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SIDRA for Roundabouts

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SIDRA INTERSECTION is the most popular software for roundabouts (multi-lane and single-lane) and other intersections in the USA and Canada, Australia and New Zealand, South Africa, Malaysia, and many countries in Europe, Arabian Peninsula, South America and elsewhere around the world.

SIDRA INTERSECTION is the most widely-used software tool in the USA according to a recent NCHRP survey of state transport agencies as reported in the following publication which is available for download:

TRB (2016). Roundabout Practice. A Synthesis of Highway Practice, NCHRP SYNTHESIS 488.  Transportation Research Board, Washington, DC, USA.

The report also states 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.

See information about our Customers and Testimonials about SIDRA INTERSECTION.

SIDRA INTERSECTION is recognized by the Highway Capacity Manual, TRB-FHWA Roundabout Guide and many local roundabout guides in the USA (e.g. Florida Roundabout Guide, Oregon DOT Roundabout website), and the AUSTROADS guides in Australia and New Zealand. 

The following articles by Dr Akçelik discuss alternative roundabout capacity models:

Roundabout Model Comparison Table
This short document presents a table comparing main features of three roundabout capacity models, namely the Australian model as implemented in the SIDRA INTERSECTION software, the HCM model described in Highway Capacity Manual Edition 6, and the UK TRL model implemented in the RODEL and ARCADY software. The HCM Edition 6 model has been implemented in the SIDRA INTERSECTION software. The terms SIDRA Standard and HCM Edition 6 models are used to distinguish between these model options in SIDRA INTERSECTION. Most HCM Edition 6 model features apply to the HCM 2010 model as well. 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.

ROUNDABOUTS - Comments on the SIDRA INTERSECTION Model and the TRL (UK) Linear Regression Model
There has been some controversy in relation to the lane-based gap-acceptance model used in SIDRA INTERSECTION software vs the approach-based linear regression (UK TRL "empirical") model used in RODEL and ARCADY software for estimating the capacity of roundabouts. This detailed discussion note presents Dr Akçelik's views on the subject. A section on lessons learned from the US research on roundabouts leading to the HCM 2010 roundabout capacity model is included, and reference is made to the HCM 2010 model implemented in SIDRA INTERSECTION where relevant. This document is now dated in some respects as the RODEL and ARCADY sofware developers claim lane-based model applications.

Also see: About Roundabouts


SIDRA INTERSECTION Software for Roundabouts in the USA

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SIDRA INTERSECTION offers the Highway Capacity Manual Edition 6 roundabout capacity model based on US research on roundabouts. The model is implemented as described in Chapter 22 of the Highway Capacity Manual Edition 6 with numerous extensions.

The HCM software setups of SIDRA INTERSECTION use the HCM Edition 6 roundabout capacity model as the default model. The software also continues to offer the HCM 2010 as well as the SIDRA Standard model with a default Environment Factor of 1.2  for two-lane roundabouts to match the lower capacity estimates based on US roundabout research.

Detailed information on the HCM roundabout model in SIDRA INTERSECTION

General information on the HCM Version of SIDRA INTERSECTION

Traffic Analysis Software Tools (TRB)

SIDRA INTERSECTION also offers several additional roundabout capacity models for comparison with its main model. These include the FHWA 2000 roundabout model which is similar to the UK TRL linear regression ("empirical" model) used in the UK roundabout software packages (ARCADY and RODEL).

SIDRA INTERSECTION is the most widely-used software tool in the USA according to a recent NCHRP survey of state transport agencies as reported in the following publication which is available for download:

TRB (2016). Roundabout Practice. A Synthesis of Highway Practice, NCHRP SYNTHESIS 488.  Transportation Research Board, Washington, DC, USA.

The report also states 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.

Florida Roundabout Guide

"Several methods of roundabout modeling have been developed, most of them in other countries where roundabouts are common intersection treatments. The Australian methods are most comparable with HCM methods, and are implemented in software that is most compatible with the computational structure that has been developed in Florida for comparing other control modes. For example, the SIDRA program offers an option to implement the HCM procedures for many computations. SIDRA is used in the Florida Roundabout Guide as the primary model for evaluating roundabout performance. ... Like all of the other evaluation models, SIDRA has its own data entry and editing capability. Its user interface is graphics based, and is very well documented and user-friendly. "

Modern Roundabout Practice in the United States
NCHRP Synthesis 264, Transportation Research Board, 1998, Section titled Roundabout Capacity Methods and Software Used in the United States

