Circulatory Roadway Research Articles

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.

Intersection Sight Distance Requirements for Basic Turbo Roundabouts

Turbo roundabouts, emerging as a solution to conventional multilane roundabouts’ safety and operations concerns, are prevalent in Europe and are still new to North America. However, few studies and guidelines on intersection sight distance (ISD) requirements exist. This paper focuses on the basic turbo roundabouts, examines the circulatory roadway and approach design features, and develops ISD models with North American practices. The ISD requirements were identified for the vehicle approaching and entering the roundabout, which conflicts with the circulating vehicle in the circulatory roadways, and the entering vehicle at the upstream entry approach. Analytical models were developed to identify the required lateral clearance at the central island and clear sight triangles at the roundabout quadrants to satisfy the required ISD. The analyses were completed for four typical sizes of basic turbo roundabouts per Dutch design criteria. The design guidelines are provided for the required lateral clearance at the central island and the required clear sight triangles at the roundabout quadrants. They would be beneficial in establishing landscaping restrictions and determining the roadway right of way and property impacts at the intersections during the roadway planning stage.

The Design Vehicle Steering Path Construction Based on the Hairpin Bend Geometry—Application in Roundabout Design

A prerequisite for a well-designed roundabout is the definition of the design vehicle movement geometry and its turning envelopes. This is because kinematic and dynamic requirements lose their importance due to low driving speed. In this paper, the investigation of the deviations of design vehicle steering paths for straight passage through the roundabout constructed according to the two approaches is presented. The first approach (A-1) includes the application of short, straight lines between the adjacent arcs on the path, and the second (A-2) implies the application of consecutive circular arcs. A computer simulation of vehicle movement was conducted using the Autodesk Vehicle Tracking 2020 software. The results showed that A-2, a faster and simpler approach, is applicable with caution, as the resulting turning envelopes derived from this approach are more offset from the roundabout centers compared to the A-1 approach. A total of 72% of right-hand-side path and resulting right-hand-side design vehicle body and chassis turning envelope deviations were between 0.00 and 0.15 m, 27% were over 0.15 m, and 7% were over 0.25 m. This results in larger roundabout entry and exit widths as well as the need for large lateral clearances for entries and exits on roundabouts where heavy pedestrian traffic on sidewalks is expected. The noted offset of left-hand-side design vehicle trajectories on the circulatory roadway (which was mostly between 0.05 and 0.15 m) could have a positive impact on the deflection around the central island.

Two Stop-Line Method for Modern T-Shape Roundabout: Evaluation of Capacity and Optimal Signal Cycle

Uncoordinated traffic flows at the traditional roundabouts, especially with a small circumference and fewer lanes, are often heavily affected by congestion, which escalates fuel consumption, CO2 emissions, idling, and travel delay. An intriguing way to mitigate such uncoordinated flows at junctions would be facilitated through optimal traffic signalization. For this purpose, this paper presents a novel holistic Three-Leg Signalized Roundabout (TLSR) model based on two signalized stop lines (2SL). The first stop line is placed at each entry curve of a roundabout with effectual lane markings as usual. Hereafter, the second stop line is set exclusively in the circulatory roadway to improve left-turning mobility with an additional “short-lane model” to deal with heavy traffic, following specific patterns for smooth vehicle merging. The capacity and optimal signal cycle relationships are derived to evaluate the performance of the proposed TLSR-2SL, considering the internal space constraints of the roundabout. Under the various scenarios, the parameters’ sensitivity tests demonstrate that signal cycle and central radius play a significant role in enhancing the roundabout’s operational performance. From the executed simulation, the proposed framework improves the traffic flow by 15% and controls the relative error within 10% compared to benchmark methods.

Geometric Design of Suburban Roundabouts

A modern roundabout is an intersection with a circulatory roadway at which the vehicle speed is low, and the traffic is continuous and circulating in one direction around the central island towards the exits at the approach legs. Modern roundabout design is an iterative process that is composed of the following steps: (1) the identification of the roundabout as the optimal traffic solution; (2) the definition of the number of lanes at the intersection based on the required capacity and the level of service; (3) the initial design of the roundabout geometry; (4) design vehicle swept path, the fastest path analysis, and visibility performance checks; and (5) detailed roundabout design if the results of the performance checks are in line with the design recommendations. Initial roundabout geometry design elements are not independent of each other; therefore, care must be taken to provide compatibility between them. An overview and a comparative analysis of the initial geometric design elements for suburban single-lane roundabouts defined in roundabout design guidelines and norms used in Croatia, Austria, France, the Netherlands, Germany, Serbia, and Switzerland is given in this entry.

