Conversion of a Semi-two Lanes Roundabout into a Turbo-roundabout: A Performance Comparison

In the last years, as regards the functional design of at-grade intersections, near to classic layouts (signal-controlled junctions, roundabouts, etc.) a new solution has born: the “turbo-roundabout”. It is a canalized multilane oval intersection with a non-traversable or partially traversable center island and with a spiral circulatory carriageway. This kind of roundabout is also characterized by a predictable lane use: some direction flows are physically separated by curbs. Several roundabouts with spiraling circular carriageways were built in northern Europe (in particular in Netherlands) and they have further allowed to extend the notable advantages of this functional solution against multilane roundabouts, such as: 1) no lane changing on the circulatory carriageway; 2) no need to yield to traffic flow on more than two lanes; 3) low driving speed along the through movement because of raised lane dividers and, consequently, a high reduction of accident risk. In this paper a careful literature review on turbo-roundabouts is proposed. Furthermore, the Authors examine the potentialities offered by the transformation of an existing semi-two lanes roundabout into a “virtual” configuration of a turbo-roundabout. In particular, they also evaluate and compare the performance parameters in the two configurations by using a microsimulation software. The case study roundabout is placed in the city of Cosenza (Southern Italy) and it is characterized by great problems of congestion during peak hours. Experimental measures of traffic flows (O/D matrixes), critical gaps, queue lengths and approaching and circulating speeds represent input data for calibration procedures. Afterwards, derived calibration parameters are used as input variables for the new configuration of the intersection as a turbo roundabout. The Authors highlight that the conversion of the existing roundabout into a virtual turbo roundabout determine an increase of capacity together with a minimization of the queue lengths.