Citation

Anna G, Tullio G. Transp. Res. Circular 2005; 2005(E-C083): 9p.

Copyright

(Copyright © 2005, U.S. National Academy of Sciences Transportation Research Board)

DOI

unavailable

PMID

unavailable

Abstract

The roundabout installations in urban areas are often conditioned by the existing constraints of different kind (particularly physical and topographical ones), compelling road engineers to make “compromise solutions” compared with the traditional geometric design standards of roundabouts; operational performances and risk conditions, in their turn, can be quite different from those known for modern roundabouts. By the way, similarly to the wide range of local situations, a wide range of geometric layouts can be recognized in the existing not-conforming roundabouts; in these installation operational conditions still maintain some typical roundabout operating characteristics (first of all, traffic along the circulatory roadway), but, on the other hand (for example, for some movements), look like those typical of stop-controlled intersections. The peculiarity of this kind of circular intersections, referred to as “Roundabout Inspired Intersections” (RII) (Granà and Giuffrè, 2004), makes very complex the conceptual formulation of intersection operations. It also increases the uncertainty to evaluate performances of the infrastructural organization, either in terms of efficiency of operational conditions or in regard to road safety. Starting from these considerations, in a previous research (Granà, 2002) the opportunity to define for the subject intersections suitable risk indexes was considered. Moreover, the general goal of this study is to explain and to value traffic operations and driver behaviours at not-conforming roundabouts, as above specified (RII). For this purpose, three real case studies in Palermo City (corresponding to the same number of existing RI intersections), different for geometric layout and for type of give-way control (old priority rule: give way to entering vehicles and off-side priority: give way to circulating vehicles), have been examined. The methodological approach assumes that operational performances at RI Intersections are intermediate between roundabouts and stop-controlled intersections and that methodologies suited to latter types of intersections, applying them to the single movement passing through the intersections, can be used in the analysis of RI Intersections. Although results are not yet generalizable because of the little number of observations, they underline the capability of the suggested methodology to analyze operational conditions and to evaluate performances of atypical not-conforming schemes of intersections, for which a large range of cases can rise depending on traffic demand, geometrical configuration of the intersection and traffic control.