SIDRA INTERSECTION 11
SIDRA INTERSECTION 11
ROUNDABOUTS
The Global Standardfor Roundabout Analysis

The most widely used roundabout analysis software in the USA — recognised by the HCM, NCHRP, FHWA and state DOTs.

Five CapacityModels, One Platform

SIDRA Standard, HCM Edition 6, HCM 6 Extended, HCM 2010, and full geometry-based gap-acceptance — all lane-by-lane.

Geometry DrivesCapacity

Inscribed diameter, entry radius, circulating width, flare geometry — the SIDRA Standard model links design parameters directly to capacity.

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According to the US Transportation Research Board, SIDRA INTERSECTION is the most widely used software tool in the USA for roundabout capacity and performance analysis.

All five models operate within SIDRA's lane-based framework, extending the HCM's traditionally approach-based roundabout method to individual lane analysis.

The SIDRA Standard model directly links geometric design parameters to capacity. Inscribed diameter, circulating road width, entry radius and angle, approach lane width, flare geometry, and number of entry and circulating lanes all feed directly into the capacity calculation.

Changing the geometry immediately changes the results — enabling an iterative design process where the engineer sees the operational effect of each modification.

Capacity constraint
Prevents unrealistically low delay estimates on oversaturated lanes.
Unbalanced flow
Models the roundabout as an interactive system rather than independent T-intersections.
Metering signals
Assessed using a unique analytical method developed by Dr Akçelik for VicRoads.
Any lane configuration
Any combination of entry and circulating lanes, bypass/slip lanes, short lane capacity with overflow effects.

WSDOT recommends the SIDRA Standard roundabout model. ODOT recommends SIDRA for non-standard configurations. GDOT lists SIDRA as approved operational analysis software.

Also recognised by the US Highway Capacity Manual, NCHRP roundabout guides (NCHRP 672, NCHRP Research Report 1043), the FHWA alternative intersections guide, and Austroads.

Roundabout Analysis

The global standardfive capacity models, 40+ years of development

No other software provides the combination of multiple capacity model options, geometry-based capacity estimation, lane-by-lane analysis depth, and integrated network modelling for roundabouts that SIDRA INTERSECTION offers.

According to the US Transportation Research Board, SIDRA INTERSECTION is the most widely used software tool in the USA for roundabout capacity and performance analysis. All five models operate within SIDRA's lane-based framework, extending the HCM's traditionally approach-based roundabout method to individual lane analysis.

SIDRA Standard (HCM)
Calibrated to match HCM Edition 6 using Environment Factors of 1.05 (single-lane) and 1.20 (two-lane) for US conditions. Default in the US HCM Setup.
HCM Edition 6
Full implementation of the HCM Edition 6 roundabout model (identical to HCM Edition 7).
HCM 6 Extended
Same equations as HCM 6 with more detailed parameter specification, developed using Wisconsin DOT data.
HCM 2010
Earlier HCM model, retained for backward compatibility.
SIDRA Standard
Full geometry-based gap-acceptance model with the most detailed sensitivity to roundabout design. The most comprehensive model option, linking capacity directly to driver behaviour and geometric parameters.
Design Workflow

Geometry drives capacitychange the design, see the operational effect

Empirical regression models predict what capacity will be. The SIDRA Standard model explains why — linking capacity to the driver behaviour and geometry that produce it.

The SIDRA Standard model directly links geometric design parameters to capacity. Inscribed diameter, circulating road width, entry radius and angle, approach lane width, flare geometry, and number of entry and circulating lanes — all feed directly into the capacity calculation.

Iterative design process
Changing the geometry immediately changes the results — enabling an iterative design process where the engineer sees the operational effect of each modification. Fundamentally different from regression-based models that treat capacity as a statistical prediction.
Geometric delay
Calculated separately from queuing delay. For roundabouts, vehicles slow to negotiate the circulating roadway even in free-flow conditions — essential for accurate comparison between roundabout and signalised alternatives.
Gap-acceptance model
Uses driver behaviour parameters — critical gap, follow-up headway, and their sensitivity to geometry — rather than aggregate regression coefficients. The engineer can see which parameter is limiting capacity.
Depth & Versatility

Every detail, any configurationthe operational modelling that matters in practice

Roundabout metering signals are assessed using a unique analytical method developed by Dr Akçelik for VicRoads — no other software provides this capability.

Beyond capacity models, SIDRA provides a comprehensive set of operational modelling capabilities that handle real-world roundabout configurations other tools cannot.

Capacity constraint on oversaturated lanes
Prevents unrealistically low delay estimates that occur when models allow theoretical capacity to exceed practical limits.
Unbalanced flow treatment
Models the roundabout as an interactive system rather than independent T-intersections — capturing flow interactions between approaches.
Upstream signal effects
Extra bunching from upstream signal releases reduces gap-acceptance capacity at the roundabout — an effect that isolated analysis cannot capture.
Metering signals
Assessed using a unique analytical method developed by Dr Akçelik for VicRoads. No other software provides this capability.
Any lane configuration
Any combination of entry and circulating lanes, bypass/slip lanes (shared, exclusive, continuous), short lane capacity with overflow effects, and exit short lane merge operations.
Fully signalised roundabouts
Including signalised circulating roads. Roundabout corridors and networks modelled using the Network model with queue spillback and signal coordination effects.
Third-Party Recognition

Recognised by the agencies that set the standardsthird-party assessments, not marketing claims

These are third-party assessments, not SIDRA marketing claims.

SIDRA INTERSECTION is recognised by the agencies and institutions that set the standards for roundabout practice in the US and internationally.

WSDOT — Washington State

Recommends the SIDRA Standard roundabout model. Explicitly states that Synchro is not recommended for roundabout analysis.

ODOT — Oregon

Recommends SIDRA for non-standard configurations and to verify analysis from other software. Guidelines indicate that Synchro's roundabout capabilities are limited.

GDOT — Georgia

Lists SIDRA as approved operational analysis software for roundabout analysis.

SIDRA INTERSECTION is also recognised by the US Highway Capacity Manual, the NCHRP roundabout guides (NCHRP 672, NCHRP Research Report 1043), the FHWA alternative intersections guide, and Austroads.

SIDRA ASSIGN

Add traffic assignment
to your roundabout networks.

SIDRA ASSIGN extends the Network model with lane-based traffic assignment — using Origin–Destination data to determine how traffic distributes across routes. Five assignment methods, one analytical environment.

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