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Roundabout Capacity Model Comparison

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The table given below presents a brief comparison of main features of three roundabout capacity models, namely the Australian model as implemented in the SIDRA INTERSECTION software (1-14), the HCM Edition 6 model described in Chapter 22 of the Highway Capacity Manual Edition 6 (15-19), and the UK (TRL) model implemented in the RODEL and ARCADY software packages (20-25)

The HCM Edition 6 model (15) and the earlier HCM 2010 (Chapter 21) model (16) areavailable as roundabout capacity model options in the SIDRA INTERSECTION software. The terms SIDRA Standard is used to distinguish between the Australian model and these two HCM models in SIDRA INTERSECTION. The HCM Edition 6 model assessments included in the comparison table apply to the HCM 2010 model as well.

A recent NCHRP survey of US state transport agencies (19) indicated that "to analyze roundabout performance, about three-quarters of the reporting states use some form of the Highway Capacity Manual model and SIDRA Standard Model; about one-quarter use some form of the UK equations".

In order to avoid misleading statements about particular software packages, the third model will be referred to as the UK TRL model to distinguish it as the original published model as opposed to the ARCADY and RODEL software packages which have implemented it since the software may include some differences from the original model. For example, recently these software packages claim to have included lane-based analysis.

The features compared include methodology, model level of detail (lane-based or approach-based), parameters used in the model to represent driver behavior and roundabout geometry, and model calibration methods. The comparison focuses on the capacity model and makes only a brief reference to modeling of performance (delay, queue length, fuel consumption and emissions, etc.) and level of service methods used.

For detailed discussions and case studies on this subject, refer to technical papers (including many of those listed in the REFERENCES section on this page) available for download from our web pages:   
Articles   SIDRA for Roundabouts


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Comparison of the main features of SIDRA Standard, HCM Edition 6 and UK TRL roundabout models

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Model Feature SIDRA Standard Model HCM Edition 6 Model UK TRL Model
Methodology

Based-on gap-acceptance theory with empirical (regression) equations to model gap-acceptance parameters including the effect of roundabout geometry.

Empirical (exponential regression) capacity model with clear basis in gap-acceptance theory.

Empirical (linear regression) capacity model with no stated basis in traffic theory.

Lane-based model: capacity and performance of individual entry lanes are modeled.

Lane-based model: capacity and performance of individual entry lanes are modeled. 

Approach-based model: all lanes aggregated. Lane capacity and performance not available.

Model FeatureSIDRA Standard ModelHCM Edition 6 ModelUK TRL Model
Model Level of Detail - Entry and Circulating Lanes

Variations in lane disciplines (exclusive and shared lanes, slip and continuous lanes) can be modeled.

Variations in lane disciplines (exclusive and shared lanes, slip and continuous lanes) can be modeled.

Variations in lane disciplines (exclusive and shared lanes, slip and continuous lanes) cannot be modeled.

Dominant and subdominant entry lanes identified. 

Dominant and subdominant entry lanes identified. 

Entry lanes not identified.

Number of circulating (conflicting) lanes affects capacity.

Number of circulating (conflicting) lanes affects capacity.

Number of circulating lanes does not affect capacity.

Circulating lane flow rates used allowing for unbalanced flows. Amount of queuing before entering circulating stream affects capacity.  

Total circulating flow rate used.  Circulating lane flows not used.

Total circulating flow used.Circulating lane flows not used.

Uses a bunched arrival headway model for the circulating stream.  Proportion bunched modeled. 

Uses a random arrival headway model for the circulating stream. 

No explicit assumptions about circulating stream headways.

Extra bunching to model upstream signal effects allowed.

Effect of upstream signals modeled as an extension to the HCM 2010 model in SIDRA INTERSECTION.

Not used.

A proportion of exiting flow can be added to circulating flow as opposing flow.

Not applicable.

Not known to the author. 

Model FeatureSIDRA Standard ModelHCM Edition 6 ModelUK TRL Model
Lane Utilization for Multilane Approaches

Entry lane flow rates are calculated.

Entry lane flow rates are calculated.

No lane flow details.

De facto exclusive lanes are identified.

De facto exclusive lanes are identified.

