roundabouts fhwa-sa-10-006 technical summary adapted from photo by lee rodegerdts (used with...

Roundabouts

FHWA-SA-10-006

Technical Summary

Adapted from photo by Lee Rodegerdts (used with permission)

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Presentation Overview

• Characteristics of modern roundabouts with emphasis on principles-based approach to design

• Key considerations for planning, analysis, and design of single-lane and multilane roundabouts

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Presentation Outline

• Characteristics of Roundabouts

• Benefits of Roundabouts

• User Considerations

• Location Considerations

• Operational Analysis

• Design Considerations

• Costs

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Key Roundabout Characteristics

• Circular shape, yield control on entry, and geometric features that create a low-speed environment

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Roundabout Category Comparison

Design ElementMini-

RoundaboutSingle-Lane Roundabout

Multilane Roundabout

Desirable maximum entry design speed

15 to 20 mph(25 to 30

km/h)

20 to 25 mph(30 to 40

km/h)

25 to 30 mph(40 to 50

km/h)

Maximum number of entering lanes per approach

1 1 2+

Typical inscribed circle diameter

45 to 90 ft(13 to 27 m)

90 to 180 ft(27 to 55 m)

150 to 300 ft(46 to 91 m)

• Roundabouts are classified into three basic categories based on size and the number of lanes

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Roundabout Category Comparison (Continued)

Design ElementMini

RoundaboutSingle-Lane Roundabout

Multilane Roundabout

Central island treatment Fully

traversable

Raised (may have

traversable apron)

Raised (may have

traversable apron)

Typical daily service volumes on 4-leg roundabout below which may be expected to operate without requiring a detailed capacity analysis (veh/day)*

Up to approximately

15,000

Up to approximately

25,000

Up to approximately

45,000 for two-lane

roundabout

*Operational analysis needed to verify upper limit for specific applications.

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• Costs

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Benefits of Roundabouts

• Traffic Safety

– Reduce total crashes by 35% and injury crashes by 76%

• Pedestrian Safety

– Reduced vehicle speeds, focus on one traffic stream – May cause issues for visually impaired pedestrians

• Traffic Calming

– Reduce vehicle speeds using geometric design

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Benefits of Roundabouts (Continued)

• Operational Performance– Lower overall delay than other controlled intersections– Specific users do not receive priority

• Ongoing Operations and Maintenance– Lower operating and maintenance costs than a traffic signal

• Approach Roadway Width– May not require lengthy turn lanes– Often have greater right-of-way needs at the intersection quadrants

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Benefits of Roundabouts (Continued)

• Environmental Factors

– Less noise, fuel consumption and fewer air quality impacts

• Access Management

– Facilitate U-turns, enabling left-turn restrictions at driveways

• Aesthetics

– Islands offer opportunity for landscaping and art displays

• Land Use

– Provide transition areas between different environments

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• Costs

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Motorist and Emergency Vehicle Considerations

• Motorist considerations: Enhance driver safety

– Allow more time to make decisions, act, and react– Reduce the number of directions of conflicting traffic– Reduce the need to judge gaps in fast traffic

• Emergency vehicle considerations

– Benefit of lower vehicle speeds– Motorists should be educated to not enter a roundabout

when an emergency vehicle is approaching

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Pedestrian and Bicyclist Considerations

• Pedestrian considerations– Consider one direction of conflicting traffic at a time– Low vehicular speeds allow more time to react– Challenges experienced by pedestrians with vision

impairments may require additional treatments to improve accessibility

• Bicyclist considerations– Can navigate roundabouts either as motor vehicles or

pedestrians depending on the size of the intersection, traffic volumes, and their experience level

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• Costs

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Location Considerations

• A modern roundabout should be considered anywhere a traffic signal or stop control is under consideration

• Roundabouts can be advantageous in a number of locations

• However, certain constraints may adversely affect their feasibility at a specific site

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Common Site Applications

• Schools: Reduce vehicle speeds

• Interchanges: More efficient use of the bridges between ramp terminals

• Commercial developments: Aesthetically pleasing

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Other Common Site Applications

• New residential subdivisions: Low-speed, low-noise

• Corridors: Opportunity to shape cross sections

• Gateway treatments: Create community focal points

• Intersections with high delay: Reduce delay

• Rural intersections: Significantly reduce crashes

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Potential Site Constraints

• Physical complications may make construction infeasible

• Proximity of generators of heavy vehicles

• Proximity of conditions that require pre-emption

• Creation of unacceptable major road delay

• Proximity of bottlenecks may cause operations to fail

• Heavy pedestrian or bicycle movements may require supplemental traffic control

• Intersections within a coordinated signal network may operate better with signalized intersection

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Methods to Address Site Constraints

• Additional operational analysis

• Design modifications

• Coordination with affected parties

• Consideration of other intersection types if these constraints cannot be overcome

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• Costs

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Levels of 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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Capacity of Single-Lane and Multilane Entries

• The capacity of each entry lane is calculated based on conflicting traffic flow

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• Costs

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Key Objectives of Roundabout Design

• Slow entry speeds

• Appropriate number of lanes

• Smooth channelization

• Adequate accommodation for design vehicles

• Meeting needs of pedestrians and bicyclists

• Appropriate sight distance and visibility

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Horizontal Design

• Key considerations that affect horizontal design

– Design speed– Path alignment– Design vehicle

• Design elements influenced by these considerations

– Size– Central island– Splitter island

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Consideration 1: Design Speed

