Planning Steps

The following steps may be followed when deciding whether to implement a roundabout at an intersection:

• Step 1: Consider the context. What are there regional policy constraints that must be addressed? Are there site-specific and community impact reasons why a roundabout of any particular size would not be a good choice?
• Step 2: Determine a preliminary lane configuration and roundabout category based on capacity requirements. Exhibit 3-1 will be useful for making a basic decision on the required number of lanes. If Exhibit 3-1 indicates that more than one lane is required on any approach, refer to Chapters 4 and 6 for the more detailed analysis and design procedures. Otherwise, proceed with the planning procedure.
• Step 3: Identify the selection category. This establishes why a roundabout may be the preferred choice and determines the need for specific information.
• Step 4: Perform the analysis appropriate to the selection category. If the selection is to be based on operational performance, use the appropriate comparisons with alternative intersections.
• Step 5: Determine the space requirements. Refer to Section 3.6 for the right-of-way widths required to accommodate the inscribed circle diameter. Determine the space feasibility. Is there enough right-of-way to build it? This is a potential rejection point. There is no operational reason to reject a roundabout because of the need for additional right-of-way; however, right-of-way acquisition introduces administrative complications that many agencies would prefer to avoid.
• Step 6: If additional space must be acquired or alternative intersection forms are viable, an economic evaluation may be useful (Section 3.7).

The results of the steps above should be documented to some extent. The level of detail in the documentation will vary among agencies and will generally be influenced by the size and complexity of the roundabout. A roundabout selection study report may include the following elements:

• It may identify the selection category that specifies why a roundabout is the logical choice at this intersection;
• It may identify current or projected traffic control or safety problems at the intersection if the roundabout is proposed as a solution to these problems;
• It may propose a configuration, in terms of number of lanes on each approach;
• It may demonstrate that the proposed configuration can be implemented feasibly and that it will provide adequate capacity on all approaches; and
• It may identify all potential complicating factors, assess their relevance to the location, and identify any mitigation efforts that might be required.

Agencies that require a more complete or formal rationale may also include the following additional considerations:

• It may demonstrate institutional and community support indicating that key institutions (e.g., police, fire department, schools, etc.) and key community leaders have been consulted;
• It may give detailed performance comparisons of the roundabout with alternative control modes;
• It may include an economic analysis, indicating that a roundabout compares favorably with alternative control modes from a benefit-cost perspective; and
• It may include detailed appendices containing traffic volume data, signal, or all-way stop control (AWSC) warrant analysis, etc.

None of these elements should be construed as an absolute requirement for documentation. The above list is presented as a guide to agencies who choose to prepare a roundabout study report.