The continuous evolution of automotive and communication technologies can result in more efficient traffic management. At roundabouts, Autonomous and Connected Vehicles (CAVs) have the potential to ...enhance traffic operations, minimizing lost time and providing higher capacities by improving the gap acceptance process.
This research aims to take advantage of those evolving technologies in order to design a more efficient traffic management system for single-lane roundabouts. Such a system would provide optimal coordination of CAVs to negotiate the roundabout, maximizing throughput and minimizing average control delay.
The expert system developed and tested (Roundabout Manager) acts as a central controller and manages conflicts within the roundabout. It prioritizes incoming vehicles based on demand-dependent strategies and adjusts their trajectories accounting for the geometry of the roundabout. Seven strategies are proposed to coordinate CAV trajectories. The first strategy prioritizes incoming vehicles based on their Shortest-Remaining-Time-First (SRTF, i.e., the shortest time till the vehicle reaches the first conflict section it will encounter) to the conflict sections within the circular roadway, while the other strategies add to the SRTF rules to accommodate highly unbalanced traffic flows, with heavy right- or left-turning maneuvers.
To illustrate the operation of the algorithm and assess its effectiveness as a function of the demand distribution, a total of 15 oversaturated demand scenarios with 10 replications per scenario were simulated for each of the seven strategies examined. It was concluded that, under any of the strategies proposed, the system guarantees higher throughput with lower average control delay compared to conventional vehicles’ operation. The capacity of the roundabout under fully CAVs traffic flow is increased by 58 to 73% (from low to high conflicting flow, respectively). The strategy that prioritizes vehicles that have to travel through more conflict points and those that leave the system first outperforms the other strategies at all demand scenarios, becoming the optimal coordination of CAVs at roundabouts. This strategy reduces the average control delay between 80 and 97% compared to traffic with conventional vehicles, as assessed using the Highway Capacity Manual procedures.
This paper evaluates the potential impacts of various geometric and operational parameters on gap-acceptance behavior for approaches to roundabouts in the metropolitan area of Atlanta, Georgia. ...Twelve roundabouts were selected to provide a range of different conditions such as number of legs, number of circulating lanes, conflicting volumes, presence of pedestrian crossings, and so forth. Observational data were collected at these roundabouts using a remotely operated drone equipped with a stabilized high-resolution (4K) video camera operating at a sufficient altitude (just less than 400 ft/120 m) to encompass the complete facility within a single frame. The resulting videos were processed using commercial machine vision analysis supplemented by additional computer-assisted analysis to determine vehicle trajectories, spacings, and potential conflicts on a frame-by-frame (0.033 s) basis. These data, in turn, were used to establish gap-acceptance behavior of each roundabout approach and, through the use of logistic regression on the data, to establish observed critical headways (defined as the logistic inflection point t50). The variability of observed headways was compared against known parameters of both the roundabout and of the specific approach to develop a descriptive/predictive model as to how critical headways and gap-acceptance behavior were affected by variations in these parameters. Several factors were observed to have significant impacts on critical headways, including geometric (size category of the roundabout, number of legs, and visual angle to the upstream approach), environmental (presence of a state route on the upstream approach, presence of additional conflicting lanes, presence of a pedestrian crosswalk on the approach), and operational (approach speeds) factors.
A recent research project investigated the operational benefits of modern roundabouts for high-speed locations that serve heavy vehicles, while developing examples of best practices for design. The ...research team compiled proven results from designs in other states and collected and analyzed data from within and outside of Texas to develop updated design guidance, which can be used to implement roundabout designs that accommodate oversize/overweight (OSOW) vehicles in rural areas. This paper summarizes activities and findings from that project. Project activities indicate that the drone-based data collection effort, along with a computer algorithm-based method for data reduction, was an effective and efficient alternative to traditional data collection and reduction methods for this project. Findings from field data and simulation confirmed practices in other states that roundabouts can be designed to accommodate not only heavy vehicles but also OSOW vehicles in rural locations with high-speed approaches. The roundabout with the largest diameter studied (180 ft) had the best performance, but the smallest roundabout (120-ft diameter) was also able to accommodate larger vehicles at lower volumes, and roundabouts of all sizes generally outperformed two-way stop-controlled intersections in simulation results.
Over recent decades, roundabouts have become increasingly used when building new at-grade intersections or up-grade junctions all over the world. Consequently, control of traffic flows at at-grade ...intersections and up-grade junctions using roundabouts creates unique design problems. Nowadays, ‘alternative’ types of roundabouts have started to become very popular, especially because of their advantages compared with ‘standard’ roundabouts and standard types of up-grade junctions. Some of these alternative types of roundabouts are two-level roundabouts, which are still currently in the development phase. It is for this reason that they can be called ‘theoretical roundabouts’. Two-level roundabouts are particularly useful in urban and suburban areas with space limitations due to their relatively small footprint. This paper illustrates three new alternative types of two-level roundabouts—‘target’, ‘four flyover’, and ‘roundabout with left and right bypasses’—as well as their functions, capacities, and traffic safety characteristics.
