Stopping Sight Distance-Updated(2024)

This Article Covers What Is Stopping Sight Distance And Formula.

Start With What Is Sight Distance?

Sight Distance is a length of road surface which a particular driver can see with an acceptable level of clarity. Sight distance plays an important role in geometric highway design because it establishes an acceptable design speed, based on a driver’s ability to visually identify and stop for a particular, unforeseen roadway hazard or pass a slower vehicle without being in conflict with opposing traffic.

As velocities on a roadway increased, the design must catered to allowing additional viewing distances to allow for adequate time to stop.

The computation of sight distance depends on:

  1. Reaction time of the driver
  2. Speed of the vehicle
  3. Efficiency of brakes

Stopping Sight Distance

SSD defined as the distance needed for drivers to see an object on the roadway ahead and bring their vehicles to safe stop before colliding with the object.

The distances derived for various design speeds based on assumptions for driver reaction time, the braking ability of most vehicles under wet pavement conditions, and the friction provided by most pavement surfaces, assuming good tires.

A roadway designed to criteria employs a horizontal and vertical alignment and a cross section that provides at least the minimum stopping sight distance through the entire facility.

The SSD comprised of the distance to perceive and react to a condition plus the distance to stop:

Stopping Sight Distance Formula:-

SSD = 0.278 Vt +V2/254(f ±g) (Metric)

SSD = 1.47 Vt + V2/30(f ±g) (English)

Where,

SSD = required stopping sight distance, m or ft.
V = speed, kph or mph
t = perception-reaction time, sec., typically 2.5 sec. for design
f = coefficient of friction, typically for a poor, wet pavement
g = grade, decimal.

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