The operational analysis method for analyzing motor vehicles presented in Chapter 22 of the HCM, 6th Edition allow analysts to assess the operational performance of an existing or planned one-lane or two-lane roundabout given traffic-demand levels. The HCM capacity method employs simple, empirical regression models to reflect the capacity of roundabouts with up to two lanes. The HCM also includes models for estimating the performance of yield-controlled, right-turn bypass lanes.

All the software models that employ the HCM capacity methodology share the same constraint on use when oversaturation (v/c > 0.85) prevails. A volume-to-capacity ratio of 0.90 is considered the practical capacity of a roundabout entry. Higher capacities may be achievable but require more detailed operational analysis. In such cases the HCM recommends the use of alternative models or simulation. The Junctions (ARCADY) model has both HCM and an independent model, the U.K. empirical model that can address oversaturation of a roundabout entry. The ARCADY model accounts for the effect of a carryover of queues from one time slice to the next using an iterative approach that is not established in the HCM methodology. Sidra Standard also has an iterative capacity method, but it has not been field validated in the U.S.; hence, the results have not been proven to be reliable for U.S. roundabouts.

For performance measures, the HCM estimates the volume-to-capacity ratio, the control delay, and the 95th-percentile queue on a lane-by-lane basis and assigns LOS for each lane based on the control delay of that lane. The HCM also estimates control delay and LOS aggregated to the approach level and the intersection level.

The HCM identifies that its analytic methods have several key scope limitations for which it recommends using alternative tools, such as the methods described in the following sections of this chapter. These scope limitations include the following:

Pedestrian signals or hybrid beacons at roundabout crosswalks.
Metering signals on one or more approaches.
Adjacent signals or roundabouts.
Priority reversal under extremely high flows.
High pedestrian or bicyclist activity levels.
More than two entry lanes on an approach.

Flared entry lanes, such as an entry that widens from one approach lane to two entry lanes over a short distance.

SIDRA Intersection is a software package containing a deterministic model that is commonly used to analyze roundabouts. A SIDRA Intersection analysis can be conducted in Stage 2 of the ICE process and when a more detailed analysis is necessary.

Two capacity models are used to analyze roundabouts in SIDRA Intersection: the SIDRA Standard model and the US HCM model. The SIDRA Standard model (an Australian-based model) has not been field-validated, nor calibrated to U.S. roundabouts and driving conditions. Because of this, the US HCM model is prescribed for TxDOT roundabout analyses.

Due to limitations in the US HCM model for oversaturated conditions, Junctions/ARCADY or microsimulation (microscopic) analysis can supplement the Sidra-HCM analysis. Simulation (VISSIM or Sim Traffic) is also useful to account for the effects of adjacent intersections with signals or other innovative intersections and traffic control devices. The SIDRA settings and default input values shown in

Table M-1

can be used as a starting point in an analysis. Typical outputs for SIDRA include v/c ratio, control delay, LOS, and 95 percent queue length.

Synchro is a deterministic tool that can be used to analyze roundabouts. A Synchro analysis is conducted after the daily service volume evaluation and peak hour volume evaluation (computational steps) have been completed and a more detailed analysis is necessary. Roundabout analysis using Synchro is based on methodology found in the HCM 7th Edition. Default headway values provide for in the HCM model research are satisfactory until studies of TxDOT roundabouts justify calibration for local conditions. Typical outputs for Synchro include v/c ratio, control delay, LOS, and 95 percent queue length.

HCS is also a deterministic tool that can be used to analyze roundabouts. An HCM analysis is typically conducted after the daily service volume evaluation and peak hour volume evaluation (computational steps) have been completed and a more detailed analysis is necessary. The analysis time period and PHF are entered on the General window; roundabout configurations are entered on the Geometry window; and traffic volumes, pedestrian crossing volumes, critical headway, and follow-up headway parameters are entered in the Traffic window. Roundabout analysis using HCS is based on methodology found in the HCM.

HCS computes control delay and LOS for each entering lane, approach, and the whole roundabout. Additionally, the 95th percentile queue is obtained in terms of number of vehicles for each entering lane. The 95th percentile queue are multiplied by the length of a vehicle to compute the queue length in feet.

For general information on modeling roundabouts and changing other analysis parameters, refer to the HCS user guide.

The ARCADY module is a part of the Junctions 10 software created by Transport Research Laboratory (TRL). It can be used as an alternative to SIDRA-HCM and Synchro as well as a tool for validating the results of a microsimulation model. Typical ARCADY inputs are shown in

Table M-2

below. Typical outputs for Junctions include v/c ratio, control delay, LOS, 95 percent queue length, and crash exposure. Refer to the Junctions 10 user manual for information on developing a roundabout model and generating results.

Calibration to U.S. driving conditions is simple to accomplish if there are nearby roundabouts with saturated entries. In the absence of readily available field data, calibration is typically facilitated by adjusting the capacity equation Y-Intercept percentage to 90 percent for existing year conditions and 95 percent for design year conditions. Recent comparisons with HCM indicate that the two models correlate well.

It is recommended for ARCADY users to be aware that the default model relies on a ONEHOUR profile type under the Traffic Demand settings. This can be changed from ONEHOUR to PHF so that customized PHF values can be applied to the analysis.

When switching the analysis from the Junctions Standard Roundabout model to the HCM Roundabout model, lane assignments will need to be applied for multilane entries. The outside or curb lane is designated as the nearside lane type, while the inner lane or left lane is designated as the offside lane type. A right turn bypass lane is also be assigned per approach as a yielding bypass or as a nonyielding bypass (free flow).

Vissim is a microsimulation software that can be used to model roundabouts. Roundabout analysis using Vissim is often considered for oversaturated conditions; when modeling the network effects of closely spaced roundabouts; for complex roadway geometry; or when a visual representation of roundabout operations is necessary. The output of Vissim is delay, which can be converted into LOS and average travel time for vehicles. A Vissim analysis is typically not a standalone tool used for roundabout analysis and is typically accompanied by the results from a deterministic tool such as SIDRA, Synchro, or Junctions.

An advantage of Vissim over other roundabout analysis tools is the visual component that shows vehicles flowing through the network. Animations of roundabout operations can be recorded and serve as a visual perspective on operations. See

Chapter 13

for more information on microsimulation.

Headway values are an input in Vissim and have a considerable impact on roundabout performance. Headway values adjusted to match local conditions can be used as well. A visual inspection of gap acceptance and headways in the field is used to adjust values as necessary.

Typical outputs for Vissim include average vehicle travel time (sec) and average total delay per vehicle (sec).

SimTraffic is a microsimulation software that can be used to model roundabouts as standalone intersections or in networks with traffic signals nearby. Roundabout analysis using SimTraffic is often considered for oversaturated conditions; when modeling the network effects of closely spaced roundabouts; for complex roadway geometry; or when a visual representation of roundabout operations is necessary.

SimTraffic models individual cars, trucks, buses and pedestrians traveling through a network of freeways, streets and multiple types of intersections. Delays, vehicular queues, travel times and other measures of effectiveness are computed for these modeled users over a defined simulation period and can be determined for roadway segments and corridors, in addition to individual intersections. SimTraffic produces 2-D and 3-D animations of the transportation model.

Typical outputs for SimTraffic include average vehicle travel time (sec) and average total delay per vehicle (sec).