"There are two primary capacity methods and software programs used in the United States: the Australian method with the SIDRA software and the British method with either the RODEL or the ARCADY software. ... SIDRA appears to be the most commonly used in the United States. This is in line with the fact that two-thirds of the survey respondents mentioned that they followed, or at least consulted, the Australian guidelines for roundabout design. ... "

Modern Roundabouts for Oregon
The Oregon DOT Research Document OR-RD-98-17

"Currently, three major software packages from other countries are used to analyze or design roundabouts: SIDRA, ARCADY, and RODEL. Recently, a test of the SIDRA program in the US environment found agreement between SIDRA delay output and collected field data at low volume. The RODEL package has been used to design roundabouts in the US. However, there has been no study or information on the ability of this program to predict roundabout performance in the US."

Establishing Roundabout Guidelines for a State DOT
by C.S. Kinzel, ITE Annual Meeting Compendium 2002

"After much discussion, the technical committee (of the the Missouri Department of Transportation) decided that ... SIDRA would be the required software for detailed operational analysis. The SIDRA software’s increasing prevalence of use in the United States, and the committee’s comfort level with a gap-acceptance-based analysis approach, were key factors in this decision."

Roundabout Design Guidelines
State of Maryland DOT State Highway Administration

"Until more data is gathered concerning the performance of roundabouts in Maryland, the Maryland State Highway Administration recommends that designers use the Australian practice at this time. ... This section provides an analytical technique which can be expected to give quite accurate results which reflect current Australian experience and practice. SIDRA software is recommended and is available from McTrans at the University of Florida."


Alternative Intersection and Network Treatments

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SIDRA INTERSECTION allows you to compare roundabout design with signalised (actuated and pretimed/fixed-time) and sign-controlled (two-way and all-way stop, yield/give-way) intersections in one package using consistent methodology. You can calculate annual sums of variables such as operating cost, fuel consumption, carbon dioxide, other pollutant emissions, total person delay, stops and so on. This enables you to demonstrate benefits of alternative intersection and network treatments and improvements to existing intersection and network conditions in a more powerful way. 


Roundabout Geometry and Driver Behaviour

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SIDRA INTERSECTION combines the effects of driver behaviour and roundabout geometry factors on capacity and level of service. It uses a unique lane-by-lane analysis method combined with an advanced gap-acceptance model of the “yield” behaviour of drivers at modern roundabouts. The gap-acceptance parameters used by SIDRA INTERSECTION are empirically based, and allow for the effects of many parameters representing the roundabout geometry (this differs from old models that use constant critical gap and follow-up headways). You can use the "Sensitivity Analysis" method of SIDRA to obtain graphs of how capacity and level of service change with roundabout geometry and driver behaviour.

SIDRA INTERSECTION uses analytical models for estimating negotiation radius, speed and distance values, as well as acceleration and deceleration times and distances of turning and through vehicles at roundabouts. These traffic variables are important for both safety and efficiency analysis purposes. In particular, they are needed for determining geometric delays, fuels consumption, pollutant emission and operating cost values for traffic using roundabouts. The method used for determining negotiation speeds of vehicles at roundabouts allows for path smoothing by drivers, and uses the vehicle mass parameter in the negotiation speed formula.


SIDRA INTERSECTION for Multi-lane Roundabouts

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SIDRA INTERSECTION is particularly useful for the design of multi-lane roundabouts as the only widely available software that uses LANE-BY-LANE analysis methodology which is applied to approach lanes as well as circulating lanes. It treats the roundabout geometry in detail, and is the ideal tool for evaluating alternative lane arrangements and short lanes.

On the other hand, the approach-based UK linear-regression ("empirical") model used in software from UK is not sensitive to changes in lane arrangements. See various publications, including: Lane-by-lane modelling of unequal lane use and flares at roundabouts and signalised intersections, Roundabouts with Unbalanced Flow Patterns.


SIDRA INTERSECTION can identify congestion at roundabouts

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SIDRA INTERSECTION is more likely to identify potential congestion that may occur after a roundabout has been built. SIDRA INTERSECTION analysis has found that, for roundabouts under heavy demand condition, the Australian gap acceptance model reports lower capacity then the UK linear regression model (used in software packages from UK). A further factor that cause congestion on roundabout approaches is unbalanced flows (heavy entry flow from one approach causing congestion on the next entry). SIDRA INTERSECTION model has a unique method to allow for this factor, which is ignored by other software packages.


