Single-lane Roundabouts Research Articles

Effect of traffic roundabouts on accident rate and severity in Arizona

ABSTRACTAlthough roundabouts have been used at many locations around the world, the safety of roundabouts under different conditions has not been fully understood. In this study, 17 roundabouts in five cities in Arizona were evaluated, out of which 11 are single lane and six are double lane. Most of the intersections of single-lane roundabouts were controlled by two-way stop signs before roundabout conversion, whereas most of the intersections of double-lane roundabouts were controlled by traffic signals. Accident data were collected and broken down into five categories: damage without injury, minor injury, nonincapacitating injury, incapacitating injury, and fatality. Equal number of years were used before and after the roundabout conversion at each location. The average rates of accidents, damages without injury, injuries and fatalities per year and per million vehicles were evaluated. It was found that single-lane roundabouts reduced the accident rate, whereas double-lane roundabouts increased the accident rate. The results also showed that single- and double-lane roundabout conversions reduced the severity levels of accidents. Considering accident rate and severity level, warrants need to be developed for roundabout conversion and number of roundabout lanes under different traffic conditions.

Analyses of maximum-speed path definition at single-lane roundabouts

Abstract The process of designing roundabouts is an iterative process through which, in several checks, the design elements of a roundabout get optimized. Existing regulations for roundabouts involve swept path analyses, sight distance analyses and speed analyses of vehicles passing through the roundabout. Speed analyses are done mostly based on two models, Dutch and American. Each of these two models, in their own way takes into account design elements of the roundabouts, and the US model also envisions the construction of vehicle paths through the roundabout. Main assumption of both models is that vehicle paths through roundabouts consist of few connected radii. US models for path definition takes into account safety distances from marked lines and geometric elements (curbs) at the entrance and exit and through roundabout. Experimentally determined elements of the vehicle path through the roundabout, do not correspond to those recommendations. Comparison of the measured speed at the roundabouts and speed calculated according to aforementioned models at several roundabouts in Croatia, showed a significant difference. An experimental research was conducted as a first step in developing a new model for operating speed through roundabouts. The research aimed to define the basic path elements of vehicle movement in the roundabout at which the maximum speed is achieved. Results of the study are presented in this paper.

Traffic flow behavior at a single-lane urban roundabout

In this paper, we propose a stochastic cellular automata model to study the traffic behavior at a single-lane roundabout. Vehicles can enter the interior lane or exit from it via [Formula: see text] intersecting lane, the boundary conditions are stochastic. The traffic is controlled by a self-organized scheme. It has turned out that depending on the rules of insertion to the roundabout, five distinct traffic phases can appear, namely, free flow, congestion, maximum current, jammed and gridlock. The transition between the free flow and the gridlock is forbidden. The density profiles are used to study the traffic pattern at the interior lane of the roundabout. In order to quantify the interactions between vehicles in the interior lane of the roundabout, the velocity correlation coefficient (VCC) is also studied. Besides, the spatiotemporal diagrams corresponding to the entry/exit lanes are derived numerically. Furthermore, we have investigated the effect of displaying signal ([Formula: see text], as the [Formula: see text] decreases, the maximum current increases at the expense of the free flow and the jamming phase. Finally, we have investigated the effect of the braking probability [Formula: see text] on the interior lane of the roundabout. We have found that the increase of [Formula: see text] raises the spontaneous jam formation on the ring. Thus, enlarges the maximum current and the jamming phase while the free flow phase decreases.

Virtual Vehicle-Based Cooperative Maneuver Planning for Connected Automated Vehicles at Single-Lane Roundabouts

This paper presents a distributed maneuver planner for connected automated vehicles (CAVs) built upon the idea of CAVs being able to explicitly share their intentions by creating virtual vehicles (VVs) on the targeted traffic stream. Specifically, this work focuses on the decision-making process concerning the merging maneuver at single-lane roundabouts, for which a traffic model is exploited to assess the suitability of potential merging gaps. An optimization-based trajectory planner is then used to generate collision-free, smooth trajectories to successfully perform the planned maneuver. The algorithm is tested in simulation and its performance is compared with a gap-acceptance-based driving policy. The results show the potential positive impact that the proposed VV-based interaction mechanism along with the traffic-model based cooperative maneuver planner could have on traffic coordination.