Cyclists at Roundabouts: Risk Analysis and Rational Criteria for Choosing Safer Layouts

Cycling for transportation is an important resource to reduce urban traffic congestion, enhance personal health, reduce energy consumption, and improve air quality, and the safety of cyclists in the cities is becoming a topic of growing interest. As shown in the literature, an important number of cyclist fatalities is due to road crashes occurring at urban intersections. This study combines a probabilistic and a damage model to perform a risk analysis for the collisions between motor vehicles and bicycles in the merging and diverging conflict points of a single-lane conventional roundabout with four arms, characterized by a permanent traffic flow. The probabilistic model is based on Poisson’s law and is aimed to measure the probability of a collision between bikes and motor vehicles within the elementary unit of exposure in each conflict point of the roundabout. The damage model exploits the reaction time of a road user to avoid a collision and has been built to develop a danger classification for the conflict points. The goal of this study is then to estimate the so-called risk of collision at the roundabout, to compare different possible layouts for various traffic volumes with increasing bike flows and geometric configurations, and to identify the most effective solutions to improve safety for cyclists. The results demonstrate the risk reduction given by a roundabike compared to a standard layout where cyclists and motor vehicles share the circulatory roadway. Therefore, the study here presented could help road managers to implement mitigation strategies taking into consideration both geometric and functional constraints.

Modeling of a novel risk index for evaluating the geometric designs of roundabouts

In the geometric design of roundabouts, safety-oriented approaches are required rather than specification design ones that simply determine the dimensions of the geometric structural elements. We herein propose a risk index (RI) that combines the invisibility probability and the crash impact as a performance measure for evaluating the safety of the geometric designs of roundabouts, and we also describe a method for calculating this index. Invisibility probability represents the probability that an entering vehicle cannot view a vehicle coming from the upstream in a circulatory roadway and crash impact represents the amount of lost kinetic energy at the time of collision corresponds to the impact of the crash. A numerical simulation to model this RI on the basis of various geometric conditions is further presented. It is demonstrated that the invisibility probability is large when the entry angle is small and that the crash impact increases when the deviation angle decreases. The proposed approach is expected to help resolve issues with currently existing roundabouts and improve the design of future roundabouts to enhance their safety performance.

Investigating the Deviation Angle Method for Ensuring Deflection at One-Lane Rural Roundabouts

Roundabouts developed as a road intersection design option has resulted in a series of nonuniform design guidance criteria in Europe, as well as in the United States and other Countries. In addition to different design specifications about the geometry of the elements constituting a roundabout (width and lanes of the circulatory roadway, entry and exit legs, splitter island), the methods for guaranteeing that vehicle paths deflect through the roundabout are also different. These methods ensure proper travel speeds between conflicting traffic flows. Currently, the main parameters used by standards to control the deflection are the deflection radius, the entry path radius, and the deviation angle. After a comparison between International deflection methods for roundabouts, this study checks the geometric requirements of the deviation angle for more than 7.000 hypothetical one-lane rural roundabouts. The Computer-Aided Design (CAD) drawing of the roundabouts takes into account the range of variability of their main geometric parameters, according to the Italian Standard. Subsequently, a number of the considered roundabouts checked with both the entry path radius and the German methods. Some results showing the greater effectiveness of the less popular deviation angle method are discussed. The main aims of this paper are:1) to promote the deviation angle method, which is only used in Switzerland and Italy;2) to improve standards, as regards the applicability and validity of the deflection angle method;3) to help practitioners to know in advance the outcome of the deflection checks at the beginning of the iterative design process, once the boundary conditions are known.