Misleading statements about SIDRA INTERSECTION

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Various websites marketing the RODEL software package in the USA include misleading statements about the SIDRA INTERSECTION method for roundabout capacity analysis. Refer to the publications at the start of this page as well as many papers available for download from our Articles and Roundabouts pages.

A paper by Bared and Afshar published in Transportation Research Record 2096 (pp 8-15, 2009) states categorically that "SIDRA cannot account for conflicting circulating flow by lane separately and only considers the total circulating flow regardless of lane volume distribution" (page 8, col 2, para 2). Unfortunately, this is INCORRECT. The SIDRA INTERSECTION User Guide includes statements to the effect that circulating lane flow calculations are carried out by the software.  SIDRA INTERSECTION estimates circulating lane flows according to the approach lane flow rates of contributing movements. Therefore, varying approach lane disciplines, any approach lane underutilisation and de facto exclusive lanes will affect the downstream circulating flow rates. The circulating lane flow distribution is then used in estimating bunching parameters for the circulating stream. Varying values of bunching parameters affect the entry capacities for the same total circulating flow rate. This is unique to the bunched exponential model of circulating stream headways used in SIDRA INTERSECTION (bunching parameters are given in SIDRA INTERSECTION output).


Why is SIDRA INTERSECTION the best software for roundabout capacity analysis?

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You may have seen a claim from another software developer that the new version of their roundabout capacity software package “is the only commercially available tool to offer full geometric and gap acceptance modelling capability within a single product”.

For at least two decades now, SIDRA INTERSECTION software has used the gap-acceptance methodology for roundabout capacity estimation where gap-acceptance parameters are estimated from the roundabout geometry.  In short, SIDRA INTERSECTION has employed a combined (hybrid) geometry and gap-acceptance modelling approach in order to take into account the effect of roundabout geometry on driver behaviour directly through gap-acceptance modelling.  This approach accepts the importance of roundabout geometry but states that roundabout geometry alone is not good enough and modelling of driver behaviour is needed for roundabout capacity estimation, just as it is needed for modelling the capacity of any other intersection type (signals, sign control). 

There has been a lot of debate about the relative benefits of models based on "gap acceptance theory" vs "empirical models" in the past, with claims that gap-acceptance modelling does not work for roundabouts.  Much of this debate has been misleading due to simplistic model categorization based on the suggestion that these modeling approaches are mutually exclusive.  Opinion has also been expressed that it does not matter which one of these modelling approaches is used.  This has overlooked the fact that SIDRA INTERSECTION has used the gap-acceptance method not only for capacity but also for performance estimation includingunique equations for back of queue and stop rate based on modelling of gap-acceptance cycles.

With the aim to enhance understanding of the fundamental aspects of the different roundabout capacity models available around the world, Dr Rahmi Akçelik presented two papers at the TRB International Roundabout Conference in Carmel, Indiana, USA in May 2011, discussing three well-known analytical models of roundabout capacity, namely the Australian SIDRA INTERSECTION model, the US Highway Capacity Manual 2010 (HCM 2010) model and the UK TRL (linear regression) model.  Dr Akçelik pointed out the common features as well as differences of these models, and presented a detailed table comparing the features of these roundabout capacity models.  The features compared include methodology, model level of detail considering entry and circulating lanes, parameters used in the model to represent driver behaviour and roundabout geometry, and model calibration methods.  A short version of the model comparison table is presented below. Roundabout examples comparing these three models are given in the below papers by Dr Akçelik.


AKÇELIK, R. (2011).  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

AKÇELIK, R. (2011).  An Assessment of the Highway Capacity Manual 2010 Roundabout Capacity Model.  Paper presented at the TRB International Roundabout Conference, Carmel, Indiana, USA, 2011

TRB.  Highway Capacity Manual.  Transportation Research Board, National Research Council, Washington, DC, USA, 2010.  ("HCM 2010")

KIMBER, R.M. (1980).  The Traffic Capacity of Roundabouts. TRRL Laboratory Report 942. Transport and Road Research Laboratory, Crowthorne, Berkshire, UK


Unique model for timing and performance analysis of roundabout metering signals

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SIDRA INTERSECTION includes a unique model for timing and performance analysis of roundabout metering signals. 




