Trucks at Roundabouts: A Synthesis Study

Roundabouts in United States and other countries have been proved to be very successful, effective, and a safe traffic control treatment. Roundabouts in the context of regularly expected truck configuration have also been successful for effective roundabout designs;however, there are always some site specific issues for trucks accommodation at roundabouts. Therefore, there is a great need for an informational guide in the form of synthesis report for roundabout designs specific to unique trucking traffic activity. While there are not significant literatures and studies in this subject, there are few studies/reports spread out which address various issues in this subject area. This study attempts to synthesize the research and knowledge of truck accommodation at roundabouts by synthesizing various research studies, reports, and articles;various subject areas such as roundabout designs for trucking activity, roundabout designs and accommodation strategies for specialized trucking activity such as oversized trucks, truck safety at roundabouts, and motor carrier perspective of trucking at roundabouts have been analyzed and relevant knowledge has been synthesized. This synthesis can be useful o researchers, designers, planners, and decision makers to determine effective ways to address specific trucking related issues at a roundabout.

Integrating Roundabouts with Freight Roadway Networks

The demand for freight transportation in the United States has grown rapidly in the past couple of decades;the rate at which the highway capacity is growing is not able to meet the freight-trans- portation needs, resulting in congestion and delay, ultimately affecting the users and the public with increased prices for the goods delivered, unreliable delivery times, and air-pollution concerns. Freight bottlenecks are a major cause of recurring congestion which accounts for about 40% of total vehicle hours of delay in the United States. Intersections for urban freight-roadway networks are one of the major freight bottlenecks and are considered to be a significant contributor for congestion and delay. Improving the efficiency at urban intersections with high truck traffic can address the freight-traffic congestion, leading to optimized goods movement as well as decreased delays, congestion, and emissions, thus enhancing the air quality in and around the communities. With the roundabout intersection control being proven as a safe, operationally efficient, and environment-friendly control treatment, a greater use of roundabouts with urban freight- roadway networks and their feasibility is analyzed in this study. The control for most urban intersections is a signalized treatment;a performance analysis is conducted for selected signalized intersections in urban freight-roadway networks, comparing the intersections by adapting a roundabout control theoretically using SIDRA (signalized and un-signalized intersection design and research aid) Intersection software. Various parameters, such as the intersection’s level of service, the effective intersection capacity, the average control delay, vehicular emissions [carbon dioxide (CO2), hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx)], the intersection’s annual delay, and the intersection’s annual cost, are selected for comparison.

Evaluation and Improvement of Roundabouts in Changchun

In Changchun, China, roundabouts are fairly common. They are often designed to connect the main roads because of their ability to reduce conflict points, making them safer than other intersections. They can also beautify the city when the central islands are landscaped. However, with increasing traffic, they may not function well and may even paralyze the road network. This means that it is important to evaluate the performance of roundabouts promptly, and to make necessary improvements if required. Using several roundabouts in Changchun as case studies, this article uses V/C ratio and delay to evaluate roundabout performance. Based on the result of evaluation, the micro-simulation model of the poorly-performing roundabout is built and enhancement is proposed.

Delay model of signalized roundabouts

In order to evaluate the service level of roundabouts intuitively, a new delay model of signalized roundabouts is established. According to traffic flow characteristics in the signalized roundabout, vehicle delays on roundabouts are divided into two parts: entrance delay and circulating lane delay. Entrance delay is calculated by the delay model in HCM2010. Circulating lane delay is studied by analyzing conflict behavior in signalized roundabouts, and the delay models of traffic flows turning in different directions are established based on the motorcade analysis method. Finally,the simulation software Vissim, calibrated with field data, is used to verify the model. The simulation results showthat the average relative error of the model is 10. 1%, meeting the accuracy requirements. Therefore, the delay model properly reflects the operation efficiency of a roundabout, and can thus provide a scientific basis for the design of control schemes.

