Overtaking Sight Distance (OSD): Formula, IRC Values and Worked Example
Short answer: overtaking sight distance (OSD) is the clear distance a driver needs to see ahead to safely overtake a slower vehicle on a two-lane, two-way road. It is the sum of three parts: OSD = d1 + d2 + d3, where d1 = 0.278 Vb t, d2 = 0.278 Vb T + 2s and d3 = 0.278 V T (speeds in km/h, distances in metres). The IRC recommended OSD is 165 m at 40 km/h, 300 m at 60 km/h, 470 m at 80 km/h and 640 m at 100 km/h.
The three parts of OSD
| Part | What happens | Formula |
|---|---|---|
| d1 | Overtaking vehicle follows the slow vehicle at its speed Vb during the driver's reaction time t (taken as 2 s) | 0.278 Vb t |
| d2 | Overtaking manoeuvre: it accelerates, passes and returns to its lane in time T | 0.278 Vb T + 2s |
| d3 | Distance covered by the vehicle coming from the opposite direction at design speed V during the same time T | 0.278 V T |
Here s = 0.2 Vb + 6 (metres) is the minimum spacing between the two vehicles, and T = √(4s / a) is the overtaking time, with a the acceleration in m/s². If the acceleration is given in km/h per second (A), use a = A/3.6, or T = √(14.4 s / A).
Exam tip: if the question does not give the speed of the overtaken vehicle, take Vb = V − 16 km/h (about 4.5 m/s). On divided highways and one-way roads there is no opposing vehicle, so OSD = d1 + d2 only.
IRC recommended OSD values
| Design speed (km/h) | Safe OSD (m) |
|---|---|
| 40 | 165 |
| 50 | 235 |
| 60 | 300 |
| 65 | 340 |
| 80 | 470 |
| 100 | 640 |
These are the values in IRC:66 (sight distance on rural highways). OSD is measured with the driver's eye height and the object height both taken as 1.2 m, because the object is the roof of an oncoming car. That is different from stopping sight distance, where the object height is only 0.15 m.
Overtaking zones
It is rarely possible to give full OSD along the whole road, so stretches with enough sight distance are marked as overtaking zones. The minimum length of an overtaking zone is 3 × OSD and the desirable length is 5 × OSD. Signs are placed at the start and end of each zone. Where OSD cannot be given, IRC suggests at least the intermediate sight distance, which is twice the stopping sight distance.
Worked example
Find the safe overtaking sight distance on a two-lane, two-way highway with a design speed of 80 km/h. Take the acceleration of the overtaking vehicle as 2.5 km/h per second and reaction time 2 s.
- Speed of the overtaken vehicle (not given): Vb = 80 − 16 = 64 km/h.
- Spacing: s = 0.2 × 64 + 6 = 18.8 m.
- Acceleration: a = 2.5 / 3.6 = 0.694 m/s². Overtaking time: T = √(4 × 18.8 / 0.694) = 10.41 s.
- d1 = 0.278 × 64 × 2 = 35.6 m.
- d2 = 0.278 × 64 × 10.41 + 2 × 18.8 = 185.1 + 37.6 = 222.7 m.
- d3 = 0.278 × 80 × 10.41 = 231.4 m.
- OSD = 35.6 + 222.7 + 231.4 = 489.7 m, say 490 m (IRC table value for 80 km/h: 470 m). On a one-way road it would be d1 + d2 = 258.3 m.
- Overtaking zone: minimum 3 × 490 = 1470 m, desirable 5 × 490 = 2450 m.
OSD vs SSD at a glance
| Point | Stopping sight distance | Overtaking sight distance |
|---|---|---|
| Purpose | Stop before an obstruction | Pass a slower vehicle safely |
| Components | Lag distance + braking distance | d1 + d2 + d3 |
| Object height | 0.15 m | 1.2 m |
| Needed | Everywhere on the road | Only in overtaking zones of two-way roads |
Frequently asked questions
What is the formula for overtaking sight distance?
OSD = d1 + d2 + d3, where d1 = 0.278 Vb t, d2 = 0.278 Vb T + 2s and d3 = 0.278 V T, with s = 0.2 Vb + 6 and T = sqrt(4s/a). Speeds are in km/h and distances in metres.
What is the OSD for a design speed of 80 km/h as per IRC?
IRC recommends a safe overtaking sight distance of 470 m for a design speed of 80 km/h, and 640 m for 100 km/h.
What is the length of an overtaking zone?
The minimum length of an overtaking zone is three times the OSD and the desirable length is five times the OSD.
What speed is assumed for the overtaken vehicle if it is not given?
If the speed of the overtaken vehicle is not given, it is taken as 16 km/h less than the design speed of the highway.
Highway engineering is in every AE/JE paper. Our Highway Engineering notes cover sight distance, curves and pavement design with solved examples.
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