Roundabout Geometry Research Articles

Short Cyclic Algorithm for Turbo Block Construction

This paper presents perhaps the shortest possible algorithm for drafting a turbo block in a standard CAD environment. Though not defining the complete turbo roundabout geometry (the legs are defined by their centerlines only), turbo block construction stays at the very core of its geometrical definition. Relevant national policies elaborate turbo block construction at various levels of detail. Some policies skip certain definitions of spirals’ centers and some even allow for slight discontinuities in spiraling lanes’ geometries. But, as our computer program had to be based on precisely defined geometrical features, no question, be it a location of a particular center or relation between the interconnecting arcs, could be left unanswered. Consequently, while explaining the basics on which the program relies, all the elements of the turbo block are discussed. Even the problems not addressed in renowned national policies (such as alternatively matching marking and curb lines) had to be resolved before being incorporated into the program. After resolving and elaborating turbo block roundabout technics in detail, a new cyclic algorithm for turbo block construction is presented. The program is very short and could be easily understood and modified by the average programmer.

Comparison of Single-Lane Roundabout Entry Degree of Saturation Estimations from Analytical and Regression Models

Roundabout design is an iterative process consisting of a preliminary geometry design, geometry performance checks, and the estimation of intersection functionality (based on the results of analytical or regression models). Since both roundabout geometry design procedures and traffic characteristics vary around the world, the discussion on which functionality estimation model is more appropriate is ongoing. This research aims to reduce the uncertainty in decision-making during this final roundabout design stage. Its two objectives were to analyze and compare the results of roundabout performance estimations derived from one analytical and one regression model, and to quantify the model results’ susceptibility to changes in roundabout geometric parameters. For this, 60 four-legged single-lane roundabout schemes were created, varying in size and leg alignment. Their geometric parameters resulted from the assumption of their location in a suburban environment and chosen design vehicle swept path analysis. To compare the models’ results, the degree of saturation of roundabout entries was calculated based on presumed traffic flows. The results showed that the regression model estimates higher functionality and that this difference (both between the two models and regression models applied on different schemes) is more pronounced as the outer radius and angle between the legs increase.

Analytical approximation and calibration of roundabout capacity: A merging state transition-based modeling approach

This manuscript focuses on the theoretical advancement of causality between entry vehicle dynamics and roundabout capacity modeling, with a merging state transition-based analytical approximation and calibration approach. Gap acceptance models, such as the HCM model, usually ignore roundabout specific operating conditions, whereas empirical models are generally criticized for the lack of fundamental understanding of underlying traffic flow or driving behaviors. The roundabout geometry is firstly extracted into a Y-shaped network, and the traffic movements are illustrated with a state-space-time diagram. Next, we analyze the merging state space for the entry vehicles, and draw the state-transition diagram. The episode of a traffic flow is defined, and we show that the trajectory of an entry vehicle repeats one of four patterns within each episode. Then, state transition-based analytical derivation of roundabout capacity is presented. This is done by estimating the state transition probabilities, followed by an episode-based state transition chain analysis and, finally, finding the solution of state transitions under steady states. Circulating speed is used as a key variable to reflect the operating conditions in the target roundabout. For a special scenario, with all four entry approaches being saturated, we model the interactions between entry flow and circulating flow, and prove that the resulting model can be uniquely solved by classic root-finding algorithms. The accuracy of the proposed model is tested with OpenDD, a real-world high-resolution trajectory dataset collected by drones at four roundabouts. The results of the proposed model are shown to consistently outperform the HCM6 model and another gap acceptance-based model.

Implementing B-Spline Path Planning Method Based on Roundabout Geometry Elements

This paper presents a parametric B-spline-based path planning approach for roundabout exit traverse planners to handle successively generated trajectories. A roundabout is a complex driving scene because of its large curvature and the varying intersection leg angles. A few studies focus on the relationship between roundabout intersection leg angle and path performance. This paper discusses B-spline advantages to curve road conditions and the impact of roundabout geometry elements on path planning performance. Knot vectors are defined, and control points are calculated to meet the requirements for generating paths that fit in different roundabout direction maneuvers. Four countries’ roundabout design standards are summarized with their unique geometry elements. Additionally, the Stanley tracking controller was used to calculate the steering behavior. The impacts of maneuver directions and intersection angles are discussed. It is explained stepwise how the maneuver system framework was built and what notable impacts the geometric structure of the roundabout has on the paths.

