1. Description

The first roundabout in history is apparently the Circus in Bath ^[1], but modern roundabouts with the priority rule were first made mandatory in the United Kingdom in 1966. Roundabouts of this type have been widely adopted because they provide a significant reduction in the number of accidents with injuries and fatalities when compared with intersections ^[2][3] and old traffic circles where the vehicles entering had the priority. Modern roundabouts provide more safety and lower delay than signaled intersections with traffic lights ^[4]. Although different improvements in the design of roundabouts have been adopted, including turbo-roundabouts ^[5] and the so-called "magic roundabout" ^[6], the basic rule of absolute priority to the vehicles inside the roundabout has been kept because it provides increased safety and prevents blocking in most circumstances (not all). Surprisingly, while roundabouts’ physical design has improved over the decades, their signaling and control have not evolved accordingly. Nowadays, vehicular networks appear to be an important tool to improve the safety and capacity of roads ^[7][8]. However, most proposals for more efficient roundabouts and intersections are based on full driving automation and do not apply to human-driven vehicles, making the transition problematic ^[9]. The motivation of this paper is as follows: the capacity of roundabouts is currently limited by the drivers (gap acceptance), more specifically the duration of the critical headway (gap) and the follow-up headway ^[10]. The critical and follow-up headways’ value is high due to the frequent stops and speed changes at the roundabout entrance caused by the lack of priority of the incoming traffic and the internal conflicts between vehicles willing to leave the roundabout and vehicles continuing their traversal. The basic idea is that by eliminating conflicts via uniform speeds and synchronized platoons, safety and capacity will increase significantly. Our approach looks at roundabouts as synchronous, time-division switches or multiplexers of vehicles, although taking into account the non-negligible transmission (displacement) delays inside the roundabout.

Our proposal's basis is that by making the traffic regular and smooth at accesses, with constant speeds and controlled delays, and isolating the traffic from every access at a separate rotating sector, the conflict points at the entries and inside the roundabout are eliminated, and the subsequent stops and deceleration, while capacity and safety increase. The vehicles' priority inside the roundabout is no longer permanent, and it is dynamically assigned to rotating sectors of variable duration, each one associated with a different entry.

1.1. Working Principle

The vehicles from each roundabout access are grouped into platoons to maximize the lane's occupancy and avoid stops in the roundabout, such that the arrival times of the platoons of the two perpendicular
directions are staggered to avoid access conflicts. The capacity is optimized by the just-in-time, centrally controlled, platoons' arrival to the roundabout, while the synchronization of arrivals is based on the staggering of orthogonal platoons and platoon formation and control. Maximum smoothness is obtained by equating the vehicle circulation speed at accesses with the roundabout's linear speed.

The system uses vertical visual signaling in the form of circular light sectors (and optionally collocated traffic lights that confirm the information) to indicate the status of access priorities. The platoons of vehicles are formed and compacted using moving illuminated signs in front of the platoon head,  platoon located on the entrances' sides and transversely on the entrance floor or by other procedures, including wireless signaling for connected/autonomous vehicles. These platoon head signs (e.g., North and South access) approach the roundabout at a constant speed, synchronizing the entrance to the roundabout of the platoons of vehicles of each two accesses from opposite directions. The perpendicular direction signs (e.g., East and West access) have a spatial and temporal offset adjusted to match vehicles' arrival from the pair of accesses to the roundabout with the start of their access priority, avoiding most stops at the roundabout entrance. Besides, stop lines' placement at a certain distance before the roundabout allows the vehicles to enter the roundabout at the same speed as the rotating priority sector.

2. Conclusion

Roundabouts with rotating priorities open a new path for research  on roundabout evolution incorporating most technologies (sensors, V2X, IoT, scene interpretation, Intelligent Infrastructures, Smartphones, CAVs) preserving compatibility with human-driven vehicles. The analytical evaluation and SUMO simulations show lower delay and minimal delay variations, and higher capacities than conventional roundabouts. By eliminating the conflict points and subsequent speed changes, safety is also improved through driving smoothness. Drivers do not have to find a suitable gap and accelerate to enter, they are only required to enter the roundabout in the lane corresponding to their exit and follow the platoon or head of the platoon signal and to signaling sequences adapt dynamically to traffic unbalances and daily fluctuations and may alternate between a standard operation or synchronous mode with heavy traffic. Low delay variation will probably provide high Levels of Service (LOS). Adopting SYROPS roundabouts is not an easy task: as it happened with current roundabouts, the synchronous rotating priorities roundabout concept requires a learning period by the vehicle drivers and also a signaling design well adapted to human perception. Although the constant speed approach simplifies roundabout crossing, the perceptual aspects must be evaluated together with the traffic dynamics. Simulations and virtual reality platforms offer an excellent set of tools for it.