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.
Ø It is two types –
A. S.S.D – (Stopping Sight Distance)
B. O.S.D – ( Overtaking 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.
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.
¨ 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.