Development of a tracking-based system for automated traffic data collection for roundabouts

Traffic data collection is essential for performance assessment, safety improvement and road planning. While automated traffic data collection for highways is relatively mature, that for roundabouts is more challenging due to more complex traffic scenes, data specifications and vehicle behavior. In this paper, the authors propose an automated traffic data collection system dedicated to roundabout scenes. The proposed system has mainly four steps of processing. First, camera calibration is performed for roundabout traffic scenes with a novel circle-based calibration algorithm. Second, the system uses enhanced Mixture of Gaussian algorithm with shaking removal for video segmentation, which can tolerate repeated camera displacements and background movements. Then, Kalman filtering, Kemel-based tracking and overlap-based opti- mization are employed to track vehicles while they are occluded and to derive the complete vehicle trajectories. The resulting vehicle trajectory of each individual vehicle gives the position, size, shape and speed of the vehicle at each time moment. Finally, a data mining algorithm is used to automatically extract the interested traffic data from the vehicle trajectories. The overall traffic data collection system has been implemented in software and runs on regular PC. The total processing time for a 3-hour video is currently 6 h. The automated traffic data collection system can significantly reduce cost and improve efficiency compared to manual data collection. The extracted traffic data have been compared to accurate manual measurements for 29 videos recorded on 29 different days, and an accuracy of more than 90% has been achieved.

Conversion of Existing Roundabouts into Turbo-Roundabouts： Case Studies from Real World

Compared with roundabouts the main advantages of turbo-roundabouts are the reduction in the number of potential conflicting points and the lower speed of vehicles passing through the intersection, they both can improve safety conditions at the .junction. Moreover, the physical delimitation among lanes limits the side-by-side accident risk. These aspects make turbo-roundabouts more appropriate than roundabouts when a higher level of safety has to be guaranteed, particularly in presence of relevant pedestrian and two-wheels traffic volumes. The present paper has three main objectives： （1） to discuss general design criteria and functional principles of turbo roundabouts, （2） to give the geometric design principles of the central island and circulating lanes and （3） to present three case studies from real world concerning the conversion of existing roundabouts into turbo roundabouts.

Investigating the Possibilities of Specific Crash Type at Roundabouts

A comprehensive analysis of road crashes is extremely supportive in identifying existing deficiencies and in proposing new the design strategies. There are several research studies carried out for improving safety at roundabouts with the aim to altering conflict types, reducing the drivers’ speeds and crash severity, and highlighting a considerable differences in the safety performance of individual or group of roundabouts. The main purpose in this study was to explain the correlation between the types of crashes to the geometry of the roundabouts based on data collected from ten top poorly performing roundabouts in Toowoomba, Australia. Preliminary analysis revealed the following as top three types as prominent at these identified roundabouts such as angle crashes (74%), hit-object crashes (13%), and Rear-end crashes (9%). Excess proportion of specific crash method has been used as methodological tool to estimate the potential possibilities for a specific crash type exceeding threshold at each roundabout. Results show that eight out of ten roundabouts were regarded for a particular type of crash but only two roundabouts were viewed for two types of crashes such as angle and hit-object.

A review of traffic behaviour and intelligent driving at roundabouts based on a microscopic perspective

The contradiction between increasing traffic and the relatively poor roundabout infrastructure is getting stronger.The control and optimization of the macroscopic traffic flow needs to be improved to resolve congestion and safety problems at roundabouts and the connected road network.In order to better understand the gaps and trends in this field,we have systematically reviewed the main research and developments in traffic phenomena,driving behaviour,autonomous vehicles(AVs),intelligent connected vehicles and real vehicle trajectory data sets at roundabouts.The study is based on 388 papers about roundabouts,selected through a comprehensive literature search.The review demonstrates that based on a microscopic perspective,sensing,prediction,decision-making,planning and control aspects of AVs and intelligent connected vehicles can be designed and optimized to fundamentally and significantly improve traffic capacity and driving safety at roundabouts.However,the generation mechanism of traffic conflicts among traffic participants at roundabouts is complex,which is a tremendous challenge for the systematic design of AVs.Therefore,based on naturalistic driving data and machine learning theory,it is an important research direction to build driver models by learning and imitating human driver decision-making and driving behaviours.

