sight distance

• sight distance is the length of the highway a driver needs to be able to see clearly. It is importa for drivers on a highway to see far enough down the highway to recognizean object in the path and react appropriately to avoid the crash
• Sight distance available from a point is the actual distance along the road surface, over which a driver from a specified height above the carriage way has a visibility of stationary or moving objects
• Stopping Sight Distance

  SSD
• Absolute minimum sight distance

  SSD
• Intermediate Sight distance


  ISD
• As twice as the SSD

  Intermediate Sight Distance/ ISD
• type of sight distance that is for safety overtaking operation

  Passing sight distance
• A type of sight distance that refers to the headlight or the headlamp sight distance of a vehicle

  headlamp sight distance
• A type of sight distance that is applied in the safety intersection

  Safe Sight Distance
• 1. REACTION TIME
• 2. EFFICIENCY OF THE BREAKS
• 3. FRICTION RESISTANCE BETWEEN THE ROAD AND THE TIRE OF TE VEHICLE
• 4. GRADIENT OF THE ROAD
• 5. 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 instant when the brakes are applied.
• PIEV THEORY


  Perception, Intellection, Emotion, Volition
• According to many studies the drivers should have 1.5 to 2 secs of reaction under their normal conditions. However, taking into consideration the variability of driver characteristic, a higher reaction time will be required once the driver in not in their normal condition.
• The higher the speed more time will be required to stop the vehicle
• As the speed increases, sight distance also increases
• Sight distance will be required more the efficiency of the of brakes are less.
• Also for safe geometric design, they assumed that the vehicles have only 50% brake efficiency
• The more frictional resistance, the lesser the sigght distance will be required
• IRC

  Indian Roads Congress
• Value of longitudinal Friction according to Indian Roads Congress

  0.35-0.4
• As the vehicle climbing up the gradient, less of the sight distance will be needed. While descending a gradient, gravity also comes into action and more time will e required to stop the vehicle. Which means it requires more sight distance
• Minimum sight distance available on a highway at any spot having sufficient length to enable the driver to stop a vehicle traveling at design speed, safely without collision with any other obstruction

  stopping sight distance
• SSD formula


  $d+$$D$
• formula of d


  $v/t$
• formula of D


  $v^2/2g(f+G)$
• standard value of h1in road design

  3.75ft/ 1.14m
• standard value of h2 in road design

  6in (0.15m)
• distance visible to a driver during night driving under the illumination of headlamp/lights

  headlamp sight distance
• factors that affects the PSD are:

  velocities of overtaking vehicle, overtaken vehicle and of the vehicle coming in the opposite direction, skills and reaction time of the driver, rate of acceleration of overtaking vehicle, gradient of the road
• PASSING SIGHT DISTANCE


  It is the distance required for a vehicle to overtake or pass another vehicle and turn back to the same lane safety without colliding in the obstruction
• PSD D1

  the distance passing vehicle travels while contemplating the passing maneuver , and while accelerating to the point of encroachmenton the left lane.
• The length of the roadway that is traversed by the passing vehicle while it occupies the left lane.

  PSD D2
• The clearance distance between passing vehicle and the opposing vehicle when the passing vehicle returns to the right lane

  PSD D3
• PSD D4

  The distance that the opposing vehicle travels during the final 2/3 of the period when the passing vehicle is in the left lane
• At intersection where two or more roads meet, visibility should be provided for the drivers approaching the intersection from either sides

  SIGHT DISTANCE AT INTERSECTIONS