Site-specific conditions

Some conditions may preclude a roundabout at a specific location. Certain site-related factors may significantly influence the design and require a more detailed investigation of some aspects of the design or operation. A number of these factors (many of which are valid for any intersection type) are listed below:

• Physical or geometric complications that make it impossible or uneconomical to construct a roundabout. These could include right-of-way limitations, utility conflicts, drainage problems, etc.
• Proximity of generators of significant traffic that might have difficulty negotiating the roundabout, such as high volumes of oversized trucks.
• Proximity of other traffic control devices that would require preemption, such as railroad tracks, drawbridges, etc.
• Proximity of bottlenecks that would routinely back up traffic into the roundabout, such as over-capacity signals, freeway entrance ramps, etc. The successful operation of a roundabout depends on unimpeded flow on the circulatory roadway. If traffic on the circulatory roadway comes to a halt, momentary intersection gridlock can occur. In comparison, other control types may continue to serve some movements under these circumstances.
• Problems of grades or unfavorable topography that may limit visibility or complicate construction.
• Intersections of a major arterial and a minor arterial or local road where an unacceptable delay to the major road could be created. Roundabouts delay and deflect all traffic entering the intersection and could introduce excessive delay or speed inconsistencies to flow on the major arterial.
• Heavy pedestrian or bicycle movements in conflict with high traffic volumes. (These conflicts pose a problem for all types of traffic control. There is very little experience on this topic in the U.S., mostly due to a lack of existing roundabout sites with heavy intermodal conflicts).
• Intersections located on arterial streets within a coordinated signal network. In these situations, the level of service on the arterial might be better with a signalized intersection incorporated into the system. Chapter 8 deals with system considerations for roundabouts.

The existence of one or more of these conditions does not necessarily preclude the installation of a roundabout. Roundabouts have, in fact, been built at locations that exhibit nearly all of the conditions listed above. Such factors may be resolved in several ways:

• They may be determined to be insignificant at the specific site;
• They may be resolved by operational modeling or specific design features that indicate that no significant problems will be created;
• They may be resolved through coordination with and support from other agencies, such as the local fire department; and
• In some cases, specific mitigation actions may be required.

All complicating factors should be resolved prior to the choice of a roundabout as the preferred intersection alternative.

The effect of a particular factor will often depend on the degree to which roundabouts have been implemented in the region. Some conditions would not be expected to pose problems in areas where roundabouts are an established form of control that is accepted by the public. On the other hand, some conditions, such as heavy pedestrian volumes, might suggest that the installation of a roundabout be deferred until this control mode has demonstrated regional acceptance. Most agencies have an understandable reluctance to introduce complications at their first roundabout.