Traffic Flow on Single-Lane Road Networks: Multiscale Modelling and Simulation

This paper addresses the problem of modelling and simulation of the vehicular traffic flow in a road network by adopting a measure-theoretic framework. This allows to describe the system from both, a microscopic and a macroscopic point-of-view simultaneously, in the sense that two scales co-exist and are coupled, namely a set of moving agents and an average density. The resulting mathematical model consists of a strongly coupled PDE-ODE system with non-local interactions. Within this work we specify this multiscale model for a single-lane roundabout, that is, an example of a road network. Then, we focus on the computational aspects in order to develop a setup for numerical simulations and to analyse the results of some experiments. This multiscale simulator can reproduce phenomena that characterize the real physical system, e.g. self-organization effects, and thus it can be adopted as an environment for control algorithms benchmarking or as a tool for simulation-based traffic flow control and optimization.

Influence of roundabout capacity enhancement on emission production

Secondary effects of intersections insufficient capacity in urban areas are negative impacts on environment out of acceleration and deceleration of vehicles moving in long queues. The positive influence of increased intersection performance to reduce delays and queues, as well as negative impacts on the atmosphere is presented in this paper. The case study includes two single-lane roundabouts located close to each other in Žilina. Both roundabouts do not comply with the current traffic loads. This results in long queues and delays lasting not just during the peak hours. The solution to this problem is a new type of roundabout – turbo-roundabout. Capacity characteristics of both the current and new state are determined by microsimulation using PTV Vissim software. Obtained main characteristics of traffic flows are used as inputs to establish emission productions of NOx, CO and HC at the roundabout entries. The paper shows that proposed basic turbo-roundabout provides significant higher capacity performance compared with current state. Waiting times and queue lengths decrease about ten times. Due to this reduction, emission productions decrease about 50-60%.

Design Guidelines for Symmetrical Single-Lane Roundabouts Based on Intersection Sight Distance

AbstractChecking roundabout sight distance is currently performed graphically. This paper develops analytical models for lateral clearance at symmetrical single-lane roundabouts based on intersecti...

Assessing the impact of closely-spaced intersections on traffic operations and pollutant emissions on a corridor level

Traffic lights or roundabouts along corridors are usually installed to address location-specific operational needs. An understanding of the impacts on traffic regarding to highly-congested closely-spaced intersections has not been fully addressed. Accordingly, consideration should be given to how these specific segments affect corridor performance as a whole.One mixed roundabout/traffic light/stop-controlled junctions corridor was evaluated with the microscopic traffic model (VISSIM) and emissions methodology (Vehicle Specific Power – VSP). The analysis was focused on two major intersections of the corridor, a roundabout and a traffic light spaced lower than 170m apart under different traffic demand levels. The traffic data and corridor geometry were coded into VISSIM and compared with an alternative scenario where the traffic light was replaced by a single-lane roundabout. This research also tested a method to improve corridor performance and emissions by examining the integrated effect of the spacing between these intersections on traffic delay and vehicular emissions (carbon dioxide, monoxide carbon, nitrogen oxides, and hydrocarbons). The Fast Non-Dominated Sorting Genetic Algorithm (NSGA-II) was used to find the optimal spacing for these intersections.The analysis showed that the roundabout could achieve lower queue length (∼64%) and emissions (16–27%, depending on the pollutant) than the traffic light. The results also suggested that 200m of spacing using the best traffic control would provide a moderate advantage in traffic operations and emissions as compared with the existing spacing.

Urban single-lane roundabouts: A new analytical approach using multi-criteria and simultaneous multi-objective optimization of geometry design, efficiency and safety

Building safe and effective roundabouts requires optimizing traffic (operational) efficiency (TE) and traffic safety (TS) while taking into account geometric factors, traffic characteristics and local constraints. Most existing simulation-based optimization models do not simultaneously optimize all these factors. To capture the relationship among geometry, efficiency and safety, we put forward a model formulation in this paper. We present a new multi-criteria and simultaneous multi-objective optimization (MOO) model approach to optimize geometry, TE and TS of urban unsignalized single-lane roundabouts. To the best of our knowledge, this is the first model that uses the multi-criteria decision-making method known as analytic hierarchy process to evaluate and rank traffic parameters and geometric elements of urban single-lane roundabouts. The model was built based on comprehensive review of the research literature and existing roundabout simulation software, a field survey of 61 civil and traffic expert engineers in Croatia, and field studies of roundabouts in the Croatian capital city of Zagreb. We started from the basis of Kimber’s capacity model, HCM2010 serviceability model, and Maycock and Hall's accident prediction model, which we extended by adding sensitivity analysis and powerful MOO procedures of the bounded objective function method and interactive optimization. Preliminary validation of the model was achieved by identifying the optimal and most robust of three geometric alternatives (V.1-V.3) for an unsignalized single-lane roundabout in Zagreb, Croatia. The geometric parameters in variant V.1 had significantly higher values than in the existing design V.0, while approaches 1 and 3 in variant V.2 were enlarged as much as possible within allowed spatial limits and Croatian guidelines, reflecting their higher traffic demand. Sensitivity analysis indicated that variant V.2 showed the overall highest TE and TS across the entire range of traffic flow demand and pedestrian crossing flow demand at approaches. At the same time, the number of predicted traffic accidents was similar for all three variants, although it was lowest overall for V.2. The similarity in predicted accident frequency for the three variants suggests that V.2 provides the greatest safety within the predefined constraints and parameter ranges explored in our study. These preliminary results suggest that the proposed model can optimize geometry, TE and TS of urban single-lane roundabouts.

