Sight Distance in Highway Engineering
INTRODUCTION:
Ø The Sight
Distance is the actual distance along the road surface which a driver formed a
specified height from a carriage way as visibility station of any moving
object.
Ø Their
sight distance situations are considered for design of road stopping sight
distance, overtaking sight distance or passing distance.
TYPES:
Ø It is
two types –
A. S.S.D – (Stopping Sight Distance)
B. O.S.D – ( Overtaking Sight Distance)
· S.S.D (STOPPING SIGHT DISTANCE):
Ø The minimum sight distance available on a
highway at any spot should of sufficient length to stop a vehicle, travelling a
design speed, safety, without collision with any other obstruction.
Ø The Sight Distance available on a road to a
driver depends on the following factor –
i.
Features of
road ahead.
ii.
Height of
the driver’s eye above the road surface.
iii.
Height of
the object above the road surface.
iv.
Height of
the driver’s eye level 1.20 m &height of the object as 0.15 m.
Ø The distance with an motor vehicle can be
stopped depending upon the factor –
i.
Total
reaction time of the driver.
ii.
Speed of the
vehicle.
iii.
Efficiency
of brake.
iv.
Friction
resistance between the road and tyres.
v.
Gradient of
the road.
Ø TOTAL REACTION TIME OF THE DRIVER:
o It is the time taken from the instant the
object is visible to the driver to instant brakes are efficiently apply.
o The total reaction time may be split up into
two parts –
a) Perception Time
b) Break Reaction Time
o Perception
Time:
Perception
time is the time required for a driver to realize that brake must be applies.
It is the time from instant object comes on the lines of sight of the driver to
the instant he realizes that the vehicle needs to be stopped.
o The Break
Reaction Time:
The
Break Reaction time is also depends upon several factors including the skill of
the driver, the type of the problems and various other environmental factors,
obtained the total break reaction time of the driver is taken together.
Ø PIEV THEORY:
According
to this theory total reaction time of the driver split into four parts or it is
divided into four categories.
P
– Perception
I
– Intellection
E
– Emotion
V
– Volition
¨
Perception Time:
It
is the time required for the sensation received by the eyes or ears to be transmitted
to brain through the nervous system and spinal cord.
In
other words it is the time received to perceive an object or situation.
¨
Intellection Time:
It
is the time required for understanding the situation.
This
also the time required for comparing the different thoughts.
¨
Emotion Time:
It
is the time elapsed during emotion sensation and disturbance such as fear,
anger or any other emotional feeling such as superstition etc. with reference
the situation.
¨
Volition Time:
Volition is
the time taken for the final action.
·
LEG DISTANCE:
During
the total reaction time or PIEV time the vehicle may be assumed to process
forward with a uniform speed of which the vehicle has been moving and this
speed may be taken as the design speed.
If the
design speed is taken ‘v’ km/h then the leg distance work out
·
BREAKING DISTANCE:
The
co-efficient of friction ‘f’ depends upon several factors such as the type and
the condition of pavement surface or tyres also the value of decrease with
increase in speed IRC recommended.
·
O.S.D (OVERTAKING SIGHT DISTANCE):
Ø
The maximum
sight distance (OSD) is the distance measured along the centre of the road
which a driver eye level 1.20 m above the road surface can see the top of the
object 1.20 m.
Ø
Some of the
important factors on which the minimum OSD required for the safe overtaking
manoeuvre depends are –
a.
i. Speed
ii. Overtaking Vehicle
iii. The vehicle coming from opposite
direction speed.
b.
Distance between
overtaking and overtaken vehicle minimum spacing depends on speed.
c.
Skill and
reaction time of the driver.
d.
Rate of
acceleration of overtaking vehicle.
e.
Gradient of
the Road.
Ø
Analysis of O.S.D:
The overtaking manoeuvre of the vehicle ‘A’ travelling
at a design speed and another slow vehicle ‘B’ on a two lane road with road way
traffic, third vehicle ‘C’ come from the opposite direction the overtaking
manoeuvre may be split up into three operation. Thus dividing the OSD into
three parts like D1, D2 and D3.
D1 = D1 is the distance travel by
overtaking vehicle ‘A’ during the time reaction i.e. from position A1
to A2.
D2
= Distance travel by the vehicle
from A2 to A3 which is actual operation of overtaking.
D3 = The distance travel by the vehicle coming
opposite side see from C1 to C2 during the overtaking
operation.
Ø
Calculation:
¨
D1 = It is the distance travel by overtaking
vehicle ‘A’ during the reaction time ‘t’ sec of the driver position A1
to A2.
¨
D2 = It is the distance travel by ‘A’ vehicle
from A2 to A3 during the actual overtaking in the time ‘t’ sec.
S = Minimum spacing of the two vehicles while
moving with speed vb m/s. the minimum space between vehicle depending
upon their span is given by empirical formula
¨
D3 = D3 is the distance travel by
oncoming vehicle from C1 to C2 during the overtaking
operation ‘A’ i.e. T sec.
In the above figure ‘A’ is the overtaking
vehicle originally at design speed V m/sec or V Km/h.
‘B’ is the overtaken or slow moving vehicle
moving with uniform speed, Vb m8/s or Vb km/h.
‘C’ is the vehicle coming from opposite
direction at the design speed V m/s or Km/h.
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