The use of metering signals is an effective measure to alleviate the problem of excessive delay and queuing that may be observed at unsignalised roundabouts, especially with heavy unbalanced flows. The Australian roundabout and traffic signal guides acknowledge the problem of unbalanced flows and discuss the use of metering signals (AUSTROADS 1993, Sections 3.9.4 and 12.1; and AUSTROADS 2003, Section 15.7).  Case studies have been discussed in numerous papers by Akçelik (see the reference list and web page links below). 

The signalised roundabout solution has been used extensively in the UK as well (Huddart 1983, Lines and Crabtree 1988, Hallworth 1992, Shawaly, et al 1991) though the solution may differ from the Australian metering signals.  Ken Huddart (1983) explained the problem clearly: "... the proper operation of a roundabout depends on there being a reasonable balance between the entry flows ... an uninterrupted but not very intense stream of circulating traffic can effectively prevent much traffic from entering at a particular approach." and "The capacity of roundabouts is particularly limited if traffic flows are unbalanced. This is particularly the case if one entry has very heavy flow and the entry immediately before it on the roundabout has light flow so that the heavy flow proceeds virtually uninterrupted. This produces continuous circulating traffic which therefore prevents traffic from entering on subsequent approaches."

A recent US paper discusses the use of metering signals for the Clearwater roundabout in Florida (Sides 2003).

The unbalanced flow problem may not manifest itself when the overall demand level is low but may appear with traffic growth even at medium demand levels.  Demand flow patterns and demand levels may change significantly after the introduction of a roundabout, sometimes in a relatively short period of time.

Modeling of effects of heavy unbalanced flows on roundabout capacity and level of service is important in optimizing the roundabout geometry (including lane use) to alleviate the associated problems.  This can be achieved to a good extent for a new roundabout subject to the reliability of traffic demand information, or for an existing roundabout to a smaller extent, given the design constraints (see O'Brien, et al 1997).  Part-time metering signals on selected roundabout approaches, operational only when needed under peak demand conditions, can be an effective measure preventing the need for a fully signalized intersection treatment.

Extract from the Australian Traffic Signal Guide (AUSTROADS 2003, Section 15.7):

Roundabout metering signals may be used where excessive queuing and delays are observed on one or more legs of a roundabout due to heavy circulating flow rates, especially in the case of heavily directional origin-destination movements.  In this case, a dominant approach stream constitutes the major proportion of traffic in the circulating stream that causes a significant reduction in the capacity of the approach that has to give way to that circulating stream (Akçelik, Chung and Besley 1998).  These signals are usually employed on a part-time basis since they may be required only when heavy demand conditions occur during peak periods. 

Two-aspect yellow and red signals are used for roundabout metering.  The sequence of aspect display is Off to Yellow to Red to Off.  When metering is not required neither aspect is displayed.  

The signalised approach is referred to as the "metered approach", and the approach with the queue detector as the "controlling approach".  When the queue on the controlling approach extends back to the queue detector, the signals on the metered approach operate so as to create a gap in the circulating flow.  This helps the controlling approach traffic to enter the roundabout.  When the red display is terminated on the metered approach, the roundabout reverts to normal operation. 

The introduction and duration of the red signal on the metered approach is determined by the controlling approach traffic.  The duration of the blank signal is determined according to a minimum blank time requirement, or extended by the metered approach traffic if detectors are used on that approach.

A minimum of two signal faces, one primary and one tertiary, shall be installed.  A regulatory sign STOP HERE ON RED SIGNAL shall be fixed to any signal post erected adjacent to the stop line, as drivers do not expect to stop at the advance stop line location.  Stop lines shall be located not less than 3 m in advance of the approach holding line but preferably, should be positioned approximately 20 m from the holding line.  Queue detector setback distance on the controlling approach is usually in the range 50 m to 120 m. 

Various site-specific methods may also be used to meter traffic, e.g. using an existing upstream midblock signalised crossing on the metered approach.

In some cases, it may be necessary to supplement the traffic signals with explanatory fixed or variable message signposting.  Where sight restrictions exist, advance warning signals should be considered.


At a roundabout with an unbalanced flow pattern, a traffic stream with a heavy flow rate enters the roundabout against a circulating stream with a low flow rate.  Three such roundabout cases from Melbourne, Australia are described below.

Small single-lane roundabout at the intersection of Stanhope Grove with Broadway in Camberwell1524veh/h per lane entering against a circulating flow rate of 60 veh/h has been reported. Sum of entering and circulating flows is 1584 veh/h.