Turbo – Roundabouts and their Basic Evaluation at Realized Constructions in Czech Republic

The article deals with an operation issues and traffic evaluation at selected turbo-roundabouts realized in Czech Republic. Also describes redevelopment possibility of existing roundabouts with classic layout to the roundabouts with spiral arrangement of the traffic lanes on the circulatory roadway. Described are advantages and disadvantages of the spiral arrangement in compare with normal roundabouts. The article evaluates Czech methodology of turbo - roundabout design, which was published in 2015 and compares its findings with foreign methodologies, especially Dutch and Slovakian standards. Presented are basic divisions of turbo - roundabouts and how the single types of layout are different from the point of view of number of traffic lanes and estimated capacity. There is shown the list of realized turbo - roundabouts on which is made a comparison of identity of the selected parameters of geometric elements defined in the Czech methodology with the foreign standards. Evaluation criteria of realized constructions are basic design elements of turbo-roundabouts, traffic lane (roadway) width and layout and design arrangement with aiming on problematic design features.The article also briefly deals with the evaluation of effectiveness from the point of view of capacity and traffic accidents, when are compared traffic accidents data at selected locations in Czech Republic where redevelopment of conventional types of intersections to turbo-roundabouts were made.

Study of driver behaviour at turbo-roundabouts

The article presents the results of preliminary research into the behaviour of drivers at turbo-roundabouts. The subject of the research included the frequency of driver behaviour against the traffic rules, and the speed at which vehicles drive through turbo-roundabouts. One of the crucial problems which was analysed was the influence of different kinds of traffic lane division on the behaviour of drivers. The analysis results affirm that the raised lane dividers can visibly improve the propensity of drivers to stay within the designated traffic corridor. However, it does not eliminate the phenomenon of improper lane changing on circulatory roadway. The physical separation of traffic lanes has not been determined to introduce any additional hazard. The speed of vehicles encroaching upon the neighbouring traffic corridor is visibly higher than this of vehicles following traffic rules. Using crash prediction models developed for single- and multi-lane roundabouts, the authors of the research estimated that lane dividers may reduce the number of crashes from about 10% to 17%.

Rollover Propensity of Heavy Vehicles at Roundabouts: Case Study on High- and Low-Speed Roads

In Indiana, roundabout construction recently commenced on high-speed roadways (i.e., with posted speed limits of 50 mph or higher), many carrying significant truck traffic. Studies show that heavy vehicle rollovers may be an issue at roundabouts. A case study compared the safety-related behavior of truck drivers at roundabouts on low- and high-speed roads. The rollover model is more suitable for truck trailers than previous models used for design considerations because it accounts for the complex paths and vehicle tilt of semitrailers and other heavy vehicles. The model was used to determine the difference between the critical rollover speed determined from the model and the observed vehicle speed. The proximity to rollover in the roundabout circulating lane was found to be different on low- and high-speed roads. However, this difference could not be explained by actual driver approach speeds. Drivers on the high-speed approach began decelerating earlier than those on the low-speed approach, and their deceleration rates were similar. In addition, vehicle speeds on the high-speed approach were only 1 mph faster than those on the low-speed approach close to the roundabout. The average minimum proximity to rollover on the circulatory roadway was about 3 mph slower on the low-speed, one-lane roundabout than on the high-speed, two-lane roundabout, and this difference might have been the main reason for higher risk of circulatory rollover in the one-lane roundabout.

ALTERNATIVE ARRANGEMENT OF SPECIAL ROUNDABOUT

The paper deals with alternative arrangement of special roundabout. It describes used method, i.e. measurement of traffic flow characteristics, capacity calculations and traffic simulation. There is shown results of analysis for concrete roundabout with one-line circulatory roadway compared double roundabout with two mini roundabouts.

회전교차로 측면충돌 사고모형 개발

This study deals with the roundabout accidents. The goal of this study is to develop the sideswipe accident models at roundabout. In the pursuing the above, this study gives particular attentions to collecting the data of geometric structure and accidents of 54 roundabouts in Korea and developing the Poisson and negative binomial regression models. The main results are as follows. First, sideswipe accident is analyzed to be the highest frequency that is 39.5% of total accident data. Second, Poisson models which is statistically significant is developed. Finally, traffic volume per approach(<TEX>$X_1$</TEX>), number of circulatory roadway(<TEX>$X_3$</TEX>), operation of parking lot(<TEX>$X_4$</TEX>) and width of circulatory roadway(<TEX>$X_6$</TEX>) are adopted as the common variables. This study might be expected to give some implications to the accident research on the roundabout.