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.

Modeling the Influence of Roundabout Deflection on Its Efficiency as a Noise Abatement Measure

Challenges that emerge in roundabout design are mostly related to space constrictions and provision of appropriate deflection around the central island. This can result in speed profiles on roundabouts that might reduce their potential as a noise abatement measure. Because of this, the impact of a roundabout on noise levels and its applicability as a traffic calming device and a noise abatement measure should be investigated in the early design stage, by modeling noise levels. In this paper, the following hypothesis is tested: vehicle movement trajectories, defined during the fastest path performance check in roundabout early design phase, can be used as the road traffic noise sources when modeling noise at roundabouts using static noise model. This procedure (1) simplifies the preparation of the noise model, (2) results in a model that is closer to the real-world in terms of traffic flow conditions, (3) allows the noise calculations in the early stages of roundabout design, and (4) includes the influence of roundabout deflection on calculated noise levels. The abovementioned simplifications of the road traffic noise modeling process should encourage the optimization of roundabout geometry in terms of its noise reduction capabilities in the preliminary design phase of these intersections.

Predicting Agent Behaviour and State for Applications in a Roundabout-Scenario Autonomous Driving.

As human drivers, we instinctively employ our understanding of other road users’ behaviour for enhanced efficiency of our drive and safety of the traffic. In recent years, different aspects of assisted and autonomous driving have gotten a lot of attention from the research and industrial community, including the aspects of behaviour modelling and prediction of future state. In this paper, we address the problem of modelling and predicting agent behaviour and state in a roundabout traffic scenario. We present three ways of modelling traffic in a roundabout based on: (i) the roundabout geometry; (ii) mean path taken by vehicles inside the roundabout; and (iii) a set of reference trajectories traversed by vehicles inside the roundabout. The roundabout models are compared in terms of exit-direction classification and state (i.e., position inside the roundabout) prediction of query vehicles inside the roundabout. The exit-direction classification and state prediction are based on a particle-filter classifier algorithm. The results show that the roundabout model based on set of reference trajectories is better suited for both the exit-direction and state prediction.

Intersection performance evaluation and designing intersection at concourse between arterial road and ramp of Medan-Kualanamu-Tebing Tinggi Highway

In overcoming congestion problems that occur in Medan-Tebing Tinggi route, especially at certain points such as at Pasar Bengkel and Kayu Besar intersection during peak hours, an infrastructure has been built in, that is a highway which connecting Medan-Kualanamu-Tebing Tinggi. Medan-Kualanamu-Tebing Tinggi highway (MKTT) will connect Medan City and Tebing Tinggi City, so that drivers from Medan who will go to Tebing Tinggi will not pass through the main road that is prone to congestion.There is an intersection that connects arterial roads with each MKTT toll gate. At the intersection, there is a conflict between flows from the opposite direction and intersecting, resulting in delays and queues along the intersection arm. The conflict occured because of the lack of designing of the intersection that drains the flow of vehicles from ramp to arterial road or vice versa. This needs consideration because of the congestion causes queues and delays, especially during peak hours and holidays, so that it requires intersection performance analysis at concourse between arterial road and ramp of Medan-Kualanamu-Tebing Tinggi highway and design the geometric of intersection which appropriate with this intersection.From the results of the evaluation analysis, obtained degree of saturation at the intersection of the arterial road and the ramp of Lubuk Pakam toll road at 0.71 with level of service C that is still feasible to be used, so there is no need to change the intersection geometry. While for Sei Rampah, the degree of intersection saturation is 0.79 with the level of service D, which means that it is not feasible, requiring a change in the geometry of the intersection with a signalized roundabout. With the planned roundabout geometry, the degree of saturation is 0.5 which means it is feasible to use.

