Modern Roundabouts Research Articles

Contribution of the IATSS research projects to disseminating modern roundabouts in Japan

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.

Speed Reduction Capabilities of Two-Geometry Roundabouts

Several types of modern roundabouts are alternatives to standard ones. They are either in use or at the development stage today. One such intersection is the two-geometry roundabout. Its circulatory roadway has an elliptical outer edge and a circular inner edge that is defined by a circular central island resulting in variable circulatory roadway width. The investigation presented in this paper aims to determine the influence of this variable width on the design of other geometric elements and its impact on roundabouts’ speed reduction capabilities. There is not enough experimental data collected to make a comparison to other roundabout types, so this investigation is based on computer simulations and speed estimations. The investigation is conducted on 40 four-legged single-lane roundabout schemes. These were designed in the Autodesk AutoCAD 2021 software through computer simulations of vehicle movement and the resulting swept paths of a tractor with a semi-trailer generated by the Autodesk Vehicle Tracking 2020 software. The results show that truck aprons must be included in the design of two-geometry roundabouts with a major axis between 18 and 25 m to achieve appropriate circulatory roadway widths, personal car path deflection, and the resulting relative speed and speed consistency.

Network-Wide Implementation of Roundabouts Versus Signalized Intersections on Urban Streets: Analytical and Simulation Comparison

This paper examines the impact of roundabouts implemented at intersections throughout a dense urban network on its operational performance. Metrics considered include the average free-flow speed, flow-moving capacity, trip-serving capacity, and fuel consumption rate. Three intersection strategies are compared: signalized intersections allowing left turns in a permitted manner (TWs), signalized intersections prohibiting left turns (TWLs), and modern roundabouts (RBs). Using the approaches of macroscopic fundamental diagrams and network exit functions, both analytical investigations and microscopic traffic simulations for grid networks were conducted. In general, the results from both analyses agree well. The results reveal that when single-lane roundabouts are applied in networks with a single travel in each direction, the RB network outperforms the TW network for all operational metrics. The RB network also outperforms the TWL network in free-flow speed and flow-moving capacity and has a similar trip-serving capacity as the TWL network. However, when roundabouts with two travel lanes are applied on multi-lane networks, the TWL network exceeds the RB network in both flow-moving and trip-serving capacities. This decrease in the performance of the RB network could possibly come from the complexity imposed on the entering vehicle that wants to use the inner lane. Moreover, because vehicles in the RB network need to accelerate/decelerate more frequently those in the other networks, the RB network generates a higher fuel consumption rate in uncongested and capacity conditions. The findings suggest intersections of roundabouts could be beneficial for networks with a single travel lane in each direction.

Investigation of factors affecting turn signal usage at modern roundabouts in State of Qatar

ABSTRACT Drivers are expected to use turn signals to indicate other drivers of their intentions of taking turns, lane changing, merging or diverging indicating safe driving behaviour. This study investigated the rates of turn signal usage at multilane urban roundabouts along with factors affecting the usage. The results from data collected from four roundabouts in Doha city showed that the drivers have higher tendency of using turn signals while exiting the roundabout rather than while entering the roundabout. Further, turn signal usage at entry and exit of the roundabouts was affected by turn type, roundabout type, correct lane, vehicle type, age, type of clothes and distraction. Additionally, using correct lane affected turn signal usage only while exiting the roundabouts. The binary logistic regression models showed that the drivers wearing formal clothes, taking left or U-turn, using correct lane, driving without distraction, and traversing a roundabout located in recreation and commercial land use had higher odds of turn signal usage than their counterparts. The lower turn signal usage rates indicate that more emphasis should be provided on educating drivers highlighting safety implications. Further, traffic engineers can suggest additional strategies to improve turn signal usage based on the results. These findings can be applicable to other GCC countries exhibiting similar characteristics.

Operational Performance of Roundabouts at High-Speed and Rural Locations

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.

