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In this paper the performance level of basic turbo-roundabouts and double-lane roundabouts are compared. Cowan’s M3\ud headway distribution was chosen to take into account bunched vehicles moving along each circulating lane; the Hagring\ud model was adopted to evaluate entry capacity both at turbo-roundabout and at double-lane roundabout.\ud The comparative analysis was performed evaluating operational performances in terms of delay suffered by users entering the\ud intersection.\ud Results of the analyses show that efficiency of basic turbo-roundabouts highly depend on traffic situations and can be\ud significant when major road captures most of the traffic deman

Starting from assumptions regarding the arrival process of circulating streams and according to models based on the gap-acceptance theory, the paper is aimed at comparing operational performances between basic turbo-roundabouts and double-lane roundabouts. The paper proposes applications of the Hagring model for entry capacity estimations at double-lane roundabouts and turbo-roundabouts, these latter, in particular, featured by movements with only one or two conflicting traffic streams. This model allows to use, in fact, a bunched exponential distribution to quantify the distribution of major vehicle headways; it also considers specific values different by each lane for behavioural parameters, minimum headway and conflicting traffic flow on circulating lanes.The results obtained for the two cases examined, although influenced by the underlying assumptions, especially with regard to user behaviour at turbo-roundabouts, can give information about the convenience in choosing, at a design level, a basic turbo-roundabout rather than a double-lane roundabout. The comparison developed in this paper, indeed, can be helpful in selecting the type of roundabout and in particular in evaluating performance benefits that are obtainable from the conversion of an existing double-lane roundabout to a turbo-roundabout with similar footprint of space.Keywords: turbo-roundabout, traditional roundabout, operating performaces Introducation Introducing the ProblemTurbo-roundabouts represent a new type of circular intersection which were designed to improve safety performances at modern roundabouts, already widely spread in the world, without compromising their efficiency. The turbo-roundabout is a specific kind of spiralling roundabout developed in The Netherlands by Fortuijn in the late 1990's. Fortuijn developed turbo-roundabouts in an attempt to deal with the drawbacks of double-lane roundabouts: while double-lane roundabouts have a higher capacity than single-lane roundabouts, they have the disadvantage of a higher driving speed through the roundabout and lane changing on the ring, hence raising the crash risk. Turbo-roundabouts were, indeed, introduced to deal with the entering and exiting conflicts occurring at double-lane roundabouts; these conflicts are eliminated at turbo-roundabouts by directing drivers to the correct lanes before entering the intersection and introducing spiral lines that guide them to the correct exit. On design principles and geometric elements of a turbo roundabout, as well as different variants of the turbo-roundabout progressively introduced in The Netherlands, can be seen e.g. Fortuijn (2009a). Other European experiences with turbo-roundabouts are referred by Brilon (2008) and Tollazzi et al. (2001).An exhaustive evaluation of safety performances at turbo-roundabouts is not yet available because turbo-roundabout installations are still recent. It follows that the design choice between a standard double-lane roundabout or a basic turbo-roundabout can be carried out through convenience evaluatio...

Due to its geometric design, turbo-roundabouts impose greatest constraints to the vehicular trajectories; by consequence, one can expect a more unfavourable impact of heavy vehicles on the traffic conditions than on other types of roundabouts. The present paper addresses the question of how to estimate Passenger Car Equivalents (PCEs) for heavy vehicles driving turbo-roundabouts. The microsimulation approach used revealed as a useful tool for evaluating the variation of quality of traffic in presence of mixed fleets (different percentages of heavy vehicles). Based on the output of multiple runs of several scenarios simulation, capacity functions for each entry lane of the turbo-roundabout were developed and variability of the PCEs for heavy vehicles were calculated by comparing results for a fleet of passenger cars only with those of the mixed fleet scenarios. Results show a dependence of PCEs for heavy vehicles on operational conditions, which characterise the turbo-roundabout. Assuming the values of PCEs for roundabouts provided by the 2010 Highway Capacity Manual (HCM), depending on entering manoeuvring underestimation and overestimation of the effect of heavy vehicles on the quality of traffic conditions have been found.

Starting from consideration that urban intersections are sites with promise for safety and operational improvements, the paper describes the steps taken to develop a crash predictive model for estimating the safety performance of urban unsignalized intersections located in Palermo, Italy. The focus is on unsignalized four-legged one-way intersections widespread in Italian downtowns. The sample considered in the study consist of 92 intersections in Palermo, Italy. For the study were collected crashes occurred in the sites during the years 2006-2012, geometric design and functional characteristics and traffi c fl ow. Results showed that data were overdispersed and NB1 distributed. In order to account for the correlation within responses Generalized Estimating Equations (GEE) were used under different working correlation matrices.