2019 ga transportation summit 11 11 2019 - printable version · roundabouts: an informational guide...

GA Transportation Summit ‐ Nov. 12, 2019 Breakout Session C

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GDOT Guidance on Roundabouts

Chris Raymond, P.E.GDOT Office of Traffic Operations, Traffic Operations Manager

Daniel Trevorrow, P.E.GDOT Office of Traffic Operations, Alternative Intersections Supervisor (RAID)

#GTS2019


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• 60+ On state routes/ built with GDOT $$ • 45+ single lane/compact• 5+ multi‐lane/hybrid• 10+ mini

• 20+ under construction• 70+ in design• 90+ in concept

• 165+ On local roads

• 250+ Other circular intersections

Roundabouts in Georgia


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Timing, Purpose & Need for a Practical Design Guide

• Supplements GDOT Design Policy Chapter 8 and is underpinned by NCHRP 672, Roundabouts: An Informational Guide – 2nd Edition.

• GDOT’s practical design guide presents the principals and methods of achieving practical design

• Appendix A provides background on the modern roundabout.

• Appendix B discusses practical design in more detail.

• Appendix C and Appendix D provide detailed how‐to’s to undertake design performance checks.

• Appendix E explains best‐practice vertical design methods in more detail.


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Practical Design ConsiderationsRight‐sizing was a trend a few years ago, practical design is the next wave:

1. Minimizes staging, cost and construction duration by making the most use of existing pavements and profiles

2. Minimizes R.O.W. takes by employing ellipses to mitigate intersection skew angle.

3. Right‐sizing for traffic demands – staged expandability

4. Requires very little piped drainage, making use of existing ditches


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Practical Design Considerations

5. Encompasses landscape area

6. Shorter approach medians without chicanes; or offsets that are balanced, symmetrical and do not extend longer tapers upstream.

7. 2x2 or 2x1 design with single lanes in the links upstream and down – lane drops instead of continuous widening past the roundabout.

8. Quick Response projects are by nature practical to keep cost down. They demonstrate what is essential for roundabout design to function.


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Cost Reduction Committee


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Practical Alternatives – Lower Cost/More Functional


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[email protected]

www.dot.ga.gov/DS/SafetyOperation/Roundabouts


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Operational Analysis• When sizing a roundabout for the number of entering and circulating lanes, each

approach leg of the roundabout is evaluated individually to determine the number of entering lanes that are required

• The workflow for performing roundabout operational analyses:

• Analyze both AM and PM peak hours and perform the analysis for the Build Year, Build Plus Ten and the Design Year forecasted traffic.

• Development of the required lane configuration should be an iterative process to determine the appropriate number of lanes and lane assignments by checking Level of Service (LOS) results until they are within the acceptable and a desirable range


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How is circle size & number of lanes determined?

GDOT RB Analysis Toolwww.dot.ga.gov/PS/DesignManuals/DesignResources


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Right Turn Lane Options


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Geometric Design Process and Workflow


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GDOT’s Preferences on Geometric Design


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What are the Core Principles?• Circle Size:

• Space for the design vehicle

• Speed Reduction:• Circle location and alignment of arms

• Sight distances (SSD and ISD)• Accommodation of vulnerable users:

• Refuge islands, bike transitions

• Positive Guidance:• A traffic information system that is predictable and uniform:

• Legible, conspicuous signs and markings spaced to minimize driving task load and to recognize the driver’s needs vary from upstream to entry


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Left‐Offset Example• Left offset generates good ‘body language’ – driver adaptability is eased


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Practical Considerations for Approach Alignment


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Elliptical

SR 33 @ US 319, Colquitt County, GA


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Entry Curves – kept tight by design


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External Truck Aprons (Blisters)

SR 20 @ East Lake Rd, Henry County, GA

External truck apron


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Careful Attention to Rural Design


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Vertical Design Guidance for Commonly Complicated Cases


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Practical Construction Staging Principles• Attempt to achieve circulating traffic, i.e. getting the intersection to operate as a roundabout as soon as possible. It reduces speeds, improves capacity and improves safety for workers.

• Minimize the number of stages to avoid ‘construction fatigue’, excessive duration and excess cost.

• Minimize the changes to traffic control on the roundabout. Avoid changing from yield to stop control and back again, to manage stages where traffic needs to use parts of the circle.

• Avoid running traffic in the contra‐flow direction on the roundabout. • Signing, including way‐finding (D and M series signs) should be installed before the roundabout opens and especially lane designation signs for multilane roundabouts.

• Use changeable message boards: “New Control” and “Yield Ahead”.


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Design Aids and How‐to’s

(Appendices to the Practice Guide)


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Guidance on Performance Checks

Fastest Paths


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Providing Step‐by Step Guidance on Performance checks


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Design Vehicle Swept Paths


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OSOW Check Vehicle Swept Paths


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Obtain OSOW Permits then check Vehicle Swept [email protected]


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Horizontal & Vertical Clearance Checks of Low‐Boy Trucks


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Horizontal & Vertical Clearance Checks of Low‐Boy Trucks


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Meeting GDOT’s Objective for 3‐D Design


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Daniel Trevorrow, P.E.Alternative Intersections Supervisor (RAID)

Ph: 404-635-2967Email: [email protected]

[email protected]

http://www.dot.ga.gov/DS/SafetyOperation/Roundabouts

Chris Raymond, P.E.Traffic Operations Manager

Ph: 404-635-2809Email: [email protected]

2019 ga transportation summit 11 11 2019 - printable version · roundabouts: an informational guide...

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