Comparing traffic performances between signalised and give-way roundabouts: a case study in Bursa

Globally developing economies and opportunities cause an increase in the density of people in city centers, thus an extraordinary increase in the number of motor vehicles. The increase in the number of motor vehicles complicates the creation of a sustainable traffic network. Waiting times and the number of stops cause psychological, physical and environmental problems. The efficiency of intersections is vital to ensure sustainable transportation. Modern roundabouts outperform signalized roundabouts, and their popularity has been increasing in recent years. However, the geometric features of the intersections should be suitable for the location and traffic composition. In this study, Durmazlar roundabout, which is currently a signalized roundabout in Bursa, has been transformed into a modern roundabout and redesigned. One of the aims of the study is to make minimal changes in the geometry of the roundabout. One-way road applications have been made to regulate entrances and exits on problematic roads. Modeling of the roundabout and collecting result data was done with the PTV Vissim simulation program. Queue length, travel time, and speed parameters of the data obtained regarding the new scenario and the current situation were compared.

A Novel Modeling and Scheduling Method for Modern Roundabouts

This paper investigates optimal traffic flow scheduling for signalized roundabouts. First, a novel model for roundabouts is introduced based on the discretization of traffic flow. Then, the signal control problem for signalized roundabouts is formulated and further converted into a Markov Decision Process (MDP)-based optimization problem. Some fundamental properties about this problem are revealed. It is anticipated that the proposed modeling and signal designing method can provide an effective technique for traffic flow scheduling issues.

Impact of Police Enforcement on Critical Gap at Roundabouts

At modern roundabouts, entering vehicles must yield right of way to vehicles in the circulating lane and can only proceed if there is a proper gap. The critical gap is the minimum acceptable gap that a driver will use to enter the roundabout. It is thus assumed that drivers will reject gaps of a value less than it and will accept gaps of a value more than it. In some countries, due to heavy congestion, police enforcement officers monitor and regulate the movement of vehicles at critical roundabouts during the peak hours of the day as needed. In some cases, police enforcement officers will be present at the roundabout during peak hours without interfering with the traffic flow to reduce the incidence of traffic violations. The purpose of this study was to investigate if there is a change in driver behavior in the form of a change in the critical gap value. A major roundabout was used to investigate the effect of police enforcement presence. The results of the study indicated that the critical gap during the police enforcement presence was 2.13 s whereas the critical gap of the roundabout during normal conditions was 2.42 s. The reduction in the critical gap is an indication that roundabouts can perform better and handle more traffic volume when police officers are present.

Performance Analysis of Modern Roundabouts as an Alternative to Conventional Signalized Intersections: a Comprehensive Review

The intersection is one of the most important elements of roadway network. It is considered as a point of weakness in any roadway network, because it has a higher probability of accident occurrences and a lower capacity than other elements of roadway network. During the last three decades, the roundabout has begun to be highlighted as a solution to problems of conventional signalized intersections. Since then, numerous studies have been conducted in order to determine and quantify the performance and benefits of converting the conventional signalized intersection to a modern roundabout. Most of these studies have focused on certain aspects of the performance of roundabouts, while neglecting the others. In addition, most of these studies have not determined the extent of conflicts among the different factors that affect the performance of the roundabout. This has necessitated the need for a comprehensive study that addresses all the aspects of roundabout performance and analyzes the interference among the different factors that affect the roundabout performance. In this study, the literature concerning all the aspects of a roundabout performance (safety of drivers, pedestrians, cyclists, as well as capacity, delay, toxic emissions, fuel consumption, cost) have been analyzed. In addition, the degree of interference among all the factors that affect the performance of a roundabout has been determined in order to clarify the overall picture of roundabout performance and finally to present the overall findings of the study.

A Comparison of Traffic Flow Performance of Roundabouts and Signalized Intersections: A Case Study in Nigde

Background: Intersections affect the safety and capacity of urban traffic. Therefore, the design and selection of the type of intersection need to be made very carefully. According to the demand level, a different intersection can be designed. Signalized intersections are one of the intersection types in which the sequence and duration of the flow at the intersection are provided by the lights. Generally, this type of intersection is used on roads with high traffic volume. Modern roundabouts are one of the types of circular intersections that provide advantages over other types of intersection in terms of smooth operation and safety. Modern roundabouts exist in several types today worldwide. In practice, the distinction about the kinds of roundabouts would not be fully clarified; as a result, queuing and delay can be seen as negative effects. Methods: In this study, to make a distinction and clarify the kinds of roundabouts, first, the roundabouts types are introduced according to geometric and operational aspects. A signalized intersection, where a circular island is placed and also signalized, was investigated in terms of capacity, delay, and emissions located in Niğde. The traffic flow performance of the current state (nested signalized roundabout) was calculated with HCM Method (for signalized intersection) using SIDRA and compared with roundabout solutions of the intersection with HCM6 (for roundabout) method using SIDRA Intersection analysis software. Results: From the results of the intersection capacity analysis study based on HCM6, it was seen that the application of a roundabout scenario (intersection considered as a modern roundabout) showed higher performance at the intersections than the intersection having a secondary signal. Capacity increased to 67.8%, the average delay decreased to 72.8% and 95th percentile queue dropped to 82.2%. Conclusion: Roundabout controlling instead of a nested signal system can be an example of the increase in the performance of traffic flow. This highlights the importance of choosing the appropriate roundabout design.

