The document discusses the design of rotary intersections, highlighting their purpose in facilitating traffic movement in a clockwise direction while addressing safety concerns. It includes design objectives, specifications for width, capacity calculations, and operational requirements for effective functioning. Additionally, it emphasizes the importance of adequate sight distance and unobstructed views at intersections to prevent conflicts and improve traffic flow.

1. DESIGN OF
ROTARY
PREPARED BY –
SUVAM PATRA
SUBHAJIT KUNDU
SOUVIK MONDAL
SOMESH DE KARMAKAR
SAYAN ROY
SATYAKI SEN
DEPARTMENT OF CIVIL ENGINEERING (3RD YEAR)
1

2. Definition 2
 Rotary intersections or round-
abouts are special form of at-
grade intersections laid out for
the movement of traffic in one
direction around a central traffic
island.
 The vehicles entering the rotary
are gently forced to move in a
clockwise direction in orderly
fashion.

3. Intersections 3
 More complicated area for
drivers
 Main function is to provide for
change of direction
 Source of congestion in urban
areas
 Concern for safety (fender
benders in urban, fatals in
rural)

4. Types of Intersections
 Grade separated with ramps (freeway
interchange)
 Grade separated without ramps (over or
underpass with no access)
 At-grade
 Conventional
 Roundabouts
 New concepts (e.g., “continuous flow”)
4

5. Design Objectives
 “To reduce the severity of potential conflicts
between motor vehicles, pedestrians, and
facilities while facilitating the convenience, ease,
and comfort of people traversing the
intersection.” AASHTO
 Provide ease/control of access consistent with
the function of intersecting roadways
5

6. Width of Rotary 6
 IRC suggest that a two-lane of 7m width should be kept as
7m for urban roads and 6.5 for rural roads.
 Further for a three-lane road ofb10.5m is to be reduced to
7m and 7.5m respectively for urban and rural roads.
 The width of weaving section should be higher than the
width at entry and exit. The weaving with is given as,
W weaving= {(e1+e2)/2} +3.5 m
where,e1 = width of carriageway at the entry
e2 = width of carriageway at the exit

7. Capacity 7
 Transportation road research lab (TRL) proposed the
following empirical formula to find the capacity of the
weaving section
𝑸𝒘 =
280𝑤 1 +
𝑒
𝑤
[1 −
𝑝
3
]
[1 +
𝑤
𝑙
]
where is the average entry and exit width, is the weaving
width, is the length of weaving, and is the proportion of
weaving traffic to the non-weaving traffic.
 Four types of movements at a weaving section, and are the
non-weaving traffic and are the weaving traffic.
p =
𝒃+𝒄
𝒂+𝒃+𝒄+𝒅

8. Design consideration 8
 Length of Intersecting straights with minimum site dist
ances recommended by I.R.C. are: 50 km/h.
 Minimum Sight distance 15 m along minor roads
 Weaving Distance = 110 m, 145 m, 180 m and 220 m f
or speeds 50 km/h., 65 km/h., 80 km/h, 100 km/h. resp
ectively. Minimum Weaving length: 30 to 50 m
 All traffic islands are constructed 200 mm higher than
road level. They are provided with lawn, statue or foun
tains, painted with colours in contrast with the road pa
vement.
{1-(a/b)2} R4-{b2-5a2/4} R2 –(a2b2/2) = 0

9. Operational Requirements
 Provide adequate sight distance – for approac
h and departure maneuvers
 Minimize turning and through conflicts
 Provide natural paths for permitted movemen
ts
 Avoid geometry (sharp curves/steep grades)
that complicates the driving task and adverse
ly impacts acceleration or deceleration
9

10. Intersection Sight Distan
ce – ISD
 Allow drivers to have an unobstructed view of i
ntersection
 Required ISD is the length of cross road that
must be visible such that the driver of a turnin
g/crossing vehicle can decide to and complete t
he maneuver without conflict with vehicles appr
oaching the intersection on the cross road.
10

11. Adequate ISD
 Sight Triangle – area free of obstructions nece
ssary to complete maneuver and avoid collision
– needed for approach and departure (from sto
p sign for example)
 Allows driver to anticipate and avoid collisions
 Allows drivers of stopped vehicles enough view
of the intersection to decide when to enter
11

12. Sight Triangle
 Area free of obstructions necessary to comp
lete maneuver and avoid collision – needed fo
r approach and departure (from stop sign for
example)
 Consider horizontal as well as vertical, objec
t below driver eye height may not be an obst
ruction
 AASHTO assumes 3.5’ above roadway
12

13. 13

14. 14
Sight Distance Obstructions
Hidden Vehicle

15. ISD Cases
 No control: vehicles adjust speed
 Stop control: where traffic on minor roadwa
y must stop prior to entering major roadway
 Yield control: vehicles on minor roadway mu
st yield to major roadway traffic
 Signal control: where vehicles on all approac
hes are required to stop by either a stop sign
or traffic signal
 All way stop
 Stopped major roadway left-turn vehicles –
must yield to oncoming traffic
15

16. 16
Large Tree
47’
72’
Example
50 mph
25 mph
Is sufficient stopping sight dis
tance provided?

17. 17
Large T
ree
a = 47’
b = 72’
Example
50 mph
25 mph
25 mph > 15 mph, stopping sight
distance is not sufficient for
25 mph
db
da
bd
d
ad
a
a
b


da = 220 feet
db = 47’ (220’) = 69.9’
220’ – 72’

18. Sighting Rod and Target Rod
(AASHTO)
 For vertical sight
distance with vertical
curves
 Sighting rod- 3.5 feet tall
 Target rod- 4.25 feet tall
(Top portion and bottom
2 feet are painted
orange)
18
Sighting
Rod
Target
Rod

19. Measuring at an Uncontrolle
d Intersection 19
Observer
Assistant
Decision
Point
Obstruction
X
Y

20. 20
Observer
Assistant
Movement of Assistant

21. Measuring at a Stop-controlled
Intersection
21
Observer
Assistant
Stop Bar
10 ft
SightLine

22. 22
Assistant with Target Rod (4.25 ft)
Observer with Sighting Rod (3.5 ft)

23. THANK YOU
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