Perception Enhanced Deep Deterministic Policy Gradient for Autonomous Driving in Complex Scenarios

Autonomous driving has witnessed rapid advancement;however,ensuring safe and efficient driving in intricate scenarios remains a critical challenge.In particular,traffic roundabouts bring a set of challenges to autonomous driving due to the unpredictable entry and exit of vehicles,susceptibility to traffic flow bottlenecks,and imperfect data in perceiving environmental information,rendering them a vital issue in the practical application of autonomous driving.To address the traffic challenges,this work focused on complex roundabouts with multi-lane and proposed a Perception EnhancedDeepDeterministic Policy Gradient(PE-DDPG)for AutonomousDriving in the Roundabouts.Specifically,themodel incorporates an enhanced variational autoencoder featuring an integrated spatial attention mechanism alongside the Deep Deterministic Policy Gradient framework,enhancing the vehicle’s capability to comprehend complex roundabout environments and make decisions.Furthermore,the PE-DDPG model combines a dynamic path optimization strategy for roundabout scenarios,effectively mitigating traffic bottlenecks and augmenting throughput efficiency.Extensive experiments were conducted with the collaborative simulation platform of CARLA and SUMO,and the experimental results show that the proposed PE-DDPG outperforms the baseline methods in terms of the convergence capacity of the training process,the smoothness of driving and the traffic efficiency with diverse traffic flow patterns and penetration rates of autonomous vehicles(AVs).Generally,the proposed PE-DDPGmodel could be employed for autonomous driving in complex scenarios with imperfect data.

Developing crash modification factors for roundabouts using a cross-sectional method

The objective of the current study was to evaluate traffic and geometric features and their influences on the safety performance of roundabouts by developing suitable crash modification factors(CMFs).The cross-sectional method can be applied as an alternative method to estimate the CMFs when before-and-after studies are impractical to apply,e.g.,lack of data from the period after implementing treatments.To accomplish the study objective,CMFs were derived from generalised linear models(GLMs),i.e.,negative binomial(NB)regression,using data collected on regional roundabouts in Toowoomba City,Australia.Six years of crash data from 49 roundabouts included all recorded crashes as well as traffic and geometric features for the entire roundabouts.Several candidate models were developed using the GLMs.Five models were selected based on statistical significance,goodness-of-fit(GOF)measures,and cumulative residual(CURE)analysis.The results show that increasing the number of entry lanes,entry width,entry radius,traffic volume,circulatory roadway width,weaving width,and speed limit have positive effects on roundabout safety.On the other hand,increasing the number of legs,number of exit lanes,exit width,exit radius,weaving length,central island diameters,and presence of fixed object on a central island have negative effects on roundabout safety.The study shows that quantifying the risk factors can support road safety stakeholders to identify safety improvements at roundabouts more effectively and efficiently.

Effects of calibration process on the simulation of rear-end conflicts at roundabouts

A methodology for calibrating and validating VISSIM simulation model is presented that allows to replicate the observed vehicles conflicts. A roundabout case study has been selected to test the usefulness of a combined approach of VISSIM simulation package and the surrogate safety assessment model(SSAM) for providing reliable estimates of traffic conflicts. Safety performance has been assessed from the field by video-recording vehicle interactions at the roundabout, and then expressed in terms of time to collision(TTC)values.The proposed calibration procedure has been performed by a multistage methodology involving microscopic drivers' car following behavior parameters to enhance the correlation between observed and simulated queue lengths at the roundabout's entries. The calibration procedure is based on a statistical screening of inputs leading to a linear expression relating significant parameters to the queue length. The best estimates of the model's parameters have been determined using a genetic algorithm technique.The spatial distribution of the rear-end conflicts and the TTC values determined by SSAM have been finally compared with the observed ones to analyze the capability of the model of replicating rear-end conflicts.The results suggest to this calibration procedure impacts positively on the estimate of the safety performance measures obtained through the simulation processes.Notwithstanding the good results in the evaluation of the model's accuracy, the simulation seems to fail in reproducing the traffic phenomena linked to unusual driving behavior, and therefore it is not able to replicate forced drivers' maneuvers that can lead to a conflict situation.

nvestigating 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.

