Stopping Sight Distance (SSD): Formula, IRC Values and Worked Example
Stopping sight distance (SSD) is the minimum length of road a driver must be able to see ahead so that a vehicle moving at design speed can stop before hitting an object on the road. It is the sum of the lag distance (travelled during the driver's reaction time) and the braking distance. Every highway — curve, summit, junction — must give at least the SSD everywhere along its length.
SSD formula
With speed v in m/s:
SSD = v t + v² / (2 g f)
With speed V in km/h, the same formula becomes:
SSD = 0.278 V t + V² / (254 f)
| Symbol | Meaning | IRC value |
|---|---|---|
| t | Total reaction time (perception + brake reaction) | 2.5 s |
| f | Longitudinal coefficient of friction | 0.35 (high speed) to 0.40 (low speed) |
| g | Acceleration due to gravity | 9.81 m/s² |
| Driver's eye height | Used to check sight lines on summit curves | 1.2 m |
| Object height | Height of the obstruction on the road | 0.15 m |
The lag distance grows in direct proportion to speed, but the braking distance grows with the square of speed. That is why a small increase in design speed needs a much longer sight distance.
Effect of gradient
On a slope of n%, the braking distance changes because gravity helps or opposes the brakes:
Braking distance = v² / [2 g (f ± 0.01 n)] — use + for an ascending gradient and − for a descending gradient.
On a two-way road where both directions use the same lane, the sight distance needed is twice the SSD.
Types of sight distance
| Type | Where it is needed |
|---|---|
| Stopping sight distance (SSD) | Everywhere on every road — the absolute minimum |
| Intermediate sight distance (ISD = 2 × SSD) | Two-lane roads where full overtaking sight distance cannot be given |
| Overtaking sight distance (OSD) | Two-lane two-way roads, so a fast vehicle can pass a slower one safely |
| Headlight sight distance | Valley curves at night |
| Sight distance at intersections | Junctions, so drivers can see crossing traffic in time |
Exam tip: Watch the units. If V is in km/h use 0.278 V t + V²/(254 f); if v is in m/s use v t + v²/(2 g f). Mixing them is the most common mistake in SSD questions. Also remember: on a downgrade, subtract 0.01n from f.
Worked example
Find the SSD for a design speed of 80 km/h on a level road, then on a 3% downgrade. Take t = 2.5 s and f = 0.35.
- Speed in m/s: v = 80 / 3.6 = 22.22 m/s.
- Lag distance = v t = 22.22 × 2.5 = 55.6 m.
- Braking distance on level road = v² / (2 g f) = 22.22² / (2 × 9.81 × 0.35) = 493.8 / 6.867 = 71.9 m.
- SSD on level road = 55.6 + 71.9 = 127.5 m (say 128 m).
- On a 3% downgrade: braking distance = 493.8 / [2 × 9.81 × (0.35 − 0.03)] = 493.8 / 6.278 = 78.7 m.
- SSD on the downgrade = 55.6 + 78.7 = 134.2 m (say 135 m).
- Check with the km/h formula on level road: 0.278 × 80 × 2.5 + 80² / (254 × 0.35) = 55.6 + 72.0 = 127.6 m ✓.
If the same road were a single-lane road used by traffic in both directions, the sight distance needed on level ground would be 2 × 127.5 = 255 m.
How SSD is used in design
SSD is the starting point for several other design checks:
- Summit curves: the length of a vertical summit curve is chosen so that a driver's eye at 1.2 m can see a 0.15 m object at least one SSD ahead.
- Horizontal curves: buildings, trees or cut slopes on the inside of a curve must be set back far enough to keep the SSD clear.
- Junctions and level crossings: sight triangles are drawn using the stopping distances of the approaching vehicles.
Frequently asked questions
What is the reaction time used by IRC for SSD?
IRC takes a total reaction time (perception plus brake reaction) of 2.5 seconds.
What coefficient of friction is used for SSD?
IRC uses a longitudinal friction coefficient between about 0.35 at higher speeds and 0.40 at lower speeds.
What is intermediate sight distance?
Intermediate sight distance (ISD) is twice the stopping sight distance. It is provided where overtaking sight distance cannot be given.
Why is SSD more on a downgrade?
On a downgrade gravity works against braking, so the effective friction becomes f − 0.01n and the braking distance increases.
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