roundabout figueroa.pdf

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ModernRoundabout

Geometric DesignGuidelines

Dr. Alberto M. Figueroa

Associate Professor ofTransportation Engineering

UPR at Mayaguez

[email protected]

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.
mailto:[email protected]:[email protected]


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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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- -


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Wide NodesNarrow Roads

Roundabout Constraints

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-


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Two-lane

Single-laneMini-roundabout

Categories of

ModernRoundabouts

Roundabout Category Comparison

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-

-

-


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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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3

Example ofRoundabout

SigningMUTCD 2009

Part 2 Signs

Example ofRoundaboutSigningMUTCD 2009

Part 2 Signs


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3

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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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