The procedure of statistical evaluation of accident rate equation for roundabouts

Road Design Manuals propose the procedure for determining the accident rate at modern roundabouts by the probable number of major accident of types on specific sites of a roundabout per year. An assumption is made that the regression equation may not take into account the design features of modern roundabouts, climatic conditions, as well as driving style in the Russian Federation. The paper presents a step-by-step method of the regression equation validation to assess the accident rate at the approaches to the roundabouts on the basis of other statistical methods. The validation involves the statistical evaluation of the sample correlation coefficients between the accident equation and the factors affecting it. Within the work the values included in the equation were non-dimensionalized, necessary assumptions were introduced, and the features of performing correlation analysis in the particular case were analyzed. The procedure proposed enables to determine the reliability of the equation for determining the accident rate at the approaches to a modern roundabout in the conditions of the Russian Federation, taking into account climatic conditions, design features, the state of the existing road transport network, as well as driving style.

Modern Roundabouts: A Challenge of the Future

Throughout the world, research into the various aspects and types of roundabouts has spanned many decades. During this long period the number of vehicles, their sizes, and performance characteristics, including speeds, have radically changed. Similar changes are also noted for the drivers’ experiences. Tese changes have had a strong influence on the evolution of modern roundabouts. Te primary aspects of the layout of modern roundabouts in relation to capacity and traffic safety is now known.

Review of Roundabout Capacity Based on Gap Acceptance

Circulating vehicles have priority at modern roundabouts. Entrance vehicles can enter the roundabout when there is a time gap larger than the critical gap; otherwise, the vehicles need to wait until there is a large enough gap. The gap acceptance theory was used to analyze the entrance capacity of roundabouts, which can be derived by queuing theory involving two vehicle streams. The paper introduces the main styles of headway distribution, which are named as bunched exponential distribution or M3 distribution. The calculation model of free stream ratio is also introduced. The entrance capacity models can be classified by different entrance vehicle types, which are piecewise function or linear function, or by different critical gap types, which are constant or stochastic function. For each form, the typical capacity expressions are given. The calculation values show a very small difference between these kinds of models. The capacity value based on the critical gap of stochastic function is more realistic and more complex in function style. Some conclusions were derived that drivers’ nonhomogeneous and inconsistent character is more realistic than the fixed critical gap and following gap. The calculation results of capacity are similar to the field capacity under the assumption of homogeneity and continuance, with only a minor percent deviation. Finally, the paper points out additional problems and the suggested research in capacity of roundabouts.

Optimising total entry delay at roundabouts with unbalanced flow: a dynamic strategy for smart metering

Modern roundabouts are widely used at intersections with light traffic, generally providing safety and other advantages. However, large entry delays are often observed at roundabouts with unbalanced flow patterns, even though the entry traffic flow is not high. A metering signal-based strategy is examined to mitigate the above problems. A mathematical optimisation model is formulated firstly with the objective of minimising the total entry delay, subject to the metering signal thresholds. Then a solution algorithm based on VISSIM simulation is developed. Finally, a case study is carried out to testify the feasibility and applicability of the proposed model. Extended scenarios analyses under different levels of approach volume, different demand combinations and different proportions of right-turn vehicles (left-side driving) are also conducted. Results show that the methodology can effectively improve the operational performance, and a delay reduction of up to 25.7% can be expected using the metering signal-based strategy. This can provide a criterion for the use of metering at the roundabout.

