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ModernRoundabout
Geometric DesignGuidelines
Dr. Alberto M. Figueroa
Associate Professor ofTransportation Engineering
UPR at Mayaguez
ITE-Puerto Rico SpringMeeting
March 16, 2012
Seminar Objectives
Identify general characteristics of
modern roundabouts.
Understand user and location
considerations for modernroundabouts.
Learn the geometric design basics
of modern roundabouts.
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Main References
AASHTO.A Policy on Geometric Design ofHighways and Streets. 2011.
NCHRP. Roundabouts: An InformationalGuide, 2nd Edition. Report 672. 2010.
FHWA. Manual on Uniform Traffic ControlDevices. 2009.
NCHRP. Roundabouts in the United States.
Report 572. 2007.FHWA. Roundabouts: An Informational
Guide. June 2000.
Intersection Design Alternative:The Modern Roundabout
Roundabouts are becoming a popular option toconventional intersection design in the UnitedStates.
FHWA recommends it as proven safetycountermeasure.
Reduction in vehicle speeds
Reduction in conflict points
Improved crash rate and injury rate records
Lower overall delay than other controlledintersections in typical conditions
Driver behavior affects the performance ofroundabouts
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Key Roundabout Characteristics
Geometric features that create a low-speedenvironment
Circular shape
Yield controlon entry
Deflectionat entry
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Roundabout Operation Rules
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Source Unknown
ROTARY
United States Geographical Survey
TRAFFIC CIRCLE
Other Circular Roadway Designs
Lee Rodegerdts
NEIGHBORHOODTRAFFIC CIRCLE
Photo:LeeR
odegerdts
Photo: Maryland SHA
Other Roundabouts?
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Slow entry speeds
Appropriate number of lanes
Smooth channelization
Adequate accommodation for designvehicles
Meeting needs of pedestrians andbicyclists
Appropriate sight distance and visibility
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Key Objectives of Roundabout
Geometric Design
RoundaboutGeometric
Elements
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Location Considerations
A modern roundabout should beconsidered anywhere a traffic signal orstop control is under consideration
Roundabouts can be advantageous ina number of locations
However, certain constraints mayadversely affect their feasibility at aspecific site
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Roundabout Application
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Wide NodesNarrow Roads
Roundabout Constraints
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Two-lane
Single-laneMini-roundabout
Categories of
ModernRoundabouts
Roundabout Category Comparison
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Levels of Operational Analysis
Planning level
Based on daily volumes
Determine necessary number of lanes
Operational level
Based on peak hour volumes
Determine capacity to analyze operations
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Planning Level - Number of Lanes
Number of lanes affects capacity and size
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Rule of Thumb for Single-Lane Roundabouts
If the sum of the entering and conflicting volumesis less than 1,000 vehicles per hour (veh/h), then asingle-lane entry can be reasonably assumed tooperate within its capacity.
Example: Estimating Number of LanesUsing Turning-Movement Volumes
Entering volume + Circulating volume = X
250 + 617 = 867 < 1,000
534 + 224 = 758 < 1,000
317 + 534 = 851 < 1,000
751 + 203 = 954 < 1,000
SINGLE LANE OK
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Exhibit 3-14 Volume Thresholds forDetermining the Number of Entry Lanes Required
Operational LevelCapacity of Single-Lane and Multilane Entries
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Key Considerations in Horizontal Design
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1. Design speed
2. Design vehicle
3. Size
4. Path alignment
5. Central Island
6. Splitter Islands
7. Sight Distance
8. Cross andLongitudinalSlopes
9. Signs andMarkings
10. Non-motorizedTransportation
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Consideration 1: Design Speed
Design speed is defined by the theoreticalspeed drivers could achieve through theroundabout
Achieve an appropriate design speed forthe entry movements and consistentspeeds for all movements
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Consideration 2: Design Vehicle
The design vehicle is bestidentified at the start ofthe project and evaluatedearly in the design process
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Consideration 3: Size
The size of a roundabout is determined by a
number of design objectives, such as design
speed, path alignment, and design vehicles
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Consideration 4: Path Alignment
Multilane roundabouts should align vehicles into theappropriate lane within the circulatory roadway
A good design balances entry speed and pathalignment
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Position of Roundabout
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Entry Alignments
Entry Alignments
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Entry Alignments
Entry Alignment Design Entry widths for single-lane entrances range from
14-18 ft
Entry radii at urban single-lane roundabouts rangefrom 50-100 ft
Typical entry angles are between 20 and 40
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Exit Alignment Design
Exit curb radii should be no less than 50 feet, withvalues of 100-200 feet being more common
Issues at Entry and Exit with Large Vehicles
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Multi-lane Roundabout (Path Overlap)
Desirable Vehicle Path
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Path Alignment
Entry and exitlanes shouldbe designedwithout toomuchseparation toavoidproblems
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Issues in Path Alignment
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Vehicle Path Radii Consistency
R1- the entry path radius, is theminimum radius on the fastestthrough path prior to the entranceline.
