traffic

1. DESIGN OF TRAFFIC ROTARIES
AND ROUNDABOUTS
Dr. Samson Mathew
Professor of Civil Engineering
National Institute of Technology, Tiruchirappalli, Tamilnadu

2. DESIGN OF TRAFFIC ROTARIES
IRC 65-1976
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3. Designing for Road Safety: Junctions
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4. ROTARY ISLAND
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5. Advantages of Rotary
• Orderly and regimented traffic flow – one way movement
• All traffics proceeds simultaneously and continuously at uniform low
speed
• Weaving movement replace the angular crossing
• All turns can be made with ease
• All turns can be made with ease
• Suited for intersections with five or more intersection legs and/or heavy
turning movement
• Self- governing – no need of police/ signal
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6. Disadvantages
• Flow reaches capacity, weaving gives way to stop and go
motion
• Rotary can accommodate no more traffic than a properly
designed channelized layout.
• Requires more land – may not be feasible in built up locations.
Requires more land – may not be feasible in built up locations.
• Topographic condition in some localities may make
impracticable
• If pedestrian traffic is large, rotary itself is not sufficient
• High speed roads require large rotary
• Where provided at close intervals, they are trouble some
• Not suitable for stage development
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7. GUIDELINES / WARRANT
• Volumes entering from different intersections legs are
approximately equal
• Total volume of 3000 pcu/hr is max practical capacity
• Total volume of 3000 pcu/hr is max practical capacity
• Appropriate when turning traffic is high
• When more then four approaches to the junction
• Not warranted for intersection carrying less traffic
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8. Different shapes
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9. 9

10. 10

11. 11

12. Radius of curve at entry
- design speed, superelevation and coefficient of friction
Rotary Design speed Radius at entry
Rural areas 40 kmph 20-35
Urban areas 30 kmph 15-25
Urban areas 30 kmph 15-25
• Radius of curve at exit =1.5 to 2 times Radius at entry
> Radius of the Rotary Island
• Radius of central Island = 1.33 times Radius at entry
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13. WEAVING LENGTH
• Weaving length -
– width of weaving section
– average width at entry
– total traffic
– proportion of weaving traffic
– proportion of weaving traffic
• 4 times weaving width
• Minimum: 45m for 40 kmph
30m for 30 kmph
• Maximum: 90m for 40 kmph
(twice of minimum length) 60m for 30 kmph
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14. Width of carriageway at entry and exit
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15. Width of Rotary Carriageway
• Width of non-weaving section = Widest single entry into the
rotary
• Width of weaving section = w = (e1+e2)/2 + 3.5m
• Entry angle = 60º
• Entry angle = 60º
• Entry angle = 30º
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16. 16

17. Capacity of Rotary
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18. 18

19. The following adjustments in the capacity calculated by the above
formula are suggested:
1. Where the entry angle is between 0⁰ and 15⁰, deduct 5% from the
capacity of the weaving section.
2. Where the entry angle is between 15⁰ and 30⁰, deduct 2.5% from the
capacity of the weaving section.
3. Where the exit angle is between 60⁰ and 75⁰, deduct 2.5% from the
capacity of the weaving section.
capacity of the weaving section.
4. Where the exit angle is greater than 75⁰, deduct 5% from the
capacity of the weaving section.
5. Where the internal angle is greater than 95⁰, deduct 5% from the
capacity of the weaving section.
6. Where the pedestrian flow at exit from the roundabout exceeds
300 per hour, an arbitrary deduction of 1/6th should be made in the
practical capacity of the preceding weaving section.
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20. Channelizing Islands
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21. Outer Curb Line
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22. Camber and Superelevation
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23. Sight Distance
• A Stopping Sight Distance appropriate to the approach
speed should be ensured.
Speed Sight Distance
30-40 kmph 30-45 m
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24. Grades
• A rotary should preferably be located on level ground.
• It may be sited to lie on a plane which is inclined to the
horizontal at not more than 1 in 50.
horizontal at not more than 1 in 50.
• Rotary may be located on summit or in valley. But, it is
essential that sufficient Sight Distance is available.
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25. Curbs
• The curbs for channelizing and central islands should be
either vertical curbs or mountable curbs.
Height of curb at central island
Rural Sections <225 mm
Rural Sections <225 mm
(Mountable type is preferable)
Urban Sections should not obstruct visibility
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26. Pedestrians and Cycles
• It is desirable to segregate the cyclist by providing
separate cycle tracks as per IRC:11-1962.
• Where the channelizing island is short(A), the cycle
track should be led behind its tail.
• Where the channelizing island is long(B), a gap should
be left in the island to accommodate the cycle track.
• It is desirable to provide flashing signals to warn about
pedestrians and cyclist-crossings at rotary legs.
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27. Layout of cycle tracks and footpaths at a rotary
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28. Signs and Markings
• Rotaries require to be adequately designed for both day
and night travel.
• A red reflector about one metre above the road level or
a vertical cluster of such reflectors at a height of 0.3 to
1.0 m should be fixed on the nose of each directional
1.0 m should be fixed on the nose of each directional
island and on the curb of the central island.
• Curbs of islands should be painted with vertical black
and white stripes, each 500 m wide, to improve visibility.
• All pavement markings should be as per IRC: 35-1997.
• Exit roads should be indicated by signs and directional
arrows.
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29. • The standard warning sign indicating the presence of
the rotary should be put up in advance.
Warning Sign- Rotary
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30. Illumination
• Illumination of the rotary junction at night is
very desirable.
Small central island Single lantern
Small central island Single lantern
mounted
(<20 m diameter) centrally at height of
8 m or more
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31. 31