Upgrading Traffic Circles to Modern Roundabouts to Improve Safety and Efficiency – Case Studies from Italy

This paper presents a procedure for analysing safety and operational improvements made possible by converting traffic circles to modern roundabouts. An Italian case study is presented for alternative layouts under various traffic demand scenarios. In the application of the procedure, the average waiting times and queue lengths at entries are computed with an analytical capacity model, using default values for gap parameters. Then, the roundabout is dynamically simulated. The simulation results in a revised set of gap parameters that are in turn used as inputs to a second trial of the capacity model, and in turn fed back into the simulation. The two steps are repeated until the parameters reach a pre-selected convergence criterion, so that gap parameter values for both the static capacity and dynamic microsimulation models are in equilibrium. Therefore, the applied procedure can conduct both static and dynamic roundabout design, usually applied separately. One can start with default values in guidelines and couple them with limited field data, improving both the expected results and cost-effectiveness of solutions. Next, safety is estimated using dynamic simulation software and a compatible conflict counting model to acquire surrogate measures of safety. Level-of-serviceand surrogate safety indicators for the existing and redesigned roundabouts are then compared. The procedure is first demonstrated on an old “ultra-large” roundabout. The procedure is tested on this roundabout using the Highway Capacity Manual 2016 (HCM2016), AimsunTM, and Surrogate Safety Assessment Model (SSAM) software. A redesign is shown to be far superior in efficiency and safety. Finally, two cases are described where large first generation roundaboutswere upgraded to modern standards.

Traffic Efficiency Evaluation of Elliptical Roundabout Compared with Modern and Turbo Roundabouts Considering Traffic Signal Control

This paper compared the performance of elliptical roundabout with turbo and modern roundabouts. It considers the effects of increasing the central island radius and speed limit on delay and capacity. Three types of roundabouts (modern, turbo and elliptical roundabouts) with different numbers of lanes (single lane, two-lane and three-lane) were designed. Unsignalized and signalized controls were applied for these roundabouts. The robustness of the designed roundabouts was investigated for saturated and unsaturated flow conditions. Based on the obtained results, increasing the central island radius had both positive and negative effects on delay and capacity. However, a positive effect on these variables was observed in all roundabouts when increasing the speed limit. In unsignalized and signalized control under unsaturated flow conditions, a modern roundabout had lower delay time than an elliptical roundabout. Moreover, in saturated flow, the elliptical roundabout had the best performance in terms of delay. Overall, in comparison with the turbo roundabouts, modern and elliptical roundabouts had the highest capacities in unsignalized and signalized controls. This study can provide useful information for engineers who decide to design a roundabout.

Methodological Frontier in Operational Analysis for Roundabouts: A Review

Several studies and researches have shown that modern roundabouts are safe and effective as engineering countermeasures for traffic calming, and they are now widely used worldwide. The increasing use of roundabouts and, more recently, turbo and flower roundabouts, has induced a great variety of experiences in the field of intersection design, traffic safety and capacity modelling. As for unsignalized intersections which represent the starting point to extend knowledge about the operational analysis to roundabouts, the general situation in capacity estimation is still characterized by the discussion between gap acceptance models and empirical regression models. However, capacity modelling must contain both the analytical construction and then solution of the model, and the implementation of driver behavior. Thus, issues on a realistic modelling of driver behavior by the parameters that are included into the models are always of interest for practioners and analysts in transportation and road infrastructure engineering. Based on these considerations, this paper presents a literature review about the key methodological issues in the operational analysis of modern roundabouts. Focus is made on the aspects associated with the gap acceptance behavior, the derivation of the analytical-based models and the calculation of parameters included into the capacity equations, as well as steady state and non-steady state conditions and uncertainty in entry capacity estimation. At last, insights on future developments of the research in this field of investigation will be also outlined.

