Planning

Chapter 1 presented a range of roundabout categories, and suggested typical daily service volume thresholds below which four-leg roundabouts may be expected to operate, without requiring a detailed capacity analysis. Chapter 2 introduced roundabout performance characteristics, including comparisons with other intersection forms and control, which will be expanded upon in this chapter. This chapter covers the next steps that lead up to the decision to construct a roundabout with an approximate configuration at a specific location, preceding the detailed analysis and design of a roundabout. By confirming that there is good reason to believe that roundabout construction is feasible and that a roundabout offers a sensible method of accommodating the traffic demand, these planning activities make unnecessary the expenditure of effort required in subsequent chapters.

Planning for roundabouts begins with specifying a preliminary configuration. The configuration is specified in terms of the minimum number of lanes required on each approach and, thus, which roundabout category is the most appropriate basis for design: urban or rural, single-lane or double-lane roundabout. Given sufficient space, roundabouts can be designed to accommodate high traffic volumes. There are many additional levels of detail required in the design and analysis of a high-capacity, multi-lane roundabout that are beyond the scope of a planning level procedure. Therefore, this chapter focuses on the more common questions that can be answered using reasonable assumptions and approximations.

Feasibility analysis requires an approximation of some of the design parameters and operational characteristics. Some changes in these approximations may be necessary as the design evolves. A more detailed methodology for performing the operational evaluation and geometric design tasks is presented later in Chapters 4 and 6 of this guide, respectively.