Estimation of Passenger Car Equivalents for single-lane roundabouts using a microsimulation-based procedure

Passenger car equivalents for heavy vehicles are required to carry out capacity calculations and perform operational analysis of any road entity (roadway segments or intersections). At single-lane roundabouts, the constraints to the vehicular trajectories imposed by the curvilinear geometric design and the driver's gap acceptance behaviour are expected to produce an impact of the heavy vehicles on the quality of traffic flow different from that produced on freeways and two-lane highways or other at-grade intersections. This is also because entering flow is opposed by the circulating flow which has priority and travels in an anticlockwise direction on a single-lane path around the central island.This paper addresses the question of how to estimate the passenger car equivalents for heavy vehicles on single-lane roundabouts. First, a comparison was performed between the empirical capacity functions based on a meta-analytic estimation of the critical and the follow up headways and the simulation outputs manually obtained for a single-lane roundabout built in Aimsun microscopic simulator. A genetic algorithm-based calibration procedure, therefore, was used to reach a better convergence between the simulation outputs and the empirical capacities. Based on the calibrated model, the passenger car equivalents were determined by comparing the capacity functions built for a fleet of passenger cars with the capacity functions calculated for different percentages of heavy vehicles. Differently from HCM 2010 which assumes a heavy vehicle to be equivalent to two passenger cars and sets as 2.0 the passenger car equivalents for heavy vehicles for roundabouts, a higher PCE effect would be expected on the quality of traffic conditions when the traffic stream contains a high number of heavy vehicles; this effect should be accounted for when calculating capacity and level-of-services.

Investigating the role of exiting vehicles and turn indicator usage in gap acceptance at single-lane roundabouts

Roundabouts are becoming a preferred form of intersection control type due to their safety and capacity benefits. Yield control at roundabout entries inherently reduces fuel consumption and emissions when compared to other conventional intersection types. However, these benefits can be limited by poor driver behavior and judgment when entering the roundabout. This research addresses the nature of gap distributions and use of turn indicators by exiting vehicles at three single-lane roundabouts in Vermont, New York, and Alaska. Presented here is a comparison of vehicle headways measured at two different locations in each roundabout. Rejected headways are analyzed in the context of priority abstaining events when entering drivers yield to vehicles exiting on the same leg of the roundabout suggesting that “true” critical gaps are being overestimated. Results indicate that exiting vehicles, particularly those that do not use their turn indicators when departing from the major-stream of traffic have an influence on the entry decision of drivers on the same approach. This behavior is of particular concern for intersection efficiency (i.e., delay) and sustainability (i.e., excess fuel consumption and emissions). Results prompt the consideration of more consistent guidance on and enforcement of turn indicator use during roundabout negotiations.

Operational Analysis of a Single-Lane Roundabout with a Mix of Driverless Vehicles

Driverless cars (DLCs) are vehicles capable of managing several driving tasks by themselves under different degrees of human intervention. DLCs are able to maintain shorter headways in entering a roundabout and circulating in the roundabout. The Highway Capacity Manual (HCM) provides a regression-based gap acceptance model for estimating the capacity of roundabouts. The gap acceptance model captures driver behavior, which is characterized by headways. As automotive manufacturers and technology companies develop driverless capabilities for roadway vehicles, DLCs at various traffic compositions will likely have different effects on the traffic operation of roundabouts. This paper presents the operational analysis for a single-lane roundabout with driverless vehicles under different traffic compositions. Considering the uncertainty of the technological capabilities and the legal and behavioral acceptability of DLCs, sensitivity analyses were conducted. The simulation results for one sample case study show that when the roundabout operates at a Level of Service B (LOS B) or better, DLCs will not have an effect on either the capacity or the control delay. A reduction in control delay will occur when DLCs are set with shorter headways (as assumed for the connected DLC) under low traffic composition (1% to 10%) on a single-lane roundabout operating near capacity. When the share is below 1%, the effect of DLCs is very small. When the LOS of the roundabout approaches E, more reduction in delay is expected when the portion of DLCs in traffic increases.

