Capacity at intersection

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

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

 An intersection layout which permits crossing
manoeuvres 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

 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.

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.

 Design speed
 Entry , exit & island
rotary
 Width of the rotary
 Weaving length
Figure: Design elements of a rotary

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

Entry Radius:
 For rural design, entry radius of about 20-25m
 For urban design, entry radius of about 15-
20m 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.

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.

 IRC 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,
Wweaving ={(e₁+e₂)/2}+3.5m
Where e₁ = width of carriageway at the entry
e₂ = width of the carriageway at exit

 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.
Sl.no Design speed , kmph Min^ weaving length , m
1. 40 45
2. 30 30
Table: Current Indian practice as regard to weaving length.

 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 non-weaving
traffic

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,

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.

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.
Approaches Left turning StraightTraffic Right Turning
North 400 700 300
South 350 370 420
East 200 450 550
West 350 500 520

Fig: Traffic approaching the rotary

Fig.: Traffic negotiating a rotary

 Weaving width is calculated as,
w = [(e₁+e₂)/2] + 3:5 = 13.5 m
 Weaving length is calculated as
l = 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 non-weaving
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.

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 non-weaving
traffic is highest in the East-South direction.

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

BRISBANE GATEWAY
2009
BRISBANE GATEWAY
2012

 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.

 http://textofvideo.nptel.iitm.ac.in/105105107/lec
6.pdf
 http://www.skyscrapercity.com/showthread.php
?t=1489703&page=2
 http://www.civil.iitb.ac.in/tvm/1111_nptel/566_R
otary/plain/plain.html
 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

Capacity at intersection

More Related Content

What's hot (20)

Similar to Capacity at intersection (20)

Recently uploaded (20)

Capacity at intersection