12.2 Motorized Vehicle Capacity Methodology
12.2.1 Overview and Intended Use
This section provides guidance on conducting a daily service volume capacity evaluation, a peak hour service volume capacity evaluation, and basic parameters of using software tools for roundabout analysis (e.g., data input and parameters and reporting MOEs).
Section 12.2.2
is intended for use in the ICE Stage 1 analysis of alternatives while Sections 12.2.3, 12.2.4, and 12.2.5
are applied to Stage 2 of project development and again in latter stages of project development.12.2.2 Computational Steps – Stage 1 ICE
The computational steps make use of the peak hour service volume and capacity evaluation in the FHWA CAP-X model and the NCHRP 1043, Exhibit 8.6, Planning-level sizing guide. For a step-by-step guide on conducting a peak hour volume capacity evaluation, see
Appendix M, Section 1 – Planning Level Roundabout Sizing Guide
.12.2.3 Computational Steps ICE Stage 2 and Subsequent Analyses
This section discusses software used for roundabout analysis in Stage 2 ICE and for subsequent purposes related to design refinement. It includes a basic overview of different software packages, basic input parameters, typical output parameters, and additional external guidance that can be used during analysis. The tools mentioned in this section represent common software used in roundabout analysis, but do not represent all software options that can be used in roundabout analysis. Several different software can be used for roundabout analysis. See
Appendix M, Section 2
for a more detailed scription of each analysis software package. The selection of software depends on the project type and needs. In addition, when results from one analysis software are on the boundary of indicating a need for additional lanes, use of more than one analysis software is recommended. A summary of commonly used roundabout analysis tools is shown in Tool | Type | When to use | Notes | Outputs |
|---|---|---|---|---|
Highway Capacity Software (HCM 7th Edition) | Deterministic | After CAP-X and planning-level analysis of number of lanes. Use peak hour volumes | Apply software default values for headway and capacity parameters If V/C results > 0.85 use Junctions | V/C, Control delay and LOS, 95% queue lengths by approach |
SIDRA Intersection | Deterministic | After CAP-X and planning-level analysis of number of lanes. Use peak hour volumes | Use US HCM model and default HCM headway parameters | V/C ratio, control delay, LOS, 95% queue length |
Synchro | Deterministic | After CAP-X and planning-level analysis of number of lanes. Use peak hour volumes | Use software default HCM gap parameters | V/C ratio, control delay, LOS, 95% queue length |
Junctions ( Assessment of Roundabout Capacity and Delay (ARCADY)) | Deterministic | After computational steps are complete and more detail is necessary. For analyzing more than two lanes. | Analyze both the US HCM model and the ARCADY model to create a range of results. Calibrate the Y-Intercept by -10% for build-year conditions analysis and -5% for design year analysis. | V/C ratio, control delay, LOS, 95% queue length |
Vissim | Microsimulation | For oversaturated conditions; closely spaced signals and/or roundabouts; complex roadway geometry; or when a visual representation is necessary | Can be accompanied by a deterministic tool (SIDRA, Synchro, or Junctions). Use default headway values | Delay and average network travel time |
Synchro/ SimTraffic | Microsimulation | For oversaturated conditions; closely spaced signals and/or roundabouts; complex roadway geometry; or when a visual representation is necessary | Can be accompanied by a deterministic tool (HCM, SIDRA, Synchro, or Junctions). Use default headway values | Delay and average network travel time |
12.2.4 Reporting MOEs
Capacity, delay, queue length, and LOS are typical MOEs used in roundabout analysis. Other MOEs may be used if deemed appropriate. Improved operational performance equates to a low volume-to-capacity (V/C) ratio, low control delay, LOS A (best) to LOS D (acceptable), and short queue lengths. shows tools used for roundabout analysis and their corresponding MOE outputs.
Tool | Outputs |
|---|---|
SIDRA Intersection | V/C ratio, control delay (sec), LOS, 95% queue length (feet) |
Synchro | V/C ratio, control delay (sec), LOS, 95% queue length (feet) |
Vissim | Average vehicle travel times (sec) and average total delay per vehicle (sec) |
Junctions | V/C ratio, control delay (sec), LOS, 95% queue length (feet), crash exposure |
12.2.4.1 Entry Capacity
12.2.4.1 Entry Capacity
Within the context of this chapter, capacity is the maximum number of vehicles that can pass through a roundabout entry during an interval of time. The computational steps in
Section 12.2.2
explain how to determine approximate daily service volumes and peak hour service volumes for a roundabout. Software such as SIDRA, Synchro, or Junctions may also be used for a more detailed determination of capacity. Vissim and SimTraffic do not have a direct entry capacity output and cannot be used to determine exact capacity values for a roundabout. The volume-to-capacity (V/C) ratio describes the number of vehicles entering from one approach to the roundabout (volume) divided by the maximum number of vehicles that can pass through that same entry during an interval of time, usually one hour. The V/C ratio is typically used as an indicator of the level of saturation of a roundabout entry. If V/C ratio exceeds 0.85, consider delay and queue length more carefully and conduct sensitivity testing using alternate models, geometry and/or unanticipated traffic flows).
12.2.4.2 Delay
12.2.4.2 Delay
Delay is the sum of the deceleration delay, stopped delay, and acceleration delay caused by a roundabout. Geometric delay is not accounted for in the models identified in this section. Delay is calculated using formulas found in the HCM to compute:
- The average control delay for each approach; and
- The overall roundabout control delay
Delay is reported in seconds per vehicle. Values may be rounded to the nearest tenth of a second.