This paper examined driving behavior, vehicle operational performance, and exhaust emissions data at a roundabout level. The relative positive acceleration, power demand, speed, acceleration, ...vehicular jerk, Vehicle Specific Power modes, mass carbon dioxide and nitrogen oxide emissions were evaluated by roundabout type (single-lane, compact two-lane, and multi-lane) and sub-segment (downstream, circulating area, and upstream). Real-world vehicle tailpipe emissions, internally observable variables, dynamic and location data were measured along six conventional roundabouts in Portugal (1 Hz resolution). Although the multilane roundabout yielded the lowest amounts of carbon dioxide, it resulted in highly volatile behaviors based on vehicular jerk thresholds. The emission hotspots were found downstream roundabouts while drivers tend to adopt highly volatile behaviors close to the circulatory roadway. This paper brings valuable knowledge to researchers and transportation practitioners, who are involved in intersection design and control strategies, about the impacts of roundabout design and location based on several criteria.
Until early 2000s, modern roundabouts (RABs) were poorly understood in Japan and were not fully recognized as a form of at-grade intersections. The RAB research projects of the International ...Association of Traffic and Safety Sciences (IATSS), which began in 2009, started with examining the applicability of RABs in Japan and have steadily progressed to various stages of field experiments, social implementation, and dissemination promotion activities. These RAB research projects, which lasted for approximately seven years, are considered to have played a significant role in the practical application of RAB in Japan. This paper provides a chronological overview of the history, content, and development status of these projects and records and discusses the role that IATSS research projects played in the early days of RABs in Japan.
•Triggers for breakthroughs at modern roundabouts in Japan•Chronological overview of the history and content of the IATSS roundabout projects•Archive from applicability examinations and field experiments to practical deployment.
Conventional intersections that have been replaced by multi-lane roundabouts have experienced a substantial reduction in both injuries and fatal crashes. Property-damage-only (PDO) crashes have also ...been reduced at some of these roundabouts. However, some multi-lane roundabouts have experienced a significant increase in PDO crashes. The question arises as to why some U.S. multi-lane roundabouts have substantially higher numbers of PDO crashes while other outwardly similar roundabouts do not. In this paper, we examine the hypothesis that the cause of the higher numbers of PDO crashes at some U.S. multi-lane roundabouts is driver confusion, owing to the lack of adherence to horizontal geometric safety design principles related to entry angles. We review research and explore three case studies where adherence to the safety principles related to entry angles—Phi and left view angle—contributed to improved safety.
A growing number of studies, presented in scientific and professional literature, point out a poor traffic safety characteristic of "standard" two-lane roundabouts, and lower capacity than expected. ...These problems are resolved in different ways in different countries; however, the most successful solution has proven to be reducing the number of conflict points. Lower number of conflict points is one of the characteristics of alternative (or unconventional or no-widespread) types of roundabouts. Alternative types of roundabout differ from "standard" two-lane roundabouts in one (or more) design elements, while the purpose of their implementation is also specific. This paper illustrates two established alternative types of roundabouts (turbo and flower roundabout), and two alternative types of roundabouts in development phase (reduced-turbo and semi-turbo roundabout), offering their detailed functional description, and comparison of their capacity and traffic safety characteristics. Comparative analyses of turbo, reduced-turbo, flower and semi-turbo roundabouts was made by evaluation approach based on simulation of traffic operating at four types of alternative design layouts, including exact geometric layout of the traffic site and the precise representation of traffic flows, with turning movements, through origin-destination matrixes. The capacity comparison was conducted by a software tool VISSIM, while the traffic safety comparison was made by a software tool SSAM. In traffic safety analyses, microsimulation was used to simulate traffic operations at various levels of traffic volume. Performance measures were obtained, including measures of traffic safety, based on conflicts estimated from trajectories generated in microsimulation. According to the results, level of traffic safety (as well as capacity) of analysed alternative types of roundabouts depends on traffic flow strength, and on numbers of right-hand and left-hand turning vehicles. Consequently, for different circumstances, there are different optimal alternative types of roundabouts.
•A criterion for comparing conventional with innovative one- or two-level roundabout intersections is presented.•We applied closed-form capacity models and CORINAIR methodology.•Annual delays, and ...CO2, NOx, PM2.5 and PM10 pollutant emissions show different distributions for various types of roundabouts.•Estimations of environmental and overall costs for 8 types of roundabouts are evaluated.•We identified the roundabout type which best suits each traffic condition.
The article describes a criterion based on functional, environmental and economic aspects for comparing conventional roundabouts with innovative one- or two-level roundabouts. We compared the performances of eight roundabout types, differing in geometric layout, number of lanes and traffic flow regulation from each other, with regard to vehicle delays and CO2, NOx, PM2.5 and PM10 pollutant emissions. Recently-designed roundabouts – target roundabouts and flyover roundabouts – have also been studied for their undoubted practical interest. By means of closed-form capacity models and CORINAIR methodology, several traffic simulations were carried out to examine a typical annual traffic demand curve in a suburban context, three different distribution test matrices for traffic flows (ρ1, ρ2, ρ3) and maximum annual traffic flow values Qmax ranging between 1300 and 3300veh/h.
Estimating vehicle delays and annual pollutant emissions, along with construction and management costs, allowed obtaining overall costs for each roundabout examined, in function of traffic demand and several other parameters. Thanks to these analyses, we identified the roundabout types which best suit to each traffic condition.
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