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A turbo roundabout is a new type of canalized multilane intersection in which the physical separation between lanes helps to\ud prevent side collisions when crossing the roundabout. This paper presents an estimation of capacity, delays and level of service\ud of basic turbo roundabouts in undersaturation conditions, considering both vehicle flow and pedestrian traffic. The traffic\ud performance model was developed by evaluating the capacity for each entry lane. Owing to the geometric features of the\ud intersection, the total entry capacity is obtained by considering different values of the pedestrian impedance factor and degree\ud of saturation at both the right-turn and left-turn lanes

Abstract:The functional performances of conventional roundabouts (single-lane and multi-lane) and innovative roundabouts (spiral, flower, C and turbo) can be improved through right-turn bypass lanes controlled by stop, yield or free-flow signs. The article presents evaluations of the emissions of air pollutants (carbon dioxide, nitrogen oxides, particle pollution (PM10 and PM2.5)), fuel consumption and construction, management, energetic and environmental costs in roundabouts without or with bypass lanes (controlled by stop, yield or free-flow). The suggested methodology has a general character and can be applied as a multi-parametric criterion for choosing road intersections, although, in the present paper, it has been employed only for a case study. For the aims of this research, we employed recent closed-form formulations to determine roundabout performances; moreover, we used the COPERT IV ® software to estimate air emissions in nine different types of vehicles. Numerous traffic simulations were carried out. The variation in the maximum hourly traffic Qmax and annual traffic QTOT provided the appropriate domains of the examined geometric layouts, both in functional and environmental terms and with regard to generalized costs, estimated for a 10-year period. It resulted that the introduction of right-turn bypasses in all arms of conventional roundabouts with a one ring lane and one lane at the entries (single-lane roundabouts) is the most cost-effective when the flows entering the roundabout are higher than Qmax = 2000 veh/h. Moreover, free-flow bypass lanes always OPEN ACCESSSustainability 2015, 7 5839 provide greater capacity and lower delays than stop-or yield-signaled bypasses. However, with extremely high Qmax values, stop-controlled bypasses guarantee lower fuel consumption, while those with a yield sign lower total costs.

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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.