De facto exclusive lanes cannot be identified.

Unequal lane use can be modeled by specifying lane utilization ratios.

Unequal lane use can be modeled by specifying lane volume percentages.

Unequal lane use cannot be modeled.

Critical lane v/c ratio (degree of saturation) for a multilane approach is determined.

Critical lane v/c ratio (degree of saturation) for a multilane approach is determined.

Critical lane v/c ratio cannot be determined (only the average v/c ratio for the approach is available). This will underestimate the higher v/c ratio of the critical lane unless equal lane use exists.

Model FeatureSIDRA Standard ModelHCM Edition 6 ModelUK TRL Model
Driver Behavior Parameters

Gap-acceptance parameters (Follow-up Headway, Critical Gap), entry lane-use model and circulating stream bunching represent driver behavior. 

Driver response times determined.

Gap-acceptance parameters (Follow-up Headway, Critical Gap) and entry lane-use model represent driver behavior. 

No direct representation of any aspect of driver behavior. 

Capacity is sensitive to the circulating flow rate only. 

Follow-up Headway and Critical Gap depend on roundabout geometry. 

Follow-up Headway, Critical Gap values are constant.

Not used.

Follow-up Headway and Critical Gap values are reduced (more aggressive driver behavior) with increased circulating flow rate.

Follow-up Headway, Critical Gap values are constant.

Not used.

Priority sharing and priority emphasis effects are included in the model.

Not applicable.

Not used.

Model FeatureSIDRA Standard ModelHCM Edition 6 ModelUK TRL Model
Roundabout Geometry Parameters

(list of geometry parameters affecting capacity)

Differences in sensitivities indicated.

Average entry lane width

Not used

Total entry width

Number of entry lanes

Number of entry lanes

Not used

Number of circulating lanes

Number of circulating lanes

Not used

Inscribed diameter
With increased inscribed diameter: capacity increases and then decreases for very large roundabouts.

Not used

Inscribed diameter
With increased inscribed diameter: capacity increases with increasing inscribed diameter; capacity does not decrease for very large roundabouts.

Entry radius
With increased entry radius:
the capacity at zero circulating flow increases (more capacity), and the slope of the capacity curve decreases (more capacity); capacity remains same if the capacity at zero circulating flow is user-specified.

Not used

Entry radius
With increased entry radius:
the capacity at zero circulating flow increases (more capacity), and the slope of the capacity curve also increases (less capacity); capacity decreases if the capacity at zero circulating flow is user-specified.

Entry angle
With decreased entry angle:
the capacity at zero circulating flow increases (more capacity), and the slope of the capacity curve decreases (more capacity); capacity remains same if the capacity at zero circulating flow is user-specified.

Not used

Entry angle
With decreased entry angle:
the capacity at zero circulating flow increases (more capacity), and the slope of the capacity curve also increases (less capacity); capacity decreases if the capacity at zero circulating flow is user-specified.

Approach short lanes: capacity and overflow into adjacent lane modeled using gap-acceptance cycles and back of queue modeling. 

Short lanes modeled as an extension to the HCM 2010 model in  SIDRA INTERSECTION.

Approach flaring (Approach half width and Flare length).

Interpolation for lane width between single and multilane approach values problematic.

Number of exit lanes (can affect upstream approach use)

Not used.

Not used.

Exit short lanes (merge lanes):
effect on upstream approach lane use modeled (increased v/c ratio due to lane underutilisation)

Not used.

Not used.

Model FeatureSIDRA Standard ModelHCM Edition 6 ModelUK TRL Model
Unbalanced Flows

Capacity is sensitive to Origin-Destination demand flow pattern, lane use and level of queuing on approaches. Roundabout modeled with high level of interaction between traffic using all intersection approaches (O-D Factor method used).

Not used. O-D Factor method available as an option for the HCM 2010 metho in SIDRA INTERSECTION.

Not used.

Adjustment options exist for high Entry Flow / Circulating Flow ratio (increased entry capacity at very low circulating flow rates due to increased driver aggressiveness level).

Not used. Adjustment options for high Entry Flow / Circulating Flow ratio available in SIDRA INTERSECTION .

Not used.