Machine Learning Techniques for Undertaking Roundabouts in Autonomous Driving.

This article presents a machine learning-based technique to build a predictive model and generate rules of action to allow autonomous vehicles to perform roundabout maneuvers. The approach consists of building a predictive model of vehicle speeds and steering angles based on collected data related to driver–vehicle interactions and other aggregated data intrinsic to the traffic environment, such as roundabout geometry and the number of lanes obtained from Open-Street-Maps and offline video processing. The study systematically generates rules of action regarding the vehicle speed and steering angle required for autonomous vehicles to achieve complete roundabout maneuvers. Supervised learning algorithms like the support vector machine, linear regression, and deep learning are used to form the predictive models.

Geometric factors influencing entry capacity of roundabouts under heterogeneous traffic conditions

Roundabout entry capacity is influenced by geometric features of the roundabout, traffic flow characteristics, vehicle and driver characteristics, as well as, environmental conditions. The major methods for estimating roundabout entry capacity are based on either gap acceptance theory or on empirical relations. Roundabout geometry is the sole aspect which can be entirely manipulated by the designers to improve the entry capacity. Limited studies have been conducted to analyze the influence of geometric elements of a roundabout on its capacity, for heterogeneous traffic conditions. Many developing countries like India, Malaysia, Indonesia etc., have heterogeneous traffic conditions on their roads. Data is collected from twenty-one entries of six roundabouts, where heterogeneity in traffic is observed. Seven different vehicle categories are considered such as motorised two wheeler, three wheeler, car, mini bus, light commercial vehicle (LCV), heavy commercial vehicle (HCV) and bus. A non-linear regression model is proposed to predict entry capacity, based on the nature of variation with individual geometric elements. Various combinations of independent variables are used to estimate entry capacity. The non-linear correlations among the geometric variables are checked. In comparison with the existing empirical models such as the LR942 regression model and German empirical model, the proposed regression model produced better estimates and much lower RMSE values. Approach width is found to have the highest impact on entry capacity. The entry capacity is found to be negatively influenced by entry angle. Circulating flow is considered in terms of per metre width against the usual convention. This modification incorporates the effect of circulatory roadway width also into consideration. The circulatory roadway width has a diverse effect on entry capacity at different levels of circulating flow. Modification of inscribed circle diameter and circulatory roadway width is suggested as a potential solution for improving entry capacity. Sensitivity analysis is carried out to quantify the effect of variation of entry angle, circulatory roadway width, inscribed circle diameter and approach width on entry capacity based on the non-linear model. The sensitivity plots can be used to make subtle geometric modifications to improve capacity at congested roundabouts.

Investigating Roundabout Properties and Bicycle Accident Occurrence at Swiss Roundabouts: A Logistic Regression Approach

The positive effects of active mobility on mental and physical health as well as on air quality are widely acknowledged. Increasing the share of active travel is therefore an aim in many countries. Providing bicycle-safe infrastructure is one way to promote cycling. Roundabouts are a common traffic infrastructure and are supposed to facilitate safe and smooth traffic flow. However, data on road traffic accidents indicate an over-proportional involvement of cyclists in accidents at roundabouts. In the present study, the influence of roundabout geometry and traffic flow on bicycle accident occurrence was investigated using a logistic regression approach on twelve parameters of N = 294 mostly small- and mini-sized single-lane roundabouts in the Canton of Berne, Switzerland. Average weekday motorized traffic was identified as a major factor in explaining bicycle accident occurrence at roundabouts. Further, the radius of the central island, the location of the roundabout (in town vs. out of town) and the number of legs were significantly related to bicycle accident occurrence. While these results are in general agreement with findings from similar studies, the findings regarding the central island’s radius and the number of legs underpin the need for roundabout type-specific studies: Some parameters may not prove relevant in intermediate- to large-sized roundabouts, but become critical in small or mini roundabouts, which are common in Switzerland and numerous in the present sample.