Signalization options to improve capacity and delay at roundabouts through microsimulation approach:A case study on arterial roadways in Ghana

Roundabouts are very useful when flows are balanced but can have operational challenges when flows increase.Most roundabouts on arterial roadways in Ghana experience recurring congestion during peak traffic.The four roundabouts between Ejisu Municipality and Asokwa Municipality on the N6 highway are typical of the situation found on arterials countrywide.They experience recurring long queues and delays during the peak hours and over-representation of truck rollover accidents.In Ghana,historically,most roundabouts with capacity problems have been developed into interchanges at very high cost without consideration of other effective at-grade configurations.When roundabouts are signalized,various configurations can be explored to enhance capacity while preserving the safety benefits.Using simulation techniques,various configurations and design options were tested for their effectiveness at two of the roundabouts,located at KNUST police station and Ejisu,with similar demand and high left turning volumes.The objectives of this study were to a)evaluate the performance of the selected roundabouts on the corridor;b)explore alternative intersection configurations to improve capacity and delay.Using video data collection techniques,2-h time stamped footages of AM and PM peak traffic flows were captured for two days.Geometric design parameters of the existing roundabouts,queue lengths,travel times,and approach flows were also captured.VISSIM models of the selected roundabouts were developed and calibrated for the baseline conditions.Signalized unconventional roundabout configuration,with a phase for major road through traffic were simulated and assessed for capacity,delay and queue length.The results showed that signalized roundabout intersections effectively increased capacity by as much as 50%in some cases,reduced delay and queue length significantly,and maintained the benefits of roundabouts including safety.The performance could be further enhanced by introducing separate left turn phases to reduce conflicts or through a right turn U-turn(RTUT)concept.

Experimental analysis of operational data for roundabouts through advanced image processing

Roundabout is still the focus of several investigations due to the relevant number of variables affecting their operational performances(i.e.,capacity,safety,emissions).To develop reliable models,investigations should be supported by devices and relate d sensors to extract variables of interest(i.e.,flow,speed,gap,lag,follow-up time,vehicle classification and trajectory).Notwithstanding that several sensors and technolo gies are currently used for data collection,most of them present limitations.The paper presents the investigation carried out to survey vehicle movem ents at roundabouts as a comprehensive video image analysis system is able to derive the origin/destination(O/D)matrix,compile a vehicle classification,track individual vehicle trajectories together with corresponding speeds and accelerations along paths.To this end,the authors collected video-sequences that were analysed with a piece of software developed for that task.To minimize the problems due to perspective distortion,environmental effects,and obstructions,a number of camera set-up configurations were adopted with equipment being placed on central or external poles,and on permanent fixtures such as raised working platforms outside the confines of the intersection area.Performance of those installation set-ups with different vehicle tracking strategies has been evaluated.Particularly,speed has been successfully related to trajectory tortuosity,the result of which emphasizes the tremendous potential of image analysis and opens up to further studies on the evaluation of the operational effects of roundabout geometrics.

Driver’s Critical Gap Calibration at Urban Roundabouts: A Case Study in China

Roundabouts have been used successfully in cities throughout the world and are wildly used in China, The critical gap is a required parameter for calculating the capacity of roundabouts and deciding the needs for stop signs, While many studies on gap acceptance characteristics at roundabouts have been carried out in America and Europe, few have been done in China, This study was conducted to calibrate the critical gap in a roundabout in China, The differences between critical gaps on different days （workdays and weekends） and during different times （rush hour and non rush hour） were checked. The study was performed at a three-lane roundabout formed by a two-way and four-lane local road and a six-lane divided artery. The data was collected at this site by a video camera. The evaluations included the cumulative probability distributions of the acceptable and rejected gaps which resulted in critical gaps for all cases. The results show that the critical gap during the rush hour is shorter than after the rush hour, but the difference is not large and the cumulative probability distributions can be fit by the logit functional form with parameters estimated using linear regression.

