Stopping Distance

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.