Development and validation of a roundabout entry capacity model considering pedestrians under Japanese conditions

Besides circulating flow, pedestrian flow is another key conflict stream that significantly influences roundabout entry capacity. Due to limited space in Japan, several standard designs such as a physical splitter island cannot always be installed at the roundabout entry/exit. The impact of several influencing factors on entry capacity such as splitter islands was examined by applying a microscopic simulation to our previous research. A regression model was proposed based on the simulation results, which assumed that vehicles yielded to all pedestrians at the beginning of the crosswalk. In this paper, the regression model was validated using field data observed in Japan. The regression model was improved by adding a parameter to reflect drivers' compliance with pedestrians. The validation results indicate that the improved model shows good fitness to the field data and the results imply that roundabout geometry and traffic flow composition significantly influence entry capacity. Language: en

An Analysis of Heavy Vehicle Impact on Roundabout Entry Capacity in Japan

Heavy vehicles behave differently from ordinary passenger cars due to their characteristics such as size and operational performance. At roundabout, entry capacity estimation mainly focuses on passenger cars. In HCM 2010, the impact of heavy vehicles on entry capacity is considered through a heavy vehicle adjustment factor fHV which includes heavy vehicle percentage and passenger car equivalent (PCE hereafter) value. It is generally set as 2.0, which means a heavy vehicle is assumed to be equivalent to two passenger cars. In existing methods, the PCE value at roundabouts is estimated based on several considerations such as entry capacity and move-up time. All of these considerations are influenced by local driver behavior and geometry conditions. Thus, this paper aims to estimate entry capacity considering the characteristics of heavy vehicle behavior in Japan. In this paper, characteristics of heavy vehicle behavior such as speed and gap parameters are firstly analyzed by using data observed at several roundabouts in Japan. Then, a simulation study which is calibrated based on the observation data is conducted by applying VISSIM 5.40. Roundabout entry capacity is estimated through assuming different heavy vehicle percentage values of each entry. Based on the estimated results of entry capacity, PCE value is calculated. The PCE values from the simulation study are compared with those values adopted by several design guidelines. Through the empirical analysis, it is found that heavy vehicles move in lower speed than passenger cars in the process of travelling roundabouts. The headways which are formed by heavy vehicles are commonly greater than those only formed by passenger cars. Then, PCE values which are calculated from simulation output are shown to change with different combinations of heavy vehicle percentage of each entry. The results in this paper provide a reference for roundabout entry capacity estimation considering heavy vehicle behavior in Japan. Future works are some further analysis from the viewpoint of geometry, since heavy vehicle behavior is significantly influenced by roundabout geometry such as inscribed circle diameter and circulatory roadway width, particularly at compact roundabout which is often the case in Japan.

Driver Behavior Characterization in Roundabout Crossings

Roundabouts are widely accepted for their safety, capacity and environmental advantages. Although it can be easily recognized that the driver behavior is mostly related with the roundabout geometry.This paper presents a detailed characterization of the driver behavior while crossing three consecutive double-lane roundabouts in an arterial road.Driver behavior was described in three main levels: i) speed profiles; ii) lateral acceleration profiles; iii) roundabout geometry. It is shown that roundabouts can substantially reduce speed in the negotiation zone. The entry speed and influence zone depends on the desired speed in the upstream sections and on the roundabout geometric characteristics.

Safety analysis of roundabout designs based on geometric and speed characteristics

The study investigates data concerning crashes at roundabouts in order to identify the major factors influencing such events in South Korea. In this investigation, three stages of research tasks were carried out. First, crashes which occurred at South Korean roundabouts were categorized and subsequently compared with international cases. Second, field surveys were conducted in order to investigate how vehicle speeds influence the occurrence of accidents. Third, a statistical analysis was performed in order to investigate the correlation between roundabout geometry and crash occurrences and to reveal major geometric elements of roundabout safety. It was revealed in the study that most vehicles slow down substantially around roundabouts and recover their speed after the roundabouts. Based on the statistical analysis, it was found that negative binomial distribution would give a better performance. In addition, in contrast to cases in the US, the final selection of the independent variables includes as follows: the number of approaches, the number of entering lanes, the entry width, the flare width, the number of circulating lanes and the circulating lane width. Finally, the study provided a model with which to capture the relationship between geometric design elements and the occurrence of crashes at roundabouts. It is anticipated that this model will be applied by engineers to better predict crash occurrences at roundabouts.