Model FeatureSIDRA Standard ModelHCM Edition 6 ModelUK TRL Model
Heavy Vehicles

Circulating flow rate is increased for heavy vehicles in the circulating stream. 

Follow-up Headway and Critical Gap values are increased for heavy vehicles in the entry lane.

Capacity is decreased for heavy vehicles directly. 

Not known to the author.

Model FeatureSIDRA Standard ModelHCM Edition 6 ModelUK TRL Model
Model Calibration

Intersection-level or approach-level calibration using Environment Factor.  A general value of 1.2 used for US conditions.   

Movement-level calibration using Follow-up Headway and Critical Gap parameters. 

Method described to calibrate the model parameters using known Follow-up Headway and Critical Gap values.

The capacity at zero circulating flow (y-intercept) value of the linear regression capacity function can be adjusted. (16,21)

Problematic since the capacity decreases with improved geometry (increased entry radius, decreased entry angle, increase entry width, increased flare length) if the capacity at zero circulating flow is user-specified.

Sensitivity analysis facility is available for driver behavior and roundabout geometry parameters.

Offered as an extension in SIDRA INTERSECTION software.

Not known to author.

Model FeatureSIDRA Standard ModelHCM Edition 6 ModelUK TRL Model
Level of Service

Uses HCM and additional level of service methods (options for alternative LOS methods including HCM 2010 and HCM 2000 methods, ICU method, etc.); the LOS Target parameter to specify acceptable LOS levels for different intersection types.

HCM 2010 LOS methods define different LOS thresholds for signalized intersections and all unsignalized intersections.

Not known to the author.

Roundabout LOS options ("Same as Sign Control", "Same as Signalised Intersections" and "SIDRA Roundabout LOS") available; uses "Same as Signalised Intersections" as default. 

Same LOS thresholds for roundabouts and sign-controlled intersections.

Model FeatureSIDRA Standard ModelHCM Edition 6 ModelUK TRL Model
Drive Cycles

Detailed drive-cycle model (cruise, decelerate, idle, accelerate) of movements through the intersection. determined for queued and unqueued vehicles (light and heavy vehicles separately) for each lane.

Negotiation radius, speed and distance calculated (used for geometric delay, fuel consumption, emissions and operating cost).

Aggregate model.

Aggregate model.

Model FeatureSIDRA Standard ModelHCM Edition 6 ModelUK TRL Model
Delay, Queue and Stops

The gap-acceptance cycles are identified for modelling delay, back of queue, stop rate, proportion queued, etc. for each lane (as well as capacity). 

Geometric delay determined.

Back of queue is important for modeling short lane capacities and blocking of upstream intersections.Percentile queue values (not a single value) and probability of blockage of upstream lanes calculated.

Simple queuing theory for delay and cycle-average queue.

Geometric delay not determined.

95th percentile queue only for unsignalized intersections.

No back of queue model for unsignalized intersections.

Simple queuing theory for delay and cycle-average queue.

Geometric delay ?

No back of queue model.

Model FeatureSIDRA Standard ModelHCM Edition 6 ModelUK TRL Model
Fuel Consumption, Emissions and Operating Cost

Detailed vehicle power-based model using drive cycle information derived for queued and unqueued vehicles in each lane.  Light and heavy vehicles modelled separately.

Drive cycle model incorporating acceleration - deceleration models are important for geometric delay, fuel consumption, emissions and operating cost.

Not available.

Not available.

 


