Experimental Investigation into Speed Performance andConsistency of Urban Roundabouts: an Italian Case Study

The most important variable in roundabout geometric design is speed and its prior selection represents a fundamental step for road designers and traffic engineers. The design speed refers to that of an ideal vehicle on the shortest radius along the fastest allowable path in free-flowing conditions. It should be defined with respect to the geometry of the legs converging into the roundabout in order to ensure that the operating speed, which is conventionally set at the 85th percentile of on-site recorded values in the above mentioned flow conditions, is as close as possible to the selected design speed or, in other words, that design consistency is achieved. This is a key factor in maintaining roundabouts as a safe intersection typology. When consistency is achieved, most of the vehicles (minimum 85%) pass through the roundabout satisfying equilibrium and visibility conditions established by the designer. In this scenario, operating speeds of isolated vehicles are also similar in the circulatory roadway and relative speeds are lower compared to other intersection types. Design and operating speeds were evaluated in an experimental investigation carried out at some urban roundabouts in the Municipality of Cuneo (Italy). Operating speed data were gathered by means of a speed measuring system consisting of laser equipment, while geometric characteristics were derived by an on-site topographical survey. Speed diagrams for turn and through movements were derived in order to study the effects of the geometric characteristics of approach and exit legs. The results obtained focus mainly on possible divergences between predicted design speeds and observed operating speeds. The aim of this study is to determine if design consistency has been achieved and also to establish what kinds of defects might be responsible for any inconsistencies. Field observation highlighted the case of a roundabout characterized by tangent entering legs that generate anomalous speeds, which are greater in the circulatory roadway than in the approach legs. Comparisons with available speed equations are presented and discussed.