Sight
Distance: Three sight
distance situations are considered in the design. (at design speed).
(A) SSD:
Stopping (or) absolute minimum sight distance
(or) non passing sight distance.
(B) OSD:
Safe over taking (or) passing sight distance.
(C) Safe sight
distance for entering into uncontrolled intersections.
Apart from above,
the two additional sight distances are considered by IRC.
(a) Intermediate
Sight Distance (lSD): It
is twice the stopping sight distance. (lSD =2 x SSD). When OSD cannot be
provided, lSD is provided to given limited over taking opportunities to fast
vehicles.
(b) Head
Light Sight Distance: Distance
visible to driver during night driving under the illumination of vehicle head
lights. This is critical at up gradients and at the descending stretch of the
valley Curves.
Stopping Sight Distance (SSD): Length of road visible ahead of
driver at any instance it depends on
(a) Features
of road ahead such as horizontal alignment and vertical profile of road, traffic
etc.
(b) Height
of driver’s eye above road surface (1.2 m as per IRC).
(c) Height
of object above road surface (0.15 m).
The above
values are at summit curves.
Definition
:( as per IRC) The distance between a moving vehicles with
the eye level of the driver at a height of 1.2 m and an object of 0.15m, so that
no collision occurs in case a situation arises to stop the vehicle.
(a)Total
Reaction Time (t):Perception
Time + Break Reaction Time.
- PIEV Theory: Total reaction time is split into four parts.
- (a) Perception (b) Intellection (c) Emotion (d) Volition
>>Total
reaction time of 2.5 seconds is considered satisfactory for most situations
>>The
distance traveled by vehicle did total reaction time is known as Lag Distance
or Reaction Distance.
>>The
distance traveled after application of brakes, to a dead stop position is known
Breaking Distance.
(b)
Formulation of SSD (Units in meters)
SSD =Lag
Distance (L) +Breaking Distance (B)
(i) Lag
Distance (L) = vt,
Where y
velocity in m/sec. = 0.278 Vt,
T= reaction
time in sec, (2.5 sec as per IRC)
V = velocity
in kmph.
(ii) Braking
Distance (B) : On a level ground it is obtained by equating work done in
stopping the
vehicle to kinetic energy lost.
Let, W =
weight of vehicle
f=
coefficient of longitudinal friction
Work done in
stopping vehicle= frictional force developed x braking distance =fW x B .....(1)
Kinetic
Energy Lost = (1 / 2)mv2= Wv2/2g………(2)
equating (1)
and (2)
Breaking
distance, Sb = v2/2gf
NOTE: In case SSD provided is not
sufficient as moving vehicle will collide with the object on another parked
vehicle.
The
principle to be used in this analysis is as follows for breaking distance.
Kinetic
Energy lost = work done in skidding before collision.
i.e., W (Vi –V2 f )/2g
=f. W .B , where
Vi = initial velocity before application of brakes.
Vf = final velocity before collision.
NOTE: If coefficient of longitudinal
friction is “f” the retardation (a) developed in the process of braking is, a=fg
(iii) Equations for SSD:
Important
Note:
(A) Minimum SSD = SSD calculated for single lane
one way traffic.
= 2 x SSD calculated for single lane road with
two way traffic.
= SSD calculated for two lane two way traffic.
(ii) The ‘SSD’ should invariably be provided
throughout the length of all road and
hence this is also known as ‘Absolute minimum sight distance.
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