Operation

This chapter presents methods for analyzing the operation of an existing or planned roundabout. The methods allow a transportation analyst to assess the operational performance of a facility, given information about the usage of the facility and its geometric design elements. An operational analysis produces two kinds of estimates: (1) the capacity of a facility, i.e., the ability of the facility to accommodate various streams of users, and (2) the level of performance, often measured in terms of one or more measures of effectiveness, such as delay and queues.

The Highway Capacity Manual (1) (HCM) defines the capacity of a facility as “the maximum hourly rate at which persons or vehicles can reasonably be expected to traverse a point or uniform section of a lane or roadway during a given time period under prevailing roadway, traffic, and control conditions.” While capacity is a specific measure that can be defined and estimated, level of service (LOS) is a qualitative measure that “characterizes operational conditions within a traffic stream and their perception by motorists and passengers.” To quantify level of service, the HCM defines specific measures of effectiveness for each highway facility type. Control delay is the measure of effectiveness that is used to define level of service at intersections, as perceived by users. In addition to control delay, all intersections cause some drivers to also incur geometric delays when making turns. A systems analysis of a roadway network may include geometric delay because of the slower vehicle paths required for turning through intersections. An example speed profile is shown in Chapter 6 to demonstrate the speed reduction that results from geometric delay at a roundabout.

While an operational analysis can be used to evaluate the performance of an existing roundabout during a base or future year, its more common function in the U.S. may be to evaluate new roundabout designs.

This chapter:

• Describes traffic operations at roundabouts;
• Lists the data required to evaluate the performance of a roundabout;
• Presents a method to estimate the capacity of five of the six basic roundabout configurations presented in this guide;
• Describes the measures of effectiveness used to determine the performance of a roundabout and a method to estimate these measures; and
• Briefly describes the computer software packages available to implement the capacity and performance analysis procedures.