"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

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

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

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

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

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 the Downloads and Roundabouts pages of the Resources section of this web site.