Stopping Sight Distance
Stopping sight distance (SSD) is the minimum sight distance available on a highway at any spot having sufficient length to enable the driver to stop a vehicle travelling at design, safely without collision with any obstruction.
- The stopping sight distance is the sum of lag distance and the braking distance.
- Lag distance is the distance the vehicle travelled during the reaction time (t) and is given by ‘vt’, where v is the velocity in m/sec2.
- Braking distance is the distance travelled by the vehicle during braking operation.
- For a level road this is obtained by equating the work done in stopping the vehicle and the kinetic energy of the vehicle.
- If F is the maximum frictional force developed and the braking distance is l, then work done against friction in stopping the vehicle is Fl = fWl where W is the total weight of the vehicle. The kinetic energy at the design speed is;
½(mv2) = ½(Wv2/g)
fWl = Wv2/g
l = v2/2gf
Therefore, the SSD = lag distance + braking distance
Given by; SSD = vt + v2/2gf
Where v is the design speed in m/sec2, t is the reaction time in sec, g is the acceleration due to gravity and f is the coefficient of friction. The coefficient of friction f is given below for variation design speed
Coefficient of longitudinal friction
Speed, Kmph | <30 | 40 | 50 | 60 | 80> |
f | 0.40 | 0.38 | 0.37 | 0.36 | 0.35 |
When there is an ascending gradient of say +n%, the component of gravity adds to braking action and hence braking distance is decreased. The component of gravity acting parallel to the surface which adds to the braking force is equal to
W sinθ ≈ tanθ ≈Wn/100
Case (1): When vehicle is moving up the grade.
Equating kinetic energy and work done
(fW + Wn/100)l = Wv2/2g
l = v2/2g{f+(n/100)}
Case (2): When vehicle is moving down the grade.
SSD = vt + v2/2g(f – 0.01n)
Therefore the general formula is given by equation
SSD = vt + v2/2g(f ± 0.01n)
- Effect for grade should not be considered for undivided highway for two way traffic but must be considered for divided highway.
- On roads with restricted width or single lane road when two- way moment traffic is permitted, the minimum stopping sight distance should be equal to TWICE the minimum stopping distance to enable both vehicle coming from opposite directions.
- SSD on vertical curves should be the length which a driver from a height of 1.2m have visibility of an obstruction of height 0.15m.
- When the stopping sight distance for the design speed is not available on any section of a road, the speed should be restricted by a warning sign and a suitable speed-limit regulation sign.
Design speed, Kmph | 20 | 25 | 30 | 40 | 50 | 60 | 65 | 80 | 100 |
Safe stopping sight distance for design,m | 20 | 25 | 30 | 45 | 60 | 80 | 90 | 120 | 180 |
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