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REFERENCES

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  1. AKCELIK and ASSOCIATES. SIDRA INTERSECTION User Guide (for Version 7). Akcelik and Associates Pty Ltd, Melbourne, Australia, 2016.
  2. AKÇELIK, R. SIDRA-2 does it lane by lane. Proc. 12th ARRB Conf. 12 (4), pp 137-149, 1984.
  3. AKÇELIK, R. Lane-by-lane modelling of unequal lane use and flares at roundabouts and signalised intersections: the SIDRA solution. Traffic Engineering and Control, 38 (7/8), 1997, pp 388-399.
  4. AKÇELIK, R., CHUNG, E. and BESLEY, M. Roundabouts: Capacity and Performance Analysis. Research Report ARR No. 321. ARRB Transport Research Ltd, Vermont South, Australia, 1999 (2nd Edn).
  5. AKÇELIK, R. A Roundabout Case Study Comparing Capacity Estimates from Alternative Analytical Models. Paper presented at the 2nd Urban Street Symposium, Anaheim, California, USA, 2003.
  6. AKÇELIK, R. Roundabouts with Unbalanced Flow Patterns. Paper presented at the ITE 2004 Annual Meeting and Exhibit, Lake Buena Vista, Florida, USA, 2004.
  7. AKÇELIK, R. Roundabout Model Calibration Issues and a Case Study. Paper presented at the TRB National Roundabout Conference, Vail, Colorado, USA, 2005.
  8. AKÇELIK, R. A Review of Gap-Acceptance Capacity Models. Paper presented at the 29th Conference of Australian Institutes of Transport Research (CAITR), University of South Australia, Adelaide, Australia, 2007.
  9. AKÇELIK, R. The relationship between capacity and driver behaviour. Paper presented at the National Roundabout Conference, Transportation Research Board, Kansas City, MO, USA, 2008.
  10. AKÇELIK, R. Evaluating Roundabout Capacity, Level of Service and Performance. Paper presented at the ITE 2009 Annual Meeting, San Antonio, Texas, USA, 2009.
  11. AKÇELIK, R. An Assessment of the Highway Capacity Manual 2010 Roundabout Capacity Model. Paper presented at the TRB International Roundabout Conference, Carmel, Indiana, USA, 2011.
  12. AKÇELIK, R. Some common and differing aspects of alternative models for roundabout capacity and performance estimation. Paper presented at the TRB International Roundabout Conference, Carmel, Indiana, USA, 2011.
  13. AKÇELIK, R.  Roundabout metering signals: capacity, performance and timing.  6th International Symposium on Highway Capacity and Quality of Service, Transportation Research Board, Stockholm, Sweden.  Procedia - Social and Behavioural Sciences, Vol 16, pp 686-696, 2011.
  14. AKÇELIK, R.  Modeling Queue Spillback and Upstream Signal Effects in a Roundabout Corridor.  TRB 4th International Roundabout Conference, Seattle, WA, USA, 2014.
  15. TRB.  Highway Capacity Manual 6th Edition - A Guide for Multimodal Mobility Analysis.  Transportation Research Board, National Research Council, Washington, DC, USA, 2016.
  16. TRB. Highway Capacity Manual. Transportation Research Board, National Research Council, Washington, DC, USA, 2010.
  17. TRB. Roundabouts in the United States. NCHRP Report 572. Transportation Research Board, National Research Council, Washington, DC, USA, 2007.
  18. TRB. Roundabouts: An Informational Guide. NCHRP Report 672. Transportation Research Board, National Research Council, Washington, DC, USA, in cooperation with US Department of Transportation, Federal Highway Administration, 2010.
  19. TRB. Roundabout Practice.  A Synthesis of Highway Practice, NCHRP Synthesis 488.  Transportation Research Board, Washington, DC, USA, 2016.
  20. KIMBER, R.M. (1980). The Traffic Capacity of Roundabouts. TRRL Laboratory Report 942. Transport and Road Research Laboratory, Crowthorne, Berkshire, UK.
  21. HOLLIS, E.M., SEMMENS, M.C. and DENNISS, S.L. (1980). ARCADY: A Computer Program to Model Capacities, Queues and Delays at Roundabouts. TRRL Laboratory Report 940. Transport and Road Research Laboratory, Crowthorne, Berkshire, UK.
  22. KIMBER, R.M. (1989). Gap-acceptance and empiricism in capacity prediction. Transportation Science 23 (2), pp 100-111.
  23. BROWN, M. (1995). The Design of Roundabouts. Transport Research Laboratory State-of-the-Art-Review. HMSO, London, UK.
  24. CHARD, B. ARCADY Health Warning: Account for unequal lane usage or risk damaging the Public Purse! Traffic Eng. and Control, 38 (3), 1997, pp 122-132.
  25. LENTERS, M. and RUDY, C. HCM Roundabout Capacity Methods and Alternative Capacity Models. ITE Journal, 80 (7), pp. 22-27, 2010.

 


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