SIDRA For Roundabouts

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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.  In the USA, many Departments of Transportation have multi-site licences for the use of SIDRA INTERSECTION.

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 models (updated April 2011):

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 2010 model described in the 2010 Highway Capacity Manual, and the UK TRL model implemented in the RODEL and ARCADY software. The HCM 2010 model has been implemented in the SIDRA INTERSECTION software. The terms SIDRA Standard and HCM 2010 models are used to distinguish between the two model options in SIDRA INTERSECTION. 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 is relevant to the US scene where roundabouts are relatively new. 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.

Also see: About Roundabouts


SIDRA INTERSECTION Software for Roundabouts in the USA

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SIDRA INTERSECTION offers the HCM 2010 roundabout capacity model based on US research on roundabouts. The model is implemented as described in Highway Capacity Manual 2010 with numerous extensions (rpalces the NCHRP 572 model option used in previous versions).

The Highway Capacity Manual (HCM) versions of SIDRA INTERSECTION use the HCM 2010 roundabout capacity model as the default model. The software also continues to offer the SIDRA Standard model with a default Environment Factor of 1.2 to match the lower capacity estimates based on US roundabout research.

Detailed information on the HCM 2010 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).

A survey of US traffic professionals at the Roundabout Conference in Seattle WA in May 2000 indicated that SIDRA was by far the most popular roundabout analysis tool in the USA. At that time, there were 450 SIDRA sites in the USA, and SIDRA use in the USA has increased significantly since then (over 2000 licences today). Some quotes from US publications or web sites are presented below.

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 titledRoundabout 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 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 treatments and improvements to existing intersection 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).

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