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Your Exclusive Source of Modern Roundabout Information Edition 003 February 2012
Build Some Roundabouts, Save Some Lives A Framework for Roundabout Capacity Analysis
Constructing Roundabouts: From the Inside-Out
Radial, Left or Right Offset Alignment and Roundabout Vehicle Speeds
Slovenian Roundabouts: We Did It Our Way
Featured Article 1 1
A Framework for
Roundabout Capacity Analysis By David Stanek, PE
What capacity analysis method should you use to determine if your roundabout design will accommodate the traffic demand? The following framework provides a flowchart for identifying whether the design has sufficient capacity and what to do if it does not. By following the recommended framework, you can have a high level of confidence in the traffic operations of the proposed roundabout design. Introduction
There are numerous analysis software packages available to evaluate roundabouts, most of which use different methods to analyze capacity and identify level of service (LOS). Since the methods and are different, LOS results can be dramatically different when they are applied on the same project. Therefore, a framework has been developed to aid practitioners in the traffic analysis of roundabouts. The framework recommends the type of analysis procedure(s) to select for the given conditions, identifies the conditions that would call for modifying the design, and suggests ways to increase the confidence in the analysis results. 1
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FeaturedFeatured Article 1 Features Macroscopic and Microscopic Analysis
Two primary methods are used to evaluate intersections independent of the traffic control: macroscopic and microscopic analyses. Macroscopic models evaluate vehicle flows over a period of time (typically, the peak hour) with inputs like geometry, vehicle volumes, type of control, and capacities that were developed using theoretical equations and/or empirical data. These models are also called deterministic. Microscopic models, more commonly known as traffic simulation models, evaluate individual vehicles and are calibrated to vehicle performance and driver behavior parameters such as driver aggressiveness. Several software packages are available to provide macroscopic and microscopic capacity analysis for roundabouts. Some macroscopic programs provide several capacity models to choose from while others offer only one. Microscopic models generally use the detailed geometry and gap acceptance to model roundabout capacity for a given set of traffic flows.
David Stanek, P.E. David Stanek has analyzed traffic operations at intersections and on freeways for dozens of projects in the western US. As a registered civil and traffic engineer with Fehr & Peers transportation consultants, David has analyzed proposed roundabouts on local streets, arterial roads, state highways, and at freeway interchanges. • Platooning effects of signal coordination • Unbalanced lane use due to weaving movements on the approach
Microscopic models can address these limitations in most cases. As a practitioner, it is reasonable and responsible to assume that the situations identified above – when macroscopic models are insufficient to evaluate a particular situation – require microscopic Roundabout Capacity Analysis Software analysis independent of intersection Macroscopic Programs Website control. When these situations occur www.trlsoftware.co.uk/products/junction_signal_design/arcady with roundabout intersections, the ARCADY use of simulation models should be HCS 2010 mctrans.ce.ufl.edu/hcs/ considered. Rodel rodel-interactive.com SIDRA INTERSECTION Synchro TEAPAC Complete 2010 TRAFFIX
sidrasolutions.com
Microscopic Programs Aimsun Quadstone Paramics SimTraffic TransModeler VISSIM
Website www.aimsun.com www.paramics-online.com
www.trafficwareinc.com/transportation/product/synchro-8-0
www.strongconcepts.com/ www.ptvamerica.com/software/ptv-vision/traffix/
www.trafficwareinc.com/transportation/product/simtraffic-8-0
www.caliper.com/transmodeler www.ptvamerica.com/software/ptv-vision/vissim/
FHWA’s Traffic Analysis Toolbox Volume II provides guidance for selecting the appropriate tool for traffic analysis. The limitations of the deterministic Highway Capacity Manual methods are identified. At intersections, these limitations include the following situations. • Oversaturated conditions • Queue spill back that blocks turn pockets • Queue spill back in turn pockets that blocks through lanes
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The Framework
The intent of the framework is to provide the practitioner with an easy to follow procedure for identifying (1) whether the proposed roundabout design is feasible from a traffic perspective and (2) what type of analysis is required to provide a high level of confidence that the roundabout design will operate acceptably.
The framework begins by directing the practitioner to NCHRP Report 672 – Roundabouts: An Informational Guide, Second Edition. Specifically, the daily volumes should be compared against the thresholds shown in Exhibit 3-12 to identify if the design should have one or two lanes. If the initial check indicates that the roundabout would be near or at capacity, simulation should be used, or the roundabout should be redesigned Februarywww.RoundaboutsNow.com 2012 RoundaboutsNow
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Featured Article 1 to provide additional capacity. If the daily volumes are below the threshold and the intersection is isolated, then the practitioner can proceed with the use of macroscopic analysis. Continuing with the use of a macroscopic analysis tool, queues should be reviewed to ensure that they do not spill beyond available storage and interfere with overall operations. If there is sufficient storage, the practitioner should then review volume-to-capacity (V/C) ratios to verify that they are less than 0.85 (as recommended on Page 4-14 of NCHRP 672). If the V/C ratio is less than 0.85, the practitioner should conduct some sensitivity testing to provide a level of comfort if traffic volumes increase or decrease beyond what was forecasted. If the roundabout analysis passes all of the identified tests, the practitioner has the highest level of confidence in the analysis results.
Acknowledgements
This article is based on a paper co-authored with Jason D. Pack and Ronald T. Milam entitled “Applicability of the Round Peg: Having Confidence in Your Roundabout Analysis Results,” which was presented at the 2008 Institute of Transportation Engineers Annual Meeting and Exhibit in Anaheim, CA.
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Select Initial Roundabout Design
OR
NO
ADT Volumes Below Capacity?
Revise Design
YES
NO
Isolated Location?
Analyze Peak Hour Volumes Using Macroscopic Method
Analyze Peak Hour Volumes Using Microscopic Method
Revise Design
YES
YES
Exceeds Delay Threshold
Revise Design
YES
V/C > 0.85 or Exceeds Delay Threshold
NO
If the roundabout is at or near capacity, queues extend beyond the turn pockets, or turning movements have high V/C ratios, then the roundabout should be redesigned (with additional capacity) or the practitioner should consider using microscopic analysis. While simulation models require more time and effort to build, they can provide answers that are not available using macroscopic methods. Given their complexity, a review of the simulation analysis results is needed. The microscopic results can be compared to the macroscopic results as a reasonableness check. Finally, sensitivity testing should also be performed when results are near the delay thresholds for LOS.
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NO Review Visualization for Reasonableness OR
Compare Results to Macroscopic Results and Justify Differences
Queues Block Turn Pockets?
NO
Sensitivity Tests - 10% Increase in Volume - Model Parameter Adjustments
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Roundabout Capacity Analysis Framework
References
• Highway Capacity Manual 2010. Transportation Research Board, 2011. • NCHRP Report 672 – Roundabouts: An Informational Guide, Second Edition. Transportation Research Board, 2010. • Traffic Analysis Toolbox Volume II: Decision Support Methodology for Selecting Traffic Analysis Tools. Federal Highway Administration, 2004 www.RoundaboutsNow.com