R2 - the circulating path radius, is theminimum radius on the fastestthrough path around the centralisland.
R3 - the exit path radius, is theminimum radius on the fastestthrough path into the exit.
R4 - the left-turn path radius, is theminimum radius on the path of the
conflicting left-turn movement. R5 - theright-turn path radius, is the
minimum radius on the fastest pathof a right-turning vehicle.
Fastest Vehicle Path (Single LaneRoundabout)
Maximum entering designspeeds based on atheoretical fastest path of20 to 25 mph arerecommended.
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Fastest Vehicle Path (Double Lane
Roundabout)At multilane roundabouts, maximum entering design speeds of
25 to 30 mph are recommended based on a theoretical
fastest path assuming vehicles ignore all lane lines.
Fastest Vehicle PathCritical Right-Turn Movement
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Fastest Path Speeds
Speed-radius relationships
0
5
10
15
20
25
30
35
40
0 50 100 150 200 250 300 350 400
Radius (ft)
Speed(mph)
e =+ 0. 02 e =- 0. 02
Estimation of Entry Speed
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Estimation of Exit Speed
Speed Consistency (Cont.)
(Robinson, 2000)
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Consideration 5: Central Island
A circular central island is preferred becausethe circulatory roadway helps promoteconstant speeds
Typical circulatory roadway widths range from16 to 20 ft for single-lane roundabouts
The size plays a key role in determining theamount of deflection imposed on the throughvehicles path
The island may include enhancements,serving an aesthetic purpose and providingconspicuity of the intersection forapproaching motorists
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Truck Apron
Generally be 3-15 feet wide
Cross slope of 1%-2% away from the central island
To discourage use by passenger vehicles, theouter edge of the apron should be raised 2-3 in
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Consideration 6: Splitter Island
Splitter islands provide refuge for pedestrians,control speeds, guide traffic, separate trafficstreams, and deter wrong-way movements
Longer splitter islands can help reduceconfusion for entering motorists
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Photo:SkagitCountyPublicWo
rksDept.(usedwithpermission)
Splitter IslandDimensions
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Consideration 7:
Sight Distance and Visibility
Adequate sight distance and visibility are neededfor a roundabout to operate safely
Intersection sight distanceis evaluated at each entryto ensure a driver can seeand safety react topotentially conflictingvehicles
d1= Entering stream distance
d2= Circulating stream distance
Intersection Sight Distance
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Stopping Sight Distance @
Roundabout Approach
Stopping sight distance should be provided atevery point within a roundabout and on eachapproach
The roundabout should be visible from a distanceand focus the attention of drivers
Stopping Sight Distance @Circulatory Roadway
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Stopping Sight Distance @ Exit
Crosswalk
Consideration 8:Cross and Longitudinal Slopes
Cross slope of 2% away from the centralisland should be used for the circulatoryroadway on single-lane roundabouts
Two common methods for vertical designof circulatory roadway within a multilane
roundabout are outward sloping andcrowned circulatory roadway
Roundabouts should preferably belocated where grades through theintersection are less than 4%
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Roundabout Cross Section
(Robinson, 2000)
Consideration 9:Pavement Markings and Signs
Markings and signs areintegral to roundaboutdesign and shouldfacilitate through andturning movements
Typical pavement
markings delineate theentries, exits, andcirculatory roadway
Overall signing conceptsimilar to intersectionsigning
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Example ofRoundabout
SigningMUTCD 2009
Part 2 Signs
Example ofRoundaboutSigningMUTCD 2009
Part 2 Signs
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Consideration 10:
Pedestrian Design Treatments
Sidewalks at roundabouts should be set backfrom the circulatory roadway to discouragepedestrians from crossing to the central island
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Consideration 10:Bicycle Design Treatments
Bicycle lanesshould end inadvance ofroundabouts
Bicyclists may actas a vehicle orpedestrian
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Key Dimensions of Non-Motorized
Design Users
Other Design Details and Applications
Right-turn bypass lanes
Access management:restrict nearbydriveways
At-grade rail crossings:challenges related tocrossing control andqueue clearance
Bus stops: Can be provided on the entry or exitside of a roundabout, but not within thecirculatory roadway
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