Overtaking sight distance (OSD) is the minimum sight distance required for a driver to safely overtake a slower moving vehicle. It is measured along the center of the road according to standards set by the IRC. OSD is calculated based on factors like vehicle speed, the distance between vehicles, driver reaction time, vehicle acceleration rates, and road gradient. The calculation involves determining the distance traveled during the driver's reaction time (d1), the distance traveled during the actual overtaking maneuver (d2), and the distance traveled by an oncoming vehicle during overtaking (d3). On divided highways, d3 is not considered since there is physical separation of oncoming traffic. Overtaking zones that are a minimum of

1. OVERTAKING SIGHT DISTANCE (OSD)
 Minimum sight distance required to safely overtake the slow moving vehicle
 Measured along the centre of road
 As per the IRC
 Driver height (H) = 1.20 m
 Object height (h) = 1.20 m

2. Factors affecting OSD
 Speed
 Overtaking vehicle  Overtaken vehicle  Vehicle from opposite direction
 Distance between overtaking & overtaken vehicle
 Skill and reaction of the driver
 Overtaking vehicle rate of acceleration
 Gradient of road

3. ANALYSIS & DETERMINATION OSD
A1 A2 A3
B1 B2
C1
C2
 Overtaking vehicle speed = V kmph
 Overtaken vehicle speed = Vb kmph
d1
d2
 Time taken in actual overtaking operation = T
d3
OSD = d1 + d2 + d3

4. ASSUMPTIONS IN OSD ANALYSIS
 Assumptions made in the d1 , d2 and d3 calculations
Reduction in overtaking vehicle speed
Making decision within reaction time(t)
Accelerating & overtaking within overtaking time(T)
Distance d2 consist of three parts
Single vehicle overtake single vehicle

5. OSD Calculation/Determination
OSD = d1 +d2 +d3 d1 – distance travelled by overtaking
vehicle during reaction time
d2 – distance travelled by
overtaking vehicle during
actual overtaking operation
d3 – distance travelled by oncoming
vehicle during actual overtaking operation

6. d2 - calculation
𝒅𝟏 = 𝑽𝒃 ∗ 𝒕
Vb – Speed of overtaken vehicle (m/sec.)
t – Reaction time (sec.)
Vb = Design speed – 16 kmph
t = 2.5 sec.
d1 - calculation
s s
b
d2 = s + b + s
=> d2 = 2s+b
s – spacing maintain by overtaking vehicle

7. d2 – calculation conti…
𝒔 = 𝟎. 𝟕 ∗ 𝑽𝒃 + 𝟔
Lets T is the actual overtaking duration
Vb in kmph s in meter
Then
𝒅𝟐 = 𝟐𝒔 + 𝑽𝒃 ∗ 𝑻
Also during this time overtaking vehicle accelerate
from Vb kmph and overtaking is completed while
reaching design speed V kmph
𝒅𝟐 = 𝑽𝒃 ∗ 𝑻 +
𝟏
𝟐
∗ 𝒂 ∗ 𝑻𝟐
…equation (i)
…equation (ii)
…equation (iii)
From equations (ii) and (iii)
𝟐𝒔 + 𝑽𝒃 ∗ 𝑻 = 𝑽𝒃 ∗ 𝑻 +
𝟏
𝟐
∗ 𝒂 ∗ 𝑻𝟐
⇒ 𝟐𝒔 =
𝟏
𝟐
∗ 𝒂 ∗ 𝑻𝟐
⇒ 𝑻 =
𝟒𝒔
𝒂
Where,
a is an acceleration of
overtaking vehicle (m/sec2)
Speed (kmph) 25 30 40 50 65
a (m/sec2) 1.41 1.30 1.24 1.11 0.92

8. d3 – calculation
d3 = V*T
Distance travelled by opposing vehicle during T time
Where,
V – design speed in m/sec.
OSD = d1 +d2 +d3
OSD = d1 + (2s + Vb *T) + d3
 On divided carriage way d3
need not to be provided
So, for divided carriageway OSD become
OSD = d1 + (2s + Vb *T)

9. Overtaking Zone
 These are the stretches of the highway on which the overtaking is permitted.
 Overtaking zones are provided when OSD cannot be provided throughout the
length of the highway.
 These zones are marked with wide roads.
 The desirable length of overtaking zones is
5 time OSD and the minimum is 3 times OSD.