This paper presents a comparative analysis of the traffic performances of turbo-roundabouts and roundabouts in terms of capacity and delays. We chose to use a recent German procedure for roundabouts included in “Kreisel” software as our calculation procedure to determine the capacity of turbo-roundabouts. We found that the capacities of turbo-roundabout secondary entries were higher than roundabout capacities when the traffic flow in the inner lane of the circle is high and the traffic flow in the outer lane of the circle is in the low to lower-middle range. On the contrary, the capacities of the main entries to roundabouts are always higher than the capacities of the main entries to turbo-roundabouts. Taking into account the arithmetic or weighted mean behavior of the four entries, the total capacities of turbo-roundabouts, at both saturated and undersaturated conditions, are, in most cases, higher than the total capacities of roundabouts. In most of the important practical situations analyzed, delays we...

Comparative Analysis of Four New Alternative Types of Roundabouts: "Turbo", "Flower", "Target" and "Four-Flyover" Roundabout

Turbo roundabouts are a particular road intersection layout, designed to increase the safety of double-lane roundabouts, while maintaining their excellent capacity. The main feature of this new concept of roundabout is the impossibility to move from one lane to another, provided by physical barriers marking the lanes. The paper shows an application to turbo roundabouts of a potential accident rate model, aiming to evaluate their safety improvement. Themodel is based on the concept of potential conflict: each vehicle involved in a general intersection performs a series of maneuvers which potentially imply a crash, according to the actual traffic. The number of accidents related to each critical maneuver is proportional to the number of times this maneuver occurs at the intersection. In order to define the critical maneuvers, and hence the relevant potential conflicts, specific crash typologies for roundabouts are adopted. Traffic volumes are required, to evaluate the expected number of accidents, and also probabilities of accident for every critical maneuver. These ratios were obtained by a model calibration, based on actual accident and traffic data recorded on conventional single and double-lane roundabouts. The model was then used to compare four-leg turbo roundabouts to conventional roundabouts. The comparisons have taken into account only basic differences in layout, such as geometric elements, that also play a role in determining safety performances of a roundabout. The results obtained show that turbo roundabouts significantly decrease the accident rate with respect to conventional roundabouts, by eliminating conflicts between circulating and exiting vehicles.

The study aims to develop a support procedure to estimate the efficacy of infrastructural interventions to improve road safety. The study was carried out on a 110 km stretch of the A3 highway, in southern Italy. Data from a huge sample concerning traffic, geometry and accidents for two periods of the same duration were compared, for which cluster analysis, and in particular, the “hard c means” binary partition algorithm was employed. Using cluster analysis, all the accidents with strong similarities were aggregated. Then for each cluster, the “cluster representative” accident was identified, to find the average among the various characteristics (geometrical, environmental, accident-related). A “hazard index” was also created for each cluster, whereby it was possible to establish the danger level for each “cluster”. Using this information, an accident prediction model using a multi-variate analysis was produced. This model was used as a support for decision-making on infrastructures and to simulate situations to which the Before-After technique could be applied.