2. 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
3. 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
4. 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
5. 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
6. 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
7. 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
8. 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
9. 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)
10. 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.