Since the 1980s, the modern single-lane roundabout has been applied in the Dutch road network. The roundabout offered major safety benefits compared to the conventional signalized intersection, however, it also required more space. Influenced by society’s growing mobility, the capacity requirements to intersections increased. This quest for capacity gave rise to the concentric multi-lane roundabout: a roundabout with two or more circulatory lanes and one or more lanes on the entrance and exit legs. The concentric multi-lane roundabout offered a higher capacity, but compromised the safety benefits of the single-lane roundabout. As an answer to the safety problems on the multilane concentric roundabout, Fortuijn developed the turbo roundabout in 1996. The turbo roundabout offers the same safety level as the single-lane roundabout and a higher capacity. Nowadays, the quest for a higher capacity continues unabated. This graduation project aims to contribute to the quest by investigating the combination of turbo roundabouts and traffic lights. The central question is whether traffic signals enhance the capacity of the turbo roundabout. The two largest turbo roundabout types have been investigated: the rotor roundabout (four-leg) and the star roundabout (three-leg). Three models (the MSRV, an analytical approach and VISSIM) have been applied to evaluate the capacity of the rotor and star roundabout (signalized and un-signalized). In addition, this project states proposals for future research into other performance indicators than the capacity; namely space requirement, delay time, robustness, traffic safety and environmental issues. This research shows that the signalized turbo roundabout only allows for a leg-by-leg control. However, the lane configuration on the rotor and star roundabout can be adjusted such, that the turbo roundabout is suitable for the more efficient two-phase control, which is currently being applied to the turbo circle. The results are the partial turbo circle (adjusted rotor roundabout) and the three-leg turbo circle (adjusted star roundabout). The latter intersection types require more space than the rotor and star roundabouts. The capacity evaluation shows that the partial turbo circle has no applicability range, because the capacity for left-turning traffic streams is very limited. The three-leg turbo circle has potential, because it offers a higher capacity than the star roundabout. The geometrical design of the signalized turbo roundabout allows for phase overlap within the leg-by-leg control. Thanks to the phase overlap, two subsequent legs can obtain green simultaneously for a period in the order of 1 to 8 seconds. Consequentially, the aggregated green time within one hour can be extended compared to the leg-by-leg control without any phase overlap, enhancing the capacity of the turbo roundabout. The capacity of the signalized turbo roundabout has been evaluated analytically by means of the analytical approach, which was developed by Fortuijn simultaneously with the development of the turbo roundabout. The analytical approach calculates the capacity by multiplying the green time per hour with the saturation flow. The capacity of the signalized rotor roundabout ranges from 2800 pcu/h to 3700 pcu/h; the capacity of the signalized star roundabout ranges from 4400 pcu/h to 5200 pcu/h, depending on the demand pattern. Using the green phase overlap, the rotor roundabout may offer 5% to 6% more capacity; the star roundabout may offer 7% to 9% more capacity. Moreover, the microscopic simulation tool VISSIM has been applied to evaluate the capacity of and delay time on the signalized turbo roundabout and the (partial and three-leg) turbo circle. The capacity results of the analytical approach and VISSIM do not differ much, except for the results for the star roundabout with an inscribed radius of 20 m. No explanation has been found. In order to compare the capacities of the signalized and the un-signalized turbo roundabouts, the capacity of the un-signalized turbo roundabout has been evaluated by means of the Meerstrooksrotondeverkenner (Multilane roundabout explorer). The un-signalized rotor roundabout offers a capacity ranging from 2900 pcu/h to 4500 pcu/h. The signalized rotor roundabout offers a lower capacity; this intersection type is not recommendatory for application. The same conclusion holds for the partial turbo circle, however, the partial turbo circle offers a higher capacity in situations with low left-turning intensities (in the range of 20% of the straight-ahead intensities). The turbo circle offers a much higher capacity than the (un-)signalized rotor roundabout (ranging from 4700 pcu/h to 9300 pcu/h, depending on diameter and demand pattern), but requires more space. With regard to the three-leg intersections: the capacity of the signalized star roundabout (ranging from 4400 pcu/h to 5200 pcu/h) is larger than the capacity of the un-signalized star roundabout (ranging from 3700 to 5100 pcu/h). However, the signalized star roundabout induces longer delay times (nearly twice as long), and probably less traffic safety and higher environmental burdens. The three-leg turbo circle offers an even higher capacity than the signalized star roundabout (ranging from 5400 pcu/h to 7100 pcu/h), but requires more space. From this study, it can be recommended to pay further attention to the signalized star roundabout and the smaller three-leg turbo roundabouts (both in scientific research as well as in practical engineering), because the signalized star roundabout has a larger capacity than the un-signalized star roundabout. The signalized rotor roundabout and the partial turbo circle are not recommendatory for further research, because the un-signalized rotor roundabout offers a higher capacity. (Author/publisher)
Abstract