Turbo Roundabouts: Design, Capacity, and Comparison With Alternative Types of Roundabouts

Introduction. In turbo roundabouts traffic entry, exit, and circulatory lanes are physically delimited by kerbs (see Fig. 1A). This implies a lowerNnumber of potentially conflicting points (Fig. 1B) between vehicle trajectories than conventional roundabouts (Fortuijn, 2009). By way of an example, in a turbo roundabout with four arms and two circulatory lanes, the number of conflicting points is equal to ten, while in a conventional roundabout with similar geometric characteristics there are 22 conflicting points (see Table 1). In many European countries, turbo roundabouts are utilized more and more. The geometric guidelines have been published in the Netherlands, Slovenia, Germany, Serbia, Slovakia, and Croatia. Several turbo-roundabout layouts may be used: egg, basic turbo, knee, spiral and rotor, stretched-knee, and star. Undoubtedly, the most used is the basic turbo roundabout (Fig. 1A). The typical layout of the central island and circulatory lanes is generally designed through arcs of circumferences with different centers and radii, as shown in Fig. 2 for mini, standard, medium, and large basic turbo roundabouts (CROW, 2008). A lot of design guidelines lay down that the axis connecting the centers C1 and C2 in Fig. 2 must be inclined by around 30 degree to the axis of the minor road. In addition, ellipse arcs (Fig. 1C) or the Archimedean spiral can be used with the purpose for limiting the variation of the centrifugal acceleration around the circulatory lanes. The radii values (Fig. 2) and the lane widths must be selected in such a way that the traffic speed along the turbo roundabout does not exceed 40 km/h. Experimental measures on the Slovenian turbo roundabout of Fig. 1C (Rmax ≈ 25 m) show that instantaneous vehicle speeds at entry are rather moderate (lower than 25 km/h, 15mwell ahead of the yield line) and accelerations are always lower than 2 m/s2 at both entry and circulatory lanes (where they are generally below 1.5 m/s2). Only at exit lanes accelerations prove to be slightly higher than 2 m/s2. As a rule, the following accidents may occur at double-lane roundabouts: failure to yield collisions, crashes for vehicle control loss, rear-end at entry, carriageway run-off, circulating-exiting collisions, and side-by-side collisions. The main difference between turbo roundabouts and conventional layouts lies in the lack of circulating-exiting and side-by-side conflicts, which are avoided with lane dividers, both at entries and on the circulatory lane. Compared to double-lane roundabouts, turbo roundabouts can provide the following potential benefits: • reductions in the number of total potential accidents to between 40% and 50%; • reductions in the number of potential accidents with injuries to between 20% and 30%. Therefore, turbo roundabouts are more appropriate than conventional double-lane roundabouts in the cases requiring a higher safety level, for instance, in urban contexts where the traffic level of pedestrians and two-wheeler riders is very significant (Mauro et al., 2015).