CONVENTIONAL AND UNCONVENTIONAL ROUNDABOUTS: A REVIEW OF GEOMETRIC FEATURES AND CAPACITY MODELS

Road intersections play a key role in traffic management. Modern roundabouts require entering vehicles to yield to the circulating flow, and have proven highly effective in granting high performance levels to both new and redesigned intersections; therefore, their use has widely spread around the world over the years. The choice of the correct shape, size and signage of a roundabout is essential to accomplish the desired results in terms of performance and safety. In order to achieve these goals, designers have moved away from conventional layouts such as single-lane and multilane roundabouts, conceiving more and more unconventional solutions (spiral, turbo and two-geometry roundabouts). Alongside this design evolution, research has been conducted on functional analysis of roundabouts: several authors and authorities have developed capacity assessment models that are suitable for the analysis of unconventional roundabouts, but nowadays no complete review of such models is available. The aim of this paper is to collect descriptions of the main types of conventional and unconventional roundabouts, with a focus on their geometric features and applicable capacity assessment models.