Development of Safety Performance Measures for Urban Roundabouts in India

Roundabouts, although considered as safe among the various intersection types, have seen an increasing crash rate over the years in India. Geometric design of a roundabout directly influences the driver’s speed adoption and maneuvering behavior, and hence plays a vital role in its safety performance. Application of treatments at roundabouts by altering the geometric design requires careful safety evaluation before actual implementation. The present study identified the crash causes and devised safety performance measures for urban roundabouts located in the state of Kerala, India. Crash prediction models and crash modification factors (CMFs) are developed in this study for the safety assessment of geometric design features of roundabouts. Of the many models tried, the zero-inflated negative binomial regression model is the best model to predict the crash frequency of roundabout approaches. Geometric features of the central island, circulatory roadway, and approach roads are associated with the crash occurrence on roundabout approaches. CMFs are developed by a cross-sectional method and are useful for investigating the relationship between safety and geometric design features of roundabouts.

Methodological Approach for Evaluation of Roundabout Performances through Microsimulation

In the literature, there are many methodologies that allow the evaluation of roundabout performances (Capacity, Levels Of Service, etc.): analytical models (HCM, HBS etc.), statistical models (TRRL, SETRA) etc. Each technique considers some aspects of the roundabout in comparison to others (geometric elements, vehicular flow and behavioral parameters). Obtained results are often not comparable among themselves because of distinctive peculiarities of each method. Today, the best way to solve this problem is by using a refined simulation software of vehicular circulation. However, along with a more and more refined analysis of microsimulation software algorithms, it is frequently necessary for each user (researcher, engineer, planner, etc.) to know the real sensitivity of these packages with regard to the most important key parameters. In this paper the Authors introduce the results of a wide survey conducted on an ample range of virtual roundabout scenarios by the use of a modern simulation software. Each scenario describes a fixed roundabout phenomenon using the following variables: geometric elements (inscribed circle radius, circulatory roadway, central and splitter islands etc.); characteristics of the traffic flow (dynamic traffic assignment, approach speed, circulatory speed and reduced speed zones, etc.); behavioral features (priority rules, minimum gap, minimum headway, etc.). The results are presented from the evaluation of stop-line delays.

로터리 사고발생 위치별 사고모형 개발

PURPOSES : This study deals with Rotary by Accident Occurrence Location. The purpose of this study is to develop the accident models of rotary by location. METHODS : In pursuing the above, this study gives particular attentions to developing the appropriate models using multiple linear, Poisson and negative binomial regression models and statistical analysis tools. RESULTS : First, four multiple linear regression models which are statistically significant(their <TEX>$R^2$</TEX> values are 0.781, 0.300, 0.784 and 0.644 respectively) are developed, and four Poisson regression models which are statistically significant(their <TEX>${\rho}^2$</TEX> values are 0.407, 0.306, 0.378 and 0.366 respectively) are developed. Second, the test results of fitness using RMSE, %RMSE, MPB and MAD show that Poisson regression model in the case of circulatory roadway, pedestrian crossing and others and multiple linear regression model in the case of entry/exit sections are appropriate to the given data. Finally, the common variable that affects to the accident is adopted to be traffic volume. CONCLUSIONS : 8 models which are all statistically significant are developed, and the common and specific variables that are related to the models are derived.

Researching a Capacity Model for Multilane Roundabouts with Negotiation of the Right-of-Way between Antagonist Traffic Flows

This article summarizes a research program designed to assess operating conditions through capacity estimation at not-conventional roundabouts characterized by a central island with a large diameter and by two (or more) entering and circulating lanes. Although giving priority to vehicles on circulatory roadway is the nominal operating rule, on-field observations highlighted that in some infrastructural schemes of the type considered in this research drivers negotiate the right-of-way according to a consensus pattern that alternates between antagonist traffic flows, similarly to that observed at all-way-stop-controlled intersections. Considering the peculiarity of the way of working at roundabout schemes being examined, as well as difficulties faced in the application of models based on gap-acceptance theory to evaluate performances, an analytical capacity model derived from field observations was proposed for multilane not conventional roundabouts. Drawing inspiration from the iterative procedure proposed for capacity estimates at all-way-stop-controlled intersections, a calculation algorithm organized in 5 subsequent computational steps was developed in order to evaluate capacity at each entering/circulating lane for the type of roundabouts examined in this research. Results compared to those calculated by models for modern roundabouts show the effect on estimates of capacity raised by a more realistic operating pattern and indicate that the capacity model may be able to estimate parameters useful for planning and design purposes.

