Decision environments

There are three somewhat different policy environments in which a decision may be made to construct a roundabout at a specific location. While the same basic analysis tools and concepts apply to all of the environments, the relative importance of the various aspects and observations may differ, as may prior constraints that are imposed at higher policy levels.

A new roadway system: Fewer constraints are generally imposed if the location under consideration is not a part of an existing roadway system. Right-of-way is usually easier to acquire or commit. Other intersection forms also offer viable alternatives to roundabouts. There are generally no field observations of site-specific problems that must be addressed. This situation is more likely to be faced by developers than by public agencies.

The first roundabout in an area: The first roundabout in any geographic area requires an implementing agency to perform due diligence on roundabouts regarding their operational and design aspects, community  impacts, user needs, and public acceptability. On the other hand, a successfully implemented roundabout, especially one that solves a perceived problem, could be an important factor in gaining support for future roundabouts at locations that could take advantage of the potential benefits that roundabouts may offer. Some important considerations for this decision environment include:

• Effort should be directed toward gaining community and institutional support for the selection of a site for the first roundabout in an area. Public acceptance for roundabouts, like any new roadway facility, require agency staff to understand the potential issues and communicate these effectively with the impacted community;
• An extensive justification effort may be necessary to gain the required support;
• A cautious and conservative approach may be appropriate; careful consideration should be given to conditions that suggest that the benefits of a roundabout might not be fully realized. Collecting data on current users of the facility can provide important insights regarding potential issues and design needs;
• A single-lane roundabout in the near-term is more easily understood by most drivers and therefore may have a higher probability of acceptance by the motoring public;
• The choice of design and analysis procedures could set a precedent for future roundabout implementation; therefore, the full range of design and analysis alternatives should be explored in consultation with other operating agencies in the region; and
• After the roundabout is constructed, evaluating its operation and the public response could provide documentation to support future installations.

Retrofit to an existing intersection in an area where roundabouts have already gained acceptance: This environment is one in which a solution to a site-specific problem is being sought. Because drivers are familiar with roundabout operation, a less intensive process may suffice. Double-lane roundabouts could be considered, and the regional design and evaluation procedures should have already been agreed upon. The basic objectives of the selection process in this case are to demonstrate the community impacts and that a roundabout will function properly during the peak period within the capacity limits imposed by the space available; and to decide whether one is the preferred alternative. If the required configuration involves additional right-of-way, a more detailed analysis will probably be necessary, using the methodology described in Chapter 4.

Many agencies that are contemplating the construction of their first roundabout are naturally reluctant to introduce complications, such as double-lane, yield controlled junctions, which are not used elsewhere in their jurisdiction. It is also a common desire to avoid intersection designs that require additional right-of-way, because of the effort and expense involved in right-of-way acquisition. Important questions to be addressed in the planning phase are therefore:

• Will a minimally configured roundabout (i.e., single-lane entrances and circulatory roadway) provide adequate capacity and performance for all users, or will additional lanes be required on some legs or at some future time?
• Can the roundabout be constructed within the existing right-of-way, or will it be necessary to acquire additional space beyond the property lines?
• Can a single-lane roundabout be upgraded in the future to accommodate growth?

If not, a roundabout alternative may require that more rigorous analysis and design be conducted before a decision is made.