32. Landscaping
• A rotary provides ample scope for effective development
of landscape.
• Planting on the central island should block off the view
of approaching headlights so that an impression is not
of approaching headlights so that an impression is not
created that the road runs straight through.
• It is desirable that the motorist gets an unobstructed
view for adequate distance along the chord of the curve
to be able to pick off a particular exit road.
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33. Planting on the central island
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34. Drainage
• Adequate attention should be paid to drainage within the
area of the rotary junction.
• The water likely to accumulate at the edges of the rotary
island should be drained by means of curb and gutter
section having an outlet to underground pipes through
appropriately placed gulley traps.
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35. ROUNDABOUT
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36. Introduction
A roundabout is a form of intersection which accommodates traffic
flow in one direction around a central island, operates with yield
control at the entry points, and gives priority to vehicles within the
roundabout (circulating flow).
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NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM REPORT 672
(2010) TRANSPORTATION RESEARCH BOARD WASHINGTON, D.C.

37. Benefits of Roundabouts
Roundabouts are becoming more popular based on the multiple
opportunities to improve safety and operational efficiency, and
provide other benefits.
Traffic Safety
• Fewer conflict points. For comparison, an uncontrolled four-legged
intersection has 28 conflict points, but a roundabout has only eight
intersection has 28 conflict points, but a roundabout has only eight
conflict points.
• No Right-turn accidents, the cause of most fatal or serious accidents
at cross intersections.
• Simple decision-making at the entry point.
• Slow relative speeds of all vehicles in the conflict area.
• Splitter islands provide refuge for pedestrians and permit them to
cross one direction of traffic at a time.
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38. Operational Performance
When operating within their capacity, roundabouts typically have
lower overall delay than signalized and all-way stop-controlled
intersections. The delay reduction is often most significant during
non-peak traffic periods.
Traffic Calming
Roundabouts can have traffic calming effects on streets by reducing
vehicle speeds using geometric design rather than relying solely on
vehicle speeds using geometric design rather than relying solely on
traffic control devices.
Pedestrian Safety
Due to the reduction of vehicle speeds in and around the
intersection, roundabouts can improve pedestrian crossing
opportunities. Additionally, the splitter island refuge area provides
the ability for pedestrians to focus on one traffic stream at a time
while crossing.
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39. Ongoing Operations and Maintenance
A roundabout typically has lower operating and maintenance costs
than a traffic signal due to the lack of technical hardware, signal
timing equipment, and electricity needs.
Approach Roadway Width
Approach Roadway Width
A roundabout may reduce the amount of widening needed on the
approach roadways in comparison to alternative intersection forms.
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40. Roundabout Design Features
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41. Characteristics of Roundabout
A modern roundabout has the following distinguishing characteristics:
 Channelized approaches
 Yield control on all entries
 Circulation of all vehicles in one
 Circulation of all vehicles in one
direction around the central island
 Appropriate geometric curvature
to encourage slow travel speeds
through the intersection.
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42. TYPES
1. Normal Roundabout: A roundabout with a one-way circulating
roadway around a curbed central island 4 m or more in diameter.
2. Mini or Small Roundabout: A roundabout with a one-way
circulating roadway around a flush or slightly raised circular island
less than 4 m in diameter.
3. Double Roundabout: A single intersection with two normal or mini-
3. Double Roundabout: A single intersection with two normal or mini-
roundabouts either contiguous or connected by a central link road or
curbed island.
4. Ring Junction: A two-way circular ring road which is accessed by
external spoke roads by way of 3-leg mini-roundabouts or T-
intersections.
5. Signalized Roundabout: A roundabout in which traffic signals
regulate one or more of the entries.
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43. Mini Roundabout
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44. Double Roundabout
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45. Features of Typical Two-Lane Roundabout
Features of Typical single-Lane Roundabout
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46. Types based on number of lanes:
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47. Geometric Design of Roundabouts
Basic elements for design considerations of roundabouts are:
Design Vehicle
Roundabouts should provide a turning path for the largest vehicles
expected to use the facility in significant numbers.
Example Design Vehicle Path Check 47

