capacity of intersection -rotary-roundabout

1. Capacity at Intersection
Dr.Hana Naghawi
Done by :
Hazem al mahamid(8170713)
Ahmad mansour (8171257)

2. Intersection:
It is defined as the
general area where two
or more highways join or
cross, which includes
the roadway and
roadside facilities for
traffic movements in
that area.
Picture: Kathipara Junction in
Chennai, India

3. 1. At-grade intersection .
2. Grade separated intersection .
An intersection where all roadways join or cross at the
same level. The traffic maneuvers like merging,
diverging and crossing are involved in the intersection at
grade. It is further classified as
i. Un-channelized
ii. ii. Channelized iii. Rotary intersection
iii. iv. Signalized intersection
Classification of intersection:

5.  An intersection layout which permits crossing
maneuvers at different levels is known as grade
separated intersections. It is further classified as
i. Underpass
ii. Overpass
iii. Trumpet Interchange
iv. Diamond Interchange
v. Cloverleaf Interchange
vi. Partial Cloverleaf Interchange
vii. Directional Interchange
viii. Bridged Rotary
Grade separated at intersection:

7.  Rotary intersections or roundabouts are special form
of at-grade intersections laid out for the movement of
traffic in one direction around a central traffic island
before they can weave out of traffic flow into their
respective direction.
 In India and other countries where ‘‘keep to the left’’
regulation is followed, the vehicles entering the rotary
are gently forced to move in a clock-wise direction in
orderly fashion.
Capacity at intersection i.e rotary
intersection

10. Traffic operation at rotary :
 1) Diverging: Traffic operation when the vehicles
moving in one direction is separated into different
streams.
 2) Merging : Process of joining the traffic coming
from different approaches and going to a common
destination into a single stream.
 3) Weaving: Combined movement of both merging
and diverging movements in the same direction.

11. Design element at intersection

12. Design speed :
 Normal practice is
to keep the design
speed as
-30kmph for urban
areas and
-40kmph for rural
areas .
Fig: Traffic operation in a
rotary

13. Entry, exit and island radius :
Entry Radius:
 For rural design, entry radius of about 20-25m
 For urban design, entry radius of about 1520m is
suitable.
Exit Radius:
 Exit radius should be higher than radius of rotary
island.
 General practice is to keep exit radius as 1.5 to 2
times the entry radius.

14. Island
Radius:
 It is governed by the
rotary design speed
and theoretically
should be equal to the
radius at entry.
 Central island radius is
kept slightly higher
than that of the curve
at entry i.e. 1.3 times
that of the entry curve
is adequate for all
practical purpose.

15. Width of the rotary :
 IRC (Indian road congress) suggest that a two-lane road of 7m width
should be kept as 7m for urban roads and 6.5m for rural roads.
Further for a three-lane road of 10.5m 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 width is given as,
Where e₁ = width of carriageway at the entry e₂ = width of
the carriageway at exit

16. Weaving length:
 Determines how smoothly the traffic can merge and diverge.
 The ratio of weaving length to the weaving with i.e. 4:1 is regarded as
the minimum value suggested by IRC.
 Very large weaving length is also dangerous , as it may encourage over-
speeding .
Table: Current Indian practice as regard to weaving length.

17. :capacity
 The capacity of rotary is determined by the capacity of each weaving
section. Transportation Road Research Lab (TRL) proposed the
following empirical formula to find the capacity of the weaving section.
 Where, e=average entry & exit width i.e. =(e₁+e₂)/2 w= weaving width l
= weaving length p = proportion of weaving traffic to the nonweaving
traffic

18. Figure shows four types of movements at a weaving section, a
and d are the non-weaving traffic and b and c are the weaving
traffic.
Therefore, proportion of weaving traffic to the non-weaving
traffic,

19. This capacity formula is valid only if
following condition are satised :
1) Weaving width at the rotary is in between 6 and 18
meters.
2) The ratio of average width of the carriage way at entry
and exit to the weaving width is in the range of 0.4 to 1.
3) The ratio of weaving width to weaving length of the
roundabout is in between 0.12 and 0.4.
4) The proportion of weaving traffic to non-weaving
traffic in the rotary is in the range of 0.4 and 1.
5) The weaving length available at the intersection is in
between 18 and 90 m.

20. :Question
Q1. Width of approach for a rotary intersection is 12m.
The entry and exit width of the rotary is 10m. Find
capacity of the rotary.

21. Solution :
 Fig: Traffic approaching the rotary

22.  Fig.: Traffic negotiating a rotary

23.  Weaving width is calculated as,
w = [(e₁+e₂)/2] + 3:5 = 13.5 m
 Weaving length is calculated as
I= 4*w = 54 m
 The proportion of weaving traffic to the non-weaving
traffic in all the four approaches is found out first.
 Let the proportion of weaving traffic to the
nonweaving traffic in West-North direction be
denoted as pWN, in North-East direction as pNE, in
the East-South direction as pES, and finally in the
South-West direction as pSW.

24.  Then using equation, pES =
(510+650+500+600)/(510+650+500+600+250+375) =
2260/2885 = 0.783 .
 pWN = (505+510+350+600/505+510+350+600+400+370)
=1965/2735 = 0.718 .
 pNE = (650+375+505+370/650+375+505+370+510+408)
=1900/2818 = 0.674 .
 pSW =( 350+370+500+375/350+370+500+375+420+600)
=1595/2615 = 0.6099 .
Thus, the proportion of weaving traffic to nonweaving
traffic is highest in the East-South direction.

25. 370+420+350

26. 370
350
420

27. 370
350
420420

28. 370
350
420420
510
505
400
+400

29. 505+408
250+650 350+420
500+250
350+400
650+408
500+420
505+400
420250
400408
600375
370
510
N
S
E
W

30.  Weaving width is calculated as,
w = [(e₁+e₂)/2] + 3:5 = 13.5 m
 Weaving length is calculated as
I= 4*w = 54 m
 The proportion of weaving traffic to the non-weaving
traffic in all the four approaches is found out first.
 Let the proportion of weaving traffic to the
nonweaving traffic in West-North direction be
denoted as pWN, in North-East direction as pNE, in
the East-South direction as pES, and finally in the
South-West direction as pSW.

31.  Then using equation, pSE =
(350+420+505+408)/(350+420+505+408+370+400)
=0.686
 pEN = (500+250+350+400/ 500+250+350+400+600+420)
= 0.5952 .
 pNW = (650+408+500+420/ 650+408+500+420+375+250)
=0.7599 .
 pWS =( 505+400+350+650/ 505+400+350+650+510+408)
=0.6748 .
Thus, the proportion of weaving traffic to nonweaving
traffic is highest in the NORTH-WEST direction.

32.  Therefore, the capacity of the rotary will be capacity
of this weaving section. From the equation ,

33. BRISBANE GATEWAY 2009 BRISBANE GATEWAY 2012

34. Conclusion :
Traffic rotaries reduce the complexity of crossing
traffic by forcing them into weaving operations.
The shape and size of the rotary are determined by the
traffic volume and share of turning movements.
Capacity assessment of a rotary is done by analyzing
the section having the greatest proportion of weaving
traffic.
The analysis is done by using the formula given by
TRL.

35. Reference :
 https://www.skyscrapercity.com/showthread.php?t=14
89703&page=2 .
 L. R. Kadiyali, ‘Traffic Engineering and Transportation
Planning’, Khanna Publishers, 8th Edition:2013, New
Delhi.
 S.K.Khanna, C.E.J.Justo, A.Veeraragavan, ‘Highway
Engineering’ revised 10th edition 2014, NemChand &
Bros, Roorkee .