Small-Medium single-lane roundabout at the intersection of Grange Rd, St Georges Rd and Alexandra Avenue in Toorak1693 veh/h per lane entering against a circulating flow rate of 67 veh/h has been reported.  Sum of entering and circulating flows is 1760 veh/h.  The measured follow-up headway and critical gap values for this entry lane are 1.992 s and 2.423 s, respectively.  The maximum capacity at zero circulating flow (corresponding to the follow-up headway) is 3600 / 1.992 = 1808 veh/h.  A photo of this roundabout is shown below.

Large roundabout at the intersection of Mickleham Rd and Broadmeadows Rd in Westmeadows (this is a case described in the AUSTROADS Roundabout Guide):  1397 veh/h per lane against a circulating flow rate of 83 veh/h in am peak (sum of entering and circulating flows = 1480 veh/h) and 1501 veh/h per lane against a circulating flow rate of 112 veh/h in pm peak (sum of entering and circulating flows = 1613veh/h)s. 



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Some of these articles are available for download on this web site:

Articles    More Articles on Roundabouts

AKÇELIK, R. (2008). An investigation of the performance of roundabouts with metering signals. Paper presented at the National Roundabout Conference, Transportation Research Board, Kansas City, MO, USA, 18-21 May 2008.

AKÇELIK, R. (2006).  Operating Cost, Fuel Consumption and Pollutant Emission Savings at a Roundabout with METERING SIGNALS.  Paper presented at the ARRB 22nd Conference, Canberra, 29 Oct - 2 Nov 2006.  Updated: 22 October 2007.

E. NATALIZIO (2005).  Roundabouts with Metering Signals.  Paper presented at the Institute of Transportation Engineers 2005 Annual Meeting, Melbourne, Australia, August 2005.

AKÇELIK, R. (2005a).  Roundabout Model Calibration Issues and a Case Study.  Paper presented at the TRB National Roundabout Conference, Vail, Colorado, USA, May 2005.

AKÇELIK, R. (2005b).  Capacity and Performance Analysis of Roundabout Metering Signals.   Paper presented at the TRB National Roundabout Conference, Vail, Colorado, USA, May 2005.  

AKÇELIK, R. (2004).  Roundabouts with Unbalanced Flow Patterns.   Paper presented at the ITE 2004 Annual Meeting, Lake Buena Vista, Florida, USA, Aug 2004.  (2.8MB)

AKÇELIK, R. (2003).  A Roundabout Case Study Comparing Capacity Estimates from Alternative Analytical Models.  Paper presented at the 2nd Urban Street Symposium, Anaheim, California, USA, 28-30 July 2003.  (470KB).

AKÇELIK, R., CHUNG, E. and BESLEY, M. (1998).  Roundabouts: Capacity and Performance Analysis. Research Report ARR No. 321.  ARRB Transport Research Ltd, Vermont South, Australia (2nd Edition 1999).

AUSTROADS (1993).  Guide to Traffic Engineering Practice Part 6 - Roundabouts.  AP-G11.6, Sydney.

AUSTROADS (2003).  Guide to Traffic Engineering Practice Part 7 - Traffic Signals.  AP-G11.7, Sydney.

HALLWORTH, M.S. (1992).  Signalling roundabouts - 1. Circular arguments.  Traffic Eng. and Control, 33 (6), pp 354-363.

HUDDART, K.W. (1983). Signalling of Hyde Park Corner, Elephant and Castle and other roundabouts. PTRC 11th Summer Annual Meeting, Proceedings of Seminar K, pp 193-208.  (2100KB)

LINES and CRABTREE (1988).  The use of TRANSYT at signalised roundabouts.  Traffic Eng. and Control, 29 (6), pp 332-337.

O'BRIEN, A., AKÇELIK, R., WILLIAMSON, D. and PANTAS, T. (1997).  Three-laning a two-lane roundabout - the outcomes. Compendium of Technical Papers (CD).  67th Annual Meeting of the Institution of Transportation Engineers. (500KB)

SHAWALY, E.A.A, LI, C.W.W. and ASHWORTH, R. (1991).  Effects of entry signals on the capacity of roundabout entries - a case study of Moore Street roundabout in Sheffield.  Traffic Eng. and Control, 32 (6), pp 297-301.

SIDES. K. (2000).  Assessing the Clearwater Beach Entryway Roundabout
ITE 70th Annual Meeting Compendium.  (790KB).

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