ZAM을 이용한 원형교차로 원인별 사고모형 개발

이 연구는 국내 원형교차로에서 발생한 교차로 사고를 다루고 있다. 연구의 목적은 ZAM을 이용하여 원인별 사고모형을 개발하는데 있다. 주요결과는 다음과 같다. 첫째, 교차로 운행방법 위반에서는 ZINB 모형이 적합한 것으로 분석되었다. 둘째, 안전거리 미확보에서도 ZINB 모형이 적합한 것으로 분석되었다. 마지막으로 공통변수로는 교통량과 회전차로 폭이 선정되었다. 교통량이 많을수록 그리고 회전차로 폭이 좁을수록 사고가 많이 발생하는 것으로 분석되었다. 특정변수로는 접근로수와 감속 시설수가 채택되었고, 접근로수가 증가할수록 그리고 감속시설수가 적을수록 사고가 증가하는 것으로 분석되었다. 이 연구는 원형교차로 사고연구에 기여할 것으로 기대된다. This study deals with the traffic accidents of circular intersections in Korea. The goal of this study is to develop the traffic accident models using ZAM. The main results are as follows. First, in the case of 'violating the operational method of intersection', ZINB(zero-inflatednegative binomial) models were analyzed to be the best fit to the data. Second, in the case of' no maintaining the safe distance', ZINB models were also analyzed to be the best fit to the data. Finally, such the common variables as traffic volume and width of circular roadway were selected as the independent variables. The more traffic volume and the less width of circulatory roadway were evaluated to make the more accidents. Such the specific variables as the number of approach lanes and speed reduction facilities were selected as the explanatory variables. The more approach lanes and the less speed reduction facilities were evaluated to give the more accidents. This study might be expected to give some implications to the accident research on the circular intersections.

Optimization of Vehicle Travel Position on Roundabouts in Snowy Cold Regions

Recently, modern roundabouts have been actively introduced in various countries. Single-lane roundabouts help to prevent serious accidents because vehicles tend to slow down as a result of the transverse acceleration that acts on them during movement on the circulatory roadway. However, as the boundary between the circulatory roadway and the truck apron becomes unclear during winter in snowy cold regions, it is necessary to verify the practicality of roundabouts in consideration of winter road surface and meteorological conditions. This study examined a method of guiding vehicles on circulatory roadways in winter. Driving experiments were conducted at a roundabout with different road surface conditions and central-island diameters to allow analysis of the positions of vehicles traveling under each set of conditions. The results indicated that vehicles were able to stay on course regardless of the central-island diameter in dry road surface conditions. With winter road surface conditions, however, the number of vehicles mounting the truck apron increased when the island diameter was small, whereas a large diameter helped them to stay on course and reduced travel position variance. An examination of driving behavior under different road surface conditions and with different central-island diameters revealed that surface conditions affected travel speed and transverse acceleration, whereas the island diameter had a smaller effect. However, it was found that the adoption of a large central-island diameter in a constant inscribed circle diameter was effective in reducing travel position deviation and helped vehicles to stay on the circulatory roadway even during the winter in snowy cold regions.

Microscopic Dual-Regime Model for Single-Lane Roundabouts

Most of microsimulation tools used to model roundabouts encompass classical gap-acceptance algorithms to represent the insertion of approaching vehicles into the circulatory roadway. However, these algorithms fail to reproduce the mean priority sharing process experimentally observed when the circulatory roadway is congested. This paper fills this shortage by proposing an integrated microscopic framework with: (1) a gap-acceptance algorithm giving relevant capacity estimates in uncongested regime; and (2) a probabilistic rate-based insertion decision module in congested regime. In this framework the car-following model can be implemented independently of the insertion decision-making process. Moreover, its direct influence on the insertion decision model is released in congested regime thanks to a relaxation procedure. The obtained simulation results are convincing compared to on-field data collected at different sites for both peak and off-peak periods.