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.
Table 12-3: Analysis Tool 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

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

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

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

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.
Table 12-4: Level of Service Criteria
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.