Cost - Benefit Analysis for the Implementation of Four-arm Roundabouts in Urban Areas

The implementation of a roundabout has a mostly positive effect on the safety level of a treated site. This is based on the fact that the roundabout's geometry reduces the number of collision points, decreases the speed of vehicles, and improves the safety of pedestrians. Within the frame of the ROSEBUD project1, the cost - benefit analysis for the reconstruction of four-arm intersections into roundabouts has been calculated, and the results confirm that these reconstructions are effective and have positive effects on the reduction of accidents. The article describes this calculation on roundabouts in the Czech Republic.

Comparing Safety Performance Measures Obtained from Video Capture Data

Safety performance is evaluated by using measures obtained experimentally from a videotaping of traffic operations at an urban roundabout situated in Cosenza, Italy. Five different expressions of safety performance from the perspective of “rear-end” vehicle interactions include maximum deceleration rate to avoid a crash (DRAC), time-to-collision (TTC), proportion of stopping distance (PSD), time integrated time-to-collision (TIT), and crash potential index (CPI). Differences in safety performance are discussed with respect to the type of measure, traffic conditions, and variations in roundabout geometry. The results of this analysis suggest that safety performance is highly sensitive to the way it is measured; different measures can highlight different locations or geometric features of the roundabout as posing potential safety problems. This study underscores the usefulness of safety performance measures for providing meaningful experimental indicators of potential safety problems at roundabouts and how ...

Influence of the Roundabout Geometry on Its Safety

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The effect of roundabout design features on cyclist accident rate

Roundabouts are known to result in fewer traffic accidents than traditional intersections. However, this is to a lesser degree true for bicycles than for vehicles. In this paper, we aimed at establishing statistical relationships through Poisson regression and logistic regression analyses between yearly rate of cyclist accidents on one hand and roundabout geometry, age and traffic volume (vehicles and cyclists) on the other. We related all roundabout cyclist accidents recorded by the hospital emergency department of the town of Odense, Denmark, through the years 1999–2003 ( N = 171) to various geometric features, age and traffic volume of all roundabouts on the Danish island of Funen ( N = 88). Cyclist and vehicle volumes turned out to be significant predictors in most of our models—the higher the volumes, the more accidents. Moreover, potential vehicle speed was a significant predictor, and so was age of the roundabout—older roundabouts related to more accidents and higher accident probability. Excluding 48 single cyclist accidents strengthened the relationship between accidents on one hand and vehicle and cyclist volume and potential vehicle speed on the other. This stresses the significance of speed and traffic volume for traffic accidents with more than one partner involved. The 48 single cyclist accidents were significantly related to the traffic volume of cyclists only. Due to our limited number of observations, the models should be regarded as indicative.

Optimizing geometric design of roundabouts: multi-objective analysis

The main objectives of roundabout design are to maximize traffic safety and operational efficiency. Traditionally, because of the complexity of the system and the multiple objectives involved, the design process is iterative and time-consuming. A minor change in the geometry can result in significant changes in the system performance (operation and safety). This paper presents an optimization model that directly provides the roundabout geometry that optimizes two objectives: design consistency and operational efficiency. Design consistency is represented by the mean difference in operating speeds for various conflicting vehicle paths and operational efficiency is represented by the average roundabout delay. Vehicle paths (through, right, and left) and roundabout delay are modeled for all roundabout approaches. The input geometric data to the model can be easily obtained from an aerial photograph of the selected site using a geographic information systems (GIS) software. The system performance is optimized subject to geometric and traffic constraints. The proposed model is applicable to single-lane roundabouts (urban and rural) with four legs intersecting at right angles. Application of the model to an actual proposed roundabout site is presented. This proposed approach provides the optimum solution directly and is also more efficient than the traditional iterative approach. Key words: geometric design, roundabouts, horizontal curve, radius, optimization, consistency, capacity, traffic delay.