Stopping sight distance (SSD) is the minimum distance required for a driver to see an object on the road and bring the vehicle to a complete stop before reaching it. It is a fundamental parameter in road geometric design, determining minimum curve radii, crest vertical curve lengths, and sight line clearances.
Stopping sight distance formula
SSD is calculated as the sum of two components — the perception-reaction distance and the braking distance:
SSD = (v × t) + v² ÷ (254 × f)
Where:
v = design speed in km/h
t = perception-reaction time (typically 2.5 seconds per AASHTO)
f = longitudinal friction coefficient (depends on speed and surface)
254 = conversion factor for km/h to m/s²
On a gradient, the braking distance component is adjusted:
Braking distance = v² ÷ (254 × (f ± g/100))
Where g is the gradient percentage — positive for uphill, negative for downhill.
SSD values by design speed
| Design speed (km/h) | Reaction distance (m) | Braking distance (m) | Total SSD (m) |
| 40 | 27.8 | 16.0 | 45 |
| 50 | 34.7 | 25.0 | 65 |
| 60 | 41.7 | 36.0 | 85 |
| 80 | 55.6 | 64.0 | 130 |
| 100 | 69.4 | 100.0 | 185 |
| 110 | 76.4 | 121.0 | 220 |
| 120 | 83.3 | 144.0 | 250 |
| 130 | 90.3 | 169.0 | 285 |
Effect of gradient on SSD
Gradient significantly affects stopping distance. On downhill grades, braking distance increases because gravity assists vehicle motion. On uphill grades, stopping distance decreases.
For a 6% downhill grade at 100 km/h:
Braking distance = 100² ÷ (254 × (0.35 − 0.06)) = 135 m (vs 100 m on flat)
For a 6% uphill grade at 100 km/h:
Braking distance = 100² ÷ (254 × (0.35 + 0.06)) = 96 m (vs 100 m on flat)
This is why crest vertical curves on steep descents require longer sight distances than flat terrain.
SSD and crest vertical curve design
Crest vertical curves must be long enough to provide the required SSD. The minimum length of a crest vertical curve is determined by the K value:
L = K × A
Where L is curve length in metres and A is the algebraic difference in gradients (%). The K value depends on design speed.
Typical K values for crest curves (AASHTO):
40 km/h — K = 4
60 km/h — K = 11
80 km/h — K = 26
100 km/h — K = 51
120 km/h — K = 91
130 km/h — K = 114
Sight distance obstructions
On horizontal curves, sight lines may be obstructed by embankments, walls, vegetation, or barriers inside the curve. The required clearance from the centreline of the inner lane to any obstruction is calculated from the relationship between design speed, curve radius, and required SSD.
Common obstructions to check on drawings:
Road signs and barriers placed inside curves
Retaining walls on cut sections
Vegetation and trees on embankment slopes
Parked vehicles on urban roads
Bridge abutments and wingwalls
Passing sight distance
Passing sight distance (PSD) is considerably longer than SSD — it is the distance required for a driver to safely overtake a slower vehicle on a two-lane road. PSD is typically 4 to 8 times greater than SSD at the same design speed, and is used to determine the minimum length of passing zones and the spacing of no-overtaking lines.
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