48. Design Speed
The design speed of roundabouts should be around 40-50km/h (25-30
mph).
Speed-Radius Relationship (U.S. Customary Units) Vehicle Path Radii
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49. Sight Distance and Visibility
Evaluation of sight distance at roundabouts includes both
Intersection sight distance and stopping sight distance.
Intersection Sight Distance
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50. Stopping Sight Distance
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51. Deflection
The single most significant feature of a roundabout design is
adequate entry, through, and exit deflections. Adjusting the
geometry of the entry and exit lanes to achieve the proper deflection
will ensure the necessary reduction in speed.
Central Island
A central island consists of a raised, often landscaped, non-
traversable area and, if applicable, a truck apron. The size of the
central island is determined principally by the space available and
central island is determined principally by the space available and
the need to obtain sufficient deflection to control through vehicle
speed while providing adequate radii for required turning
movements.
Splitter Island
Their purpose is to provide refuge for pedestrians, assist in
controlling speeds, guide traffic into the roundabout, physically
separate entering and exiting traffic streams, and deter wrong-way
movements. Additionally, splitter islands can be used as a place for
mounting signs.
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52. Splitter Island
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53. Use of Longer Splitter Islands in a Rural Environment
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54. Circulating Width
Circulating width depends on the swept paths, and the layout and
width of exits and entries. Generally, consistent width throughout the
roadway is preferable. It is recommend that the width should be 1 to
1.2 times the maximum entry width.
Entry Design
Entry width is one of the most significant factors in the capacity of a
roundabout. The entry width should be designed to accommodate
roundabout. The entry width should be designed to accommodate
the design vehicle while ensuring adequate deflection.
Exit Design
While the entry curves are designed to slow vehicles down, the exit
should be as easy for vehicles to negotiate as possible. For this
reason, the exit radius should generally be greater than the
circulating radius. Ideally, a straight path tangential to the central
island is preferable for departing vehicles.
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55. Entry Design
Exit Design
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56. Grades and Super elevation
• At grades between 3% and 6%, certain conditions can be
considered unsafe.
• Areas of normal super elevation on the entrance and exit ramps,
shouldn’t have lateral slopes greater than 3%.
• On roundabouts located on a sloped plane, no slope should be
• On roundabouts located on a sloped plane, no slope should be
added to the normal slope of the circulating roadway (1 to 2%).
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57. Performance Analysis
A Performance analysis considers a precise set of geometric
conditions and traffic flow rates defined for a 15 minute analysis
period for each roundabout entry. Measure of effectiveness is
defined as a quantitative parameter whose value is an indicator of
the performance of transportation facility or service from the
perspective of the users or service.
The HCM (2000) defines three specific measures of effectiveness:
1. Degree of saturation
2. Queue length
3. Average queuing delay
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58.  Degree of Saturation
Degree of saturation = Demand at the roundabout entry
Capacity of the entry
Australian design procedure suggests that the degree of saturation,
for an entry lane should be less than 0.85 for satisfactory operation.
for an entry lane should be less than 0.85 for satisfactory operation.
When the degree of saturation exceeds this range, the operation of
roundabout will deteriorate rapidly. Queues may form and delay
begins to increase exponentially.
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59.  Queue Length
Queue length is important for assessing the adequacy of the
geometric design of the roundabout approaches.
The average queue length (L) of vehicles can be calculated by,
Where,
V = Entry flow, vph
d = Average delay, sec/hr
Average queue length is useful for comparing the roundabout
performance with other intersection forms and other planning
procedures that use intersection delay as an input.
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60.  Average Queuing Delay
Queuing delay occurs when drivers in the entry are waiting to accept a
gap in the circulatory traffic. It is the time that a driver spends queuing
and then waiting for an acceptable gap in the circulatory flow.
Average queuing delay can be calculated by the formulae recommended
by HCM:
Where,
Q e = Entry capacity, vph
x = Degree of saturation
T = Flow period in hours
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61. APPLICATION OF TRAFFIC CONTROL
DEVICES
• Pavement Markings
• Signing
• Signalization
At roundabouts, pavement markings and signs work together to
At roundabouts, pavement markings and signs work together to
create a comprehensive system to guide and regulate road users.
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62. Example of Pavement Markings for single and two lane Roundabouts
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63. Signing
The overall concept for roundabout signing is similar to general
intersection signing. Proper regulatory control, advance warning,
and directional guidance are required to avoid driver expectancy-
related problems.
Example of Regulatory and Warning Sign Layout 63

64. THANK YOU