Case Study: Capacity Characteristics Comparison of Single-lane Roundabout and Turbo-roundabouts

Turbo-roundabouts are new type of roundabouts that recently become more popular for their significant advantages. Due to their specific geometry of spiral leading circulatory lanes and raised lane dividers provide a high level of traffic safety while a relatively large capacity maintain. To compare with conventional double-lane roundabouts, turbo-roundabouts ensure above all a reduced number of conflict points moreover without waving and lower driving speed through the intersection. Challenge of this paper is to transfer existing single-lane roundabout into configuration models of basic and egg turbo-roundabouts and compare their capacity characteristics. Case study is placed in Zilina, the city on the north of Slovakia, where the single lane roundabout has a great congestion problem not just during peak hours. Configurations of the roundabout are evaluated and compared from their performance parameters using microsimulations and theoretical models. For calibration procedure of microsimulation, complex data obtained from survey of existing roundabout were used and derived as input variables for two new configurations. This example is to demonstrate the capacity increase of turbo-roundabouts comparing with single-lane roundabout and the comparison of microsimulation model and theoretical models to determine capacity of turbo-roundabouts.

An Analysis of Characteristics of Heavy Vehicle Behavior at Roundabouts in Japan

Abstract Heavy vehicles behave differently from ordinary passenger cars due to their characteristics such as larger size. Thus, it should be carefully considered when designing a road facility or evaluating its performance. This paper aims at analyzing the characteristics of heavy vehicle behavior at roundabouts under the Japanese situations. By using field data observed at a single-lane roundabout in Japan, an empirical analysis on headway parameters was conducted for examining the characteristics of heavy vehicles. It was confirmed that the headways which are formed by heavy vehicles are commonly greater than those formed by passenger cars only. The results in this paper are expected to be used for roundabout performance evaluations such as roundabout entry capacity estimation.

Capacity Limits of Basic Turbo-Roundabouts

Turbo-roundabouts with their specific design characteristics have become a suitable solution of a single-lane roundabouts capacity problem and a standard two-lane roundabouts safety problem. There is no doubt that turbo-roundabouts ensure a higher level of traffic safety in comparison to standard two-lane roundabouts due to a reduction of both conflict points and driving speed through the turbo-roundabouts. The question is if turbo-roundabouts can provide the same or a higher capacity than standard two-lane roundabouts. The answer, in the form of some important results of the basic turbo-roundabout total capacity at various traffic conditions and also in comparison with standard two-lane roundabouts, is presented in the paper. The capacity model is based on the gap-acceptance theory. Driver behaviour characteristics have been respected according to the Slovak technical guidelines. Redistribution of traffic flows into two lanes at entries and circulatory carriageway of roundabouts has been considered in the capacity calculations. An effective use of both lanes (left and right) has been assumed only at basic turbo-roundabout major entries. By contrast, low use of inner (left) lanes by drivers at entry and circulatory carriageway corresponding with real conditions in some countries has been assumed in the standard two-lane roundabout capacity calculations.

Safe roundabouts for cyclists

May roundabouts be safer for cyclists than intersections? How are safe roundabouts designed? This paper tries to answer these questions on the basis of a before-after safety study of conversions of intersections to 255 single-lane roundabouts in Denmark. The before-after study accounts for long-term accident and injury trends and regression-to-the-mean effects. In order to relate safety effects for cyclists of various roundabout design features it is crucial to split the converted sites by speed limit, because safety effects for both cyclists and other road users of converting intersections to roundabouts depend heavily on speed limits on roads entering the converted sites. If speed limits are 70km/h or higher then converting intersections to roundabouts have resulted in bicycle safety improvements in Denmark. Results show that diameter and height of central islands and type of bicycle facilities at single-lane roundabouts have considerable impacts on cyclists’ safety. Central island diameters of 20–40m are safer for cyclists than smaller or larger roundabouts. A central island, which middle is elevated 2m or more above the circulating lane, is safer for cyclists than single-lane roundabouts with lower central islands. Single-lane roundabouts with separate cycle paths, where cyclists must yield to motorists entering or exiting the roundabout, are safer than roundabouts with cycle lanes. Single-lane roundabouts are safer for cyclists than intersections regardless of speed limits when these roundabouts have high central islands and/or separate cycle paths.

