Data Requirements
The analysis method described in this chapter requires the specification of traffic volumes for each approach to the roundabout, including the flow rate for each directional movement. Volumes are typically expressed in passenger car vehicles per hour (vph), for a specified 15-minute analysis period. To convert other vehicle types to passenger car equivalents (pce), use the conversion factors given in Exhibit 4-1.
Traffic volume data for an urban roundabout should be collected for each directional movement for at least the morning and evening peak periods, since the various movements, and thus approach and circulating volumes, may peak at different times. At rural roundabouts, the analyst should check the requirements of the agency with the jurisdiction of the site. The reader is referred to the Manual of Transportation Engineering Studies (8) for a complete discussion of traffic volume data collection methods. Typically, intersection volume counts are made at the intersection stop bar, with an observer noting the number of cars that pass that point over a specified time period. However, particularly with respect to cases in which demand exceeds capacity (when queues do not dissipate within the analysis period), it is important to note that the stop bar counts reflect only the volume that is served, not the demand volume. In this case, care must be taken to collect data upstream of the end of a queue so that true demand volumes are available for analysis.
The relationship between the standard origin-to-destination turning movements at an intersection and the circulating and entry flows at a roundabout is important, yet is often complicated to compute, particularly if an intersection has more than four approaches. For conventional intersctions, traffic flow data are accumulated by directional turning movement, such as for the northbound left turn. For roundabouts, however, the data of interest for each approach are the entry flow and the circulating flow. Entry flow is simply the sum of the through, left, and right turn movements on an approach. Circulating flow is the sum of the vehicles from different movements passing in front of the adjacent uptstream splitter island. At existing roundabouts, these flows can simply be measured in the field. Right turns are included in approach volumes and require capacity, but are not included in the circulating volumes downstream because they exit before the next entrance.
For proposed or planned four-legged roundabouts, Equations 4-1 through 4-4 can be applied to determine conflicting (circulating) flow rates, as shown graphically in Exhibit 4-2.
For existing roundabouts, when approach, right-turn, circulating, and exit flows are counted, directional turning movements can be computed as shown in the following example. Equation 4-5 shows the through movement flow rate for the eastbound approach as a function of the entry flow rate for that approach, the exit flow rate for the opposing approach, the right turn flow rate for the subject approach, the right turn flow rate for the approach on the right, and the circulating flow rate for the approach on the right. Other through movement flow rates can be estimated using a similar relationship.
The left turn flow rate for an approach is a function of the entry flow rate, the through flow rate, and the right turn flow rate for that same approach, as shown in Equation 4-6. Again, other movements’ flows are estimated using similar equations.
While this method is mathematically correct, it is somewhat sensitive to errors and inconsistencies in the input data. It is important that the counts at all of the locations in the roundabout be made simultaneously. Inconsistencies in the data from counts taken on different days can produce meaningless results, including negative volumes. At a minimum, the sum of the entering and exiting volumes should be checked and adjustments should be made if necessary to ensure that the same amount of traffic enters and leaves the roundabout.