12.2.4.3 Queue Length
12.2.4.3 Queue Length
Queue length is the number of vehicles waiting to proceed through the roundabout due to congestion. Queue length is typically reported as the 95th percentile queue length and is reported in feet. Queue length is calculated with formulas found in the HCM and is dependent on V/C ratio, capacity, and analysis time period. Queue may be used:
- When calibrating simulation models;
- To determine adequate vehicular storage length based on queue length;
- To determine driveway or through lane blockages; and
- As an MOE
12.2.4.4 Level of Service
12.2.4.4 Level of Service
LOS is the letter grade assignment (A-F) of roundabout. It is based on control delay caused by a roundabout entry and is assumed to be LOS F if V/C is greater than 1.0. While the LOS A is the most favorable, scores A through D are typically acceptable ranges for LOS. Refer to HCM 7th Edition Chapter 22 for a step-by-step guide on performing manual calculations of LOS for motorized vehicles at roundabouts. SIDRA, Junctions and Synchro determine LOS automatically, while Vissim and SimTraffic can calculate LOS using control delay. The unsignalized LOS thresholds, referred to in HCM 7th Edition Chapter 22, as shown in , apply to roundabout yield control.
Control Delay (s/veh) | LOS by Volume-to-Capacity Ratio* | |
|---|---|---|
v/c≤1.0 | v/c>1.0 | |
0-10 | A | F |
> 10-15 | B | F |
> 15-25 | C | F |
> 25-35 | D | F |
> 35-50 | E | F |
> 50 | F | F |
Note: *For approaches and intersection wide assessment, LOS is defined solely by control delay
12.2.4.5 Sensitivity Testing
The anticipated operational performance for future conditions considers a sensitivity testing-approach that varies the design year peak period traffic volumes by more or less an assigned percentage (i.e., +/- 30 percent).
This accounts for the uncertainty of future traffic conditions, especially at intersections where a roundabout is installed with adjacent land use that is undeveloped or anticipated to re-develop before the design year. Sensitivity testing may reveal that the chosen entry or a specific lane would experience excessive queuing. Varying the mode to be used for analysis and varying the geometry or lane configuration are also useful sensitivity testing considerations. Often the most probable result is not the exact result produced by any given model, but with a range of results produced by more than one type of model or for more than one geometry.
12.3 Other Modes
While roundabout analysis is mostly limited to the vehicular methodology, there are some considerations available for roundabout analysis of pedestrians and bicycles. These methodologies are generally described in this section and are described in detail in the HCM, NCHRP 616, NCHRP 834, and Mineta Transportation Institute Report 11-19. For more information on multimodal analysis, see
Chapter 14
.12.3.1 Pedestrian
High volumes of pedestrians at a roundabout may warrant the need for a LOS analysis, which may be conducted using the guidance found in HCM 7th Edition, Chapter 22, or NCHRP 616, Chapter 8. The methodology in NCHRP 834, Chapter 7 may also be used as an alternative method of analyzing pedestrian crossing needs, (e.g., need for PHBs at a roundabout). Even in situations where high volumes of pedestrians are not consistent at a roundabout, it is common for a few pedestrians or groups of pedestrians to sporadically interrupt vehicular traffic. Junctions (ARCADY: Assessment of Roundabout Capacity and Delay) or Microsimulation (microscopic) analysis may be used to model the interactions in this scenario. Microsimulation can provide valuable visual and numerical insights on how pedestrians interact with and impact the operations of a roundabout.
An alternative method of multimodal analysis is based on bicycle level of traffic stress (BLTS) and may be applied to pedestrian analysis at a roundabout. The pedestrian level of traffic stress (PLTS) methodology classifies the roundabout into one of four walkability categories. Each of these categories is based on perceived safety issues near the roundabout rather than pedestrian LOS. Characteristics that affect level of traffic stress are sidewalk existence, condition and widths, buffer between the sidewalk and the road, and roadway speed. A level of traffic stress score of 1 indicates that the roundabout is suitable for most pedestrians; a level of traffic stress score of 4 indicates that only able-bodied pedestrians may attempt to cross the roundabout. This methodology is based on the Mineta Transportation Institute, Report 11-19. See
Chapter 14
of this manual for more information. Additional analysis methods can be found in NCHRP Research Report 992: Guide to Pedestrian Analysis. It presents a state-of-the-art guide to conducting pedestrian traffic analysis on the basis of volume, safety, operations, and quality of service.12.3.2 Bicycle
Bicyclists can either use the vehicular travel lanes or the pedestrian sidewalk to traverse the roundabout. In instances where large volumes of bicycles are anticipated to use the sidewalk to traverse the roundabout, slip ramps can be integrated into the roundabout to accommodate the transition from road to sidewalk. When bicycles traverse the roundabout on the road, they are analyzed for capacity similarly to other vehicular traffic. Conversely, when they traverse the roundabout on the sidewalk, they are analyzed as pedestrians. The volume of bicycles that enter the roundabout on the road are included in the vehicular volumes when analyzing roundabouts with the motorized vehicle methodology.
For bicycle LOS analysis, refer to HCM 7th Edition, Chapter 22, or NCHRP 616, Chapter 7.