Evaluation of Roundabout Capacity Models: An Empirical Case Study

AbstractBased on field data collected at nine roundabouts in Gold Coast, Australia, this paper evaluates the performance of the capacity estimation for single-lane roundabouts using analytical models [including the highway capacity manual (HCM) 2000 model, the German Highway Capacity Manual (GHCM) model, the signalized and unsignalized intersection design and research aid (SIDRA) model and a new roundabout capacity (NRC) model] and an empirical model (the HCM 2010 model). First, this study calibrates critical gaps, follow-up times, and conflicting flows. Compared with the capacities measured in the field, the authors carry out a study to analyze the accuracy of the HCM, GHCM, SIDRA, and NRC models. The results show that the five models underestimate capacity but the NRC model produces a smaller range of relative error (−1.07 to −5.74%) and root-mean-square deviation (47.68) than the HCM 2010 (−4.62 to −16.14% and 105.00, respectively), HCM 2000 (−5.76 to −17.21% and 115.29, respectively), GHCM (−8.95 to −...

Simulation study of interference of crossings pedestrian and vehicle traffic at a single lane roundabout

Abstract This paper proposes a vehicle–pedestrian cellular automata model to investigate the characteristics of the mixed traffic at a single lane roundabout. The roundabout boundary is controlled by the injecting rates α 1 , α 2 and the extracting rate β whereas pedestrians are generated with arrival probability P cr . The pedestrian and vehicle flux are calculated in terms of rates. Also, the phase diagrams of the system in the ( α 1 , P cr ) and ( α 1 , β ) spaces are constructed. The results show that, the gridlock state is reached only when the pedestrians have the right of way over vehicles, while the maximum current state is reached when including a pedestrians’ crossing decision rule. Likewise, we have found that the crosswalk location play a chief role in improving the dynamic characteristics of pedestrian and vehicle flux. Furthermore, it has turned out that the traffic is sensitive to the driver behavior.

Influence of priority taking and abstaining at single-lane roundabouts using cellular automata

Existing roundabout simulation models fail to consider all types of driver behavior which compromises their accuracy and ability to accurately evaluate roundabout performance. Further, these non-compliant driver behaviors, including priority taking and priority abstaining, are inconsistent with existing traffic flow theories. In this paper, a new cellular automata model, C.A.Rsim, is developed and calibrated with field data from five single-lane roundabouts in four northeastern states. Model results indicate that approximately 20% of the individuals in the driver population are inclined to priority taking and approximately 20% are inclined to priority abstaining behavior, though the observed levels of these types of behavior are naturally lower and vary with traffic volume. The model results also corroborate other research indicating that current models can overestimate capacity at higher circulating volumes, possibly a result of the jamming effect produced by priority taking behavior. The reduction in priority abstaining behavior, which is observed at older roundabouts, significantly reduces delay and queue length in certain traffic volumes. C.A.Rsim is also more parsimonious than many existing microsimulation models. These results provide insight on how variations in conflicting flow (i.e., traffic volume and turning movement balance) impact the amount of observed non-compliant behavior.

Assessment of Turbo and Multilane Roundabout Alternatives to Improve Capacity and Delay at a Single Lane Roundabout Using Microsimulation Model Vissim: A Case Study in Ghana

A single lane roundabout characterized by long queues during morning and evening peak periods was chosen as our study site. The objective of this study was to 1) Model and calibrate the vissim simulation model for the roundabout and 2) to model roundabout alternatives to improve capacity and assess the delay. A two hour video data collection was undertaken on a typical morning peak from which the traffic demand and turning movement data were extracted. The vissim micro simulation model was calibrated using the west approach as the target and the analysis was done for the existing single lane roundabout. A Turbo roundabout and a conventional double lane roundabout alternatives were also assessed. The capacity of the single lane roundabout was estimated as 2990 pcu/h and was performing at an ICU level of service H. Average Delay on the west approach was 232 seconds. The intersection capacity was 4392 pcu/h when the turbo roundabout alternative was assessed. Westbound vehicles experienced average delay of 87 seconds (inner lane) and 74 seconds (outer lane). The capacity of the conventional double lane roundabout was estimated to be 3690 pcu/h. The turbo roundabout concept will deliver a comparatively higher capacity and could be the most effective alternative to reduce congestion and delay.