• Design speed is defined by the theoretical speed drivers could achieve through the roundabout

• Achieve an appropriate design speed for the entry movements and consistent speeds for all movements

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Consideration 1: Design Speed (Continued)

• Speed-radius relationships are shown in the graph

0

5

10

15

20

25

30

35

40

0 50 100 150 200 250 300 350 400Radius (ft)

Sp

eed

(m

ph

)

e=+0.02 e=-0.02

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Consideration 2: Path Alignment

• Multilane roundabouts should align vehicles into the appropriate lane within the circulatory roadway

• A good design balances entry speed and path alignment

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Consideration 2: Path Alignment (Continued)

• Entry and exit lanes should be designed without too much separation to avoid problems

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Consideration 3: Design Vehicle

• The design vehicle is best identified at the start of the project and evaluated early in the design process

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Consideration 4: 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 5: Central Island

• A circular central island is preferred because the circulatory roadway helps promote constant speeds

• The size plays a key role in determining the amount of deflection imposed on the through vehicle’s path

• The island may include enhancements, serving an aesthetic purpose and providing conspicuity of the intersection for approaching motorists

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Consideration 6: Splitter Island

• Splitter islands provide refuge for pedestrians, control speeds, guide traffic, separate traffic streams, and deter wrong-way movements

• Longer splitter islands can help reduce confusion for entering motorists

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Pedestrian Design Treatments

• Sidewalks at roundabouts should be set back from the circulatory roadway to discourage pedestrians from crossing to the central island

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Bicycle Design Treatments

• Bicycle lanes should end in advance of roundabouts

• Bicyclists may act as a vehicle or pedestrian

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Sight Distance and Visibility

• Adequate sight distance and visibility are needed for a roundabout to operate safely

• Intersection sight distance is evaluated at each entry to ensure a driver can see and safety react to potentially conflicting vehicles

d1 = Entering stream distanced2 = Circulating stream distance

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Sight Distance and Visibility (Continued)

• Stopping sight distance should be provided at every point within a roundabout and on each approach

• The roundabout should be visible from a distance and focus the attention of drivers

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Vertical Design

• Cross slope of 2% away from the central island should be used for the circulatory roadway on single-lane roundabouts

• Two common methods for vertical design of circulatory roadway within a multilane roundabout are outward sloping and crowned circulatory roadway

• Roundabouts should preferably be located where grades through the intersection are less than 4%

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Pavement Markings and Signs

• Markings and signs are integral to roundabout design and should facilitate through and turning movements

• Typical pavement markings delineate the entries, exits, and circulatory roadway

• Overall signing concept similar to intersection signing

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Lighting and Landscaping

• Lighting and landscaping focus driver attention on key conflict areas

• Landscaping also discourages pedestrian traffic through center island

• Lighting should make roundabouts conspicuous and visible to approaching drivers

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Other Design Details and Applications

• Right-turn bypass lanes: Similar to those used at conventional intersections

• Access management: May restrict nearby driveways

• At-grade rail crossings: Introduce challenges related to crossing control and queue clearance

• Evacuation routes: Reverse flow to facilitate evacuation

• Bus stops: Can be provided on the entry or exit side of a roundabout, but not within the circulatory roadway

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• Costs

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Costs

• Construction and right-of-way costs vary

• Benefit-cost analysis may be useful in an alternatives analysis

• Roundabouts often have longer life cycle between upgrades than signalized intersections

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Conclusion

• Roundabouts provide safety and operational benefits compared to other types of intersections

• Location, types of users, and operations should be analyzed when designing a roundabout

• Design elements work together to provide desirable safety and operational characteristics

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References

• Federal Highway Administration. Technical Summary on Roundabouts. 2010.

• Federal Highway Administration. Roundabouts: An Informational Guide. June 2000.

• National Cooperative Highway Research Program. Roundabouts: An Informational Guide, 2nd Edition. 2010.

• Federal Highway Administration. Technical Summary on Mini-Roundabouts. 2010.

• National Cooperative Highway Research Program. Roundabouts in the United States. 2007.

• Insurance Institute for Highway Safety. Crash Reductions Following Installation of Roundabouts in the United States. March 2000.

• Maryland Department of Transportation. Maryland’s Roundabouts: Accident Experience and Economic Evaluation. March 2007.

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References (Continued)

• Transportation Research Board. Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities. 2010.

• Transportation Research Board. Highway Capacity Manual. 2010.• American Association of State Highway and Transportation

Officials. A Policy on Geometric Design of Highways and Streets. 2004.

• Kansas Department of Transportation. Kansas Roundabout Guide: A Supplement to FHWA’s Roundabouts: An Informational Guide. October 2003.

• Federal Highway Administration. Manual on Uniform Traffic Control Devices. January 2009.

• Illuminating Engineering Society. Design Guide for Roundabout Lighting. February 2008.

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For More Information

Ed RiceIntersection Safety Team Leader,FHWA Office of Safety

Phone: (202) 366-9064

E-mail: [email protected]

• This presentation is part of a set of roundabout outreach materials which includes Technical Summaries, a video and a brochure.

• Visit FHWA’s intersection safety web site to access more materials highlighting roundabouts:

http://safety.fhwa.dot.gov/intersection

• Or Contact:

roundabouts fhwa-sa-10-006 technical summary adapted from photo by lee rodegerdts (used with...

Documents

kmhmaximum number

number of directions

leg roundabout

twolane roundabout

traversable apronraised

designkey considerations

geometric features

injury crashes