A case study on multi-lane roundabouts under congestion: Comparing software capacity and delay estimates with field data

Existing studies on modern roundabouts performance are mostly based on data from singe lane roundabouts that are not heavily congested. For planners and designers interested in building multilane roundabouts for intersections with potential growth in future traffic, there has been a lack of existing studies with field data that provide reference values in terms of capacity and delay measurements. With the intent of providing such reference values, a case study was conducted by using the East Dowling Road Roundabouts in Anchorage, Alaska, which are currently operating with extensive queues during the evening peak hours. This research used multiple video camcorders to capture vehicle turning movements at the roundabouts as well as the progression of vehicle queues at the roundabout entrance approaches. With these video records, the number of vehicles in the queues can be accurately counted in any single minute during the peak hours. This study shows that unbalanced entrance flow patterns (i.e., one entrance has significant higher flow than others) can intensify the queue and delay for the overall roundabouts. Then various software packages including RODEL, SIDRA and VISSIM were used to estimate several performance measurements, such as capacity, queue length, and delay, compared with the collected field data. With the comparison, it is found that all the three software packages overestimate multi-lane roundabout capacity before calibration. With default parameters, SIDRA and VISSIM tend to underestimate delays and queue lengths for the multi-lane roundabouts under congestion, while RODEL results in higher delay and queue length estimations at most of the entrance approaches.

Passenger Car Equivalent for Heavy Vehicles Crossing Turbo-roundabouts

Turbo-roundabouts represent an innovative scheme of modern roundabouts which provides a spiraling traffic flow and requires drivers to choose their direction before entering the intersection, since raised lane separators mark the lanes on the ring. The configuration of the turbo-roundabout makes that patterns of conflict at entries with one and two conflicting traffic streams can coexist.This paper presents research efforts aimed at measuring quantitatively the effect of heavy vehicles on operational conditions of a turbo-roundabout. The study starts from the initial belief that the greatest constraints to the vehicular trajectories imposed by the turbo-roundabout necessarily imply that the impact of heavy vehicles on the quality of traffic flow is more unfavorable than on other modern roundabouts. Microsimulation revealed as a useful tool when the variation of the traffic quality in turbo-roundabouts should be evaluated in presence of mixed fleets, each having different percentages of heavy vehicles; indeed, it allowed to isolate traffic conditions difficult to observe on field and replicate them to have a number of data as much as possible numerous. Entry capacity values for each entry lane of the turbo-roundabout were obtained by microsimulation, varying the percentage of heavy vehicles for entering flows. Nonlinear regression analysis of simulation data allowed to derive the behavioral parameters for heterogeneous populations of users and, ultimately, composed exclusively of heavy vehicles. The capacity functions thus obtained allowed us to determine how the passenger car equivalent (PCE) varies with the percentage of heavy vehicles and circulating flows for each entry lane of the turbo-roundabout. The results of this study indicate that there is a need to distinguish the impact of heavy vehicles when analyzing the capacity of a turbo-roundabout. When the traffic stream contains a significant number of heavy vehicles, a larger PCE effect would be expected. This effect should be accounted for in the estimation of the turbo-roundabout capacity. Lastly it should be emphasized that an important aspect of the research consists in having identified a methodology for assessing the impact of heavy vehicles on the quality of traffic flow, that can be applied to different patterns of intersection.

Investigating Pedestrian Injury Crashes on Modern Roundabouts in Addis Ababa, Ethiopia

Pedestrian crashes represent about 40% of total fatal crashes in low-income developing countries. Although many pedestrian crashes in these countries occur at unsignalized intersections such as roundabouts, studies focusing on this issue are limited. The objective of this study was to develop safety performance functions for pedestrian crashes at modern roundabouts to identify significant roadway geometric, traffic, and land use characteristics related to pedestrian safety. Detailed data, including various forms of exposure, geometric and traffic characteristics, and spatial factors such as proximity to schools and to drinking establishments were collected from a sample of 22 modern roundabouts in Addis Ababa, Ethiopia, representing about 56% of such roundabouts in Addis Ababa. To account for spatial correlation resulting from multiple observations at a roundabout, both the random effect Poisson (REP) and random effect negative binomial (RENB) regression models were estimated. Model goodness-of-fit statistics revealed a marginally superior fit of the REP model to the data compared with the RENB model. Pedestrian crossing volume and the product of traffic volumes along major and minor roads had significant and positive associations with pedestrian crashes at roundabouts. The presence of a public transport (bus or taxi) terminal beside a roundabout was associated with increased pedestrian crashes. Although the maximum gradient of an approach road was negatively associated with pedestrian safety, the provision of a raised median along an approach appeared to increase pedestrian safety at roundabouts. Remedial measures were identified for combating pedestrian safety problems at roundabouts in the context of a developing country.