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

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.

The deviation angle for one-lane roundabouts: A general mathematical formulation and application

Properly designed roundabouts may lead to safety improvements based on both reducing approaching speeds and controlling traffic. Measurements of deflection of vehicle trajectories are commonly used to estimate roundabout speed control. One of these measurements is the deviation angle, which is mentioned in both the Italian and Swiss road standards and, in specific conditions, can be more effective than other methods.This article presents a general mathematical formulation for linking several geometric parameters with the deviation angle in different rural and urban one-lane roundabout configurations, which is currently missing in the literature. For urban roundabouts, refuge islands for pedestrians and cyclists were considered. Based on the proposed formulation, a sensitivity analysis of the influential geometric parameters was conducted. Results suggest that an insufficient deflection of trajectories(deviation angle less than 45°) is always present for roundabouts with inscribed circle diameter less than 25 m;for urban roundabouts with refuge islands for pedestrians and cyclists having inscribed circle diameter less than34 m and orthogonal legs;and for roundabouts with angles between opposite legs smaller than 140°. The main parameters which are responsible for a decrease in the deviation angle are: a decrease in the inscribed circle diameter;a decrease in the angle between opposite legs;and an increase in the width of the circulatory lane. Some optimized procedures for roundabout design, the generalized application of the deviation angle method and alternative speed control measures in cases of small deviation angles are discussed.

Analysis of motorcycle microscopic characteristics at roundabouts under mixed traffic condition:A case study of Vietnam

Recently,the demand for comprehension of mixed traffic in developing countries,particularly at roundabouts,which are highly interactive road junctions,has increased.Thus,we analyzed mixed traffic at roundabouts,considering Vietnam as a case study.The two main objectives of this study were to characterize the mixed traffic in Vietnam and to determine the microscopic characteristics of motorcycles at roundabouts.First,efforts were made to clarify the two constitutions of mixed traffic(the performance rule and the presence of small-sized vehicles),and the term“motorcycle-oriented mixed traffic”was defined.Even when satisfying the two fundamental constitutions,this traffic state has unique features,e.g.,only one type of non-lane-based vehicle(the motorcycle)and the predominance of motorcycles in the traffic composition(91.7%).Second,four microscopic characteristics of motorcycles were obtained from a large dataset:the motorcycles’continuous changes in speed,the relationship between the turning angle rate and the speed,the critical gap,and the following space.The relationship between the turning angle rate and the speed was first formulated as a power curve.The critical gap of motorcycles was estimated as a small value(1.25 s)in the case study.The following spaces varied with respect to the speed and had an oval shape.The smallest lateral and longitudinal dimensions were 1.5 and 1.74 m,respectively.While all the findings are meaningful,they are restricted to the case study(Ho Chi Minh City,Vietnam).

A Traffic Simulation Model for Analysing Capacity and Service Level at Signalized Roundabouts

This paper describes signalized roundabouts with GPSS and developa simulation model to assess the traffic performance of the signalizedroundabout and its capacity. Average delay and stop frequency are used to analyze a few signal control strategies. A real small roundabout which is assumed to be signalizedis simulated by this model. The results is inclined to show thatsignalizing theN small round about would not be a good traffic control strategy.

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 fron： singe lane roundabouts that are not heavily congested. For planners and designers interested in building multilane roundabouts for intersections with potential growth i~ 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 DowlinC Road Roundabouts in Anchorage, Alaska, which are currently operating with extensive queues during the evening peak hours. This research used multiple video camcorders t（ capture vehicle turning movements at the roundabouts as well as the progressior~ 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., ~ne entrance has significant higher flow than others） can intensify the queue and delay fo., 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 ca pacity before calibration. With default parameters, SIDRA and VISSIM tend to underes timate 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.