stopping distance and is one of the important where v = velocity in m/sec.
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 Braking distance (db) : the distance traveled by the vehicle
complete. during braking operation. For a level road this is obtained by
Drivers must have adequate time if they are to suddenly respond to a equating the work done in stopping the vehicle and the kinetic
situation. Thus in highway design, sight distance at least equal to the safe energy of the vehicle.
stopping distance should be provided. If F is the maximum frictional force developed and the braking
The stopping sight distance is the sum of perception reaction distance distance is db , then work done against the friction in stopping
(dp.r) and the braking distance (db) the vehicle is F db = f W db
SSD = PRD + BD (with final velocity V 2 = 0) where W is the total weight of the vehicle.
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Stopping Sight Distance SSD Stopping Sight Distance SSD
SSD = perception reaction distance + braking distance
f Where:
db v = design speed in m/sec,
W
t = reaction time in sec,
g = acceleration due to gravity
f = coefficient of friction.
The coefficient of friction f is given for various design speed.
Table 1: Coefficient of longitudinal friction
Speed, kmph < 30 40 50 60 > 80
f 0.40 0.38 0.37 0.36 0.35
W sin α
G
Gradient of the road. Effect of grade: W
α
W sin α = W tan α = W G/100
Gradient of the road also affects the sight
distance. Equating kinetic energy and work done.
While climbing up a gradient, the vehicle can
stop immediately. Therefore sight distance
required is less.
The same analysis could be shown for the descending grade (-G%).
While descending a gradient, gravity also Hence, the general relation for the stopping sight distance is:
comes into action and more time will be
required to stop the vehicle. Sight distance
required will be more in this case.
AASHTO represents friction f as a/g
a=deceleration rate m/s2
g=gravitational force (9.81 m/s2)
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SSD : Example SSD : Calculated /Design
Use basic assumptions to determine SSD at 100 kmph on
a) 0% grade, b) 3% grade
SSD = 0.278Vt+ _____V2______ or f=a/g
254(f ± G/100) = 3.4/9.81
= 0.346
For V= 100 kmph → f=0.35
Note:
Brake reaction
distance predicted
For 0% grade: SSD = 0.278(100)(2.5)+_(100)2___ = 181.986 m on a time of 2.5 s;
254(0.35± 0/100) 183.29 m deceleration rate of
3.4 m/s2 used to
determine calculated
For 3% grade: SSD = 0.278(100)(2.5)+ _(100)2___ = 130.07 m sight distance
254(0.35+3/100) 130.44 m
