Evaluation of traffic performance at roundabouts

Dissertação de mestrado integrado em Engenharia CivilRoundabouts have evolved into one of the most common forms of crossings. Their success in resolving traffic issues and enhancing junction performance has been critical to their adoption. In ancient times, roundabouts were the subject of study by many scientists who transformed the traffic principles by which they were governed and today they are considered one of the most widely implemented alternatives worldwide. Speed reduction, greater safety, shorter wait times, and higher capacity as compared to any other at-grade junction. These characteristics have resulted in their deployment all over the world. This dissertation aims to study and compare the performance of various models to assess the capacity of a roundabout. Thus, a comparison of the capacity calculation models developed in different countries to estimate the performance of roundabouts is done, relying its assessment on numerous elements ranging from geometric attributes to driving behaviour. Furthermore, several performance measures such as delay, degree of saturation, and others are evaluated, which will allow comparing empirical, analytical and simulation models. In this work, a microsimulation model for roundabouts with the application of the PTV VISSIM software is applied. Vissim is considered a tool that simplifies road network research and analysis. allowing to analyze the performance of a roundabout through the evaluation of several indicators in different parts of the roundabout such as entries, exits and conflicting carriageway. Finally, a case study of the roundabout performance in the city of Guimaraes, Portugal, under its geometric and traffic features is done based on real traffic data.As rotundas têm evoluído para uma das formas mais comuns de cruzamentos. O sucesso das rotundas na resolução de questões de tráfego e na melhoria do desempenho dos cruzamentos, em termos de eficiência e segurança, tem sido crítico para a sua adopção. No inicio da implementação de interseções giratórias, as rotundas eram objecto de estudo por muitos cientistas que alteraram os princípios de trânsito pelos quais eram governadas, passando por alterar a ordem de prioridade para quem entrava circulava no interior da rotunda, que alterou completamente o projeto, dimensionamento e avaliação do desempenho deste tipo de soluções. Características como a redução da velocidade, a segurança, a redução dos tempos de espera e a maior capacidade em comparação com outros tipos de cruzamento tornaram as rotundas uma excelente solução para serem implementadas em diversos contextos rodoviários, que resultou na sua disseminação e forte implantação em todo o mundo. Neste contexto, o objectivo desta dissertação é estudar e caracterizar os modelos mais comuns para avaliar o desempenho deste tipo de interseções. Para esse efeito, é feita uma comparação de modelos de cálculo de capacidade desenvolvidos em diferentes países para estimar o desempenho de rotundas, tendo por base a avaliação de diversos elementos geométricos e de circulação do tráfego. No processo comparativo são avaliadas várias medidas de desempenho, tais como o atraso, grau de saturação, comprimento das filas de espera, entre outras. Neste trabalho são comparados modelos empíricos e analíticos com os modelos de microssimulação. Para este efeito, foi desenvolvido um estudo de caso numa rotunda da cidade de Guimarães, Portugal, tendo sido utilizado o software PTV VISSIM, que é considerada uma ferramenta que simplifica o estudo e análise do funcionamento de uma rede rodoviária, particularmente ao nível das interseções. Os resultados mostraram uma grande semelhança entre os modelos de capacidade empíricos e analíticos, assim como, destes com o modelo de microssimulação

Design of the first Italian roundabout with jointed plain concrete pavement

This work presents the results of the overall design of the first Italian roundabout with jointed plain concrete pavement. The examined case study complies with current international standards and practices for geometry of roundabouts and road pavements. The construction of a concrete pavement in an urban roundabout will better manage and slow down fast vehicular flows, and increase traffic fluidity in an important junction, trafficked by heavy vehicles, where maintenance works should be reduced to avoid queues. The design of the roundabout involved several competences for: defining the geometry of the four-arm junction, designing the thickness of the jointed plain concrete pavement both on the circular crown and the arms, studying the mix design of a high resistance concrete. As regard to the pavement, the result of the study was an un-dowelled concrete pavement composed of square slabs laid on a cement concrete subbase and a granular layer. The shape of the slabs has been designed to optimize the structural performance of their material, which is a high strength concrete mix derived from an extensive laboratory test work. In general, the results summarized approaches typical of different design conditions: urban ones for traffic flow and safety needs; high-traffic ones for the chosen pavement type; airport ones for the absence of dowel and tie bars at the joints. Indeed, the article has highlighted that the design process of a concrete roundabout requires multiple analyses to consider various features and correctly re-design anexistingurbanjunction. Its geometrical design,the structural design of the concrete pavement and the theoretical and experimental design of the concrete mix were the main phases of this process and they needed different competences to conduct comprehensive and appropriate analyses

Broadening Understanding of Roundabout Operation Analysis: Planning-Level Tools and Signal Application

In United States, roundabouts have recently emerged as an effective and efficient alternative to conventional signalized intersections for the control of traffic at junctions. This thesis includes two investigations related to the operations of roundabouts. The first investigation examines the ability of a planning-level tool (the critical sum method) to serve as an indicator variable for the results of the Highway Capacity Manual’s average delay per vehicle measure for a roundabout facility; to what extent do the results of one predict the results of the other? The critical sum method was found to accurately predict the HCM average delay per vehicle for low-volume conditions, approximately up to an average delay of 15 seconds per vehicle, but the tool was found to provide inaccurate predictions for higher volume conditions. The second investigation looks at the potential of metering signals on a roundabout facility to transfer excess capacity from a low-volume approach to an adjacent higher-volume approach. The analysis indicated positive results for the theoretical benefits of the metering signal when only placing simulated traffic on two of the approaches, but the results were not duplicated when analyzing more-realistic volume scenarios with traffic on all four approaches. Advisor: John Sangster

An Overview of Converting Kabul’s Traffic Circles and Rotaries to Modern Roundabouts

Circular intersections, in the form of traffic circles and rotaries, were built around the world in the first half of the twentieth century. However, with the rise in traffic flow, regular traffic congestion and accidents forced authorities to look for alternative solutions and, in many cases, signalized intersections became the preferred alternative. In the 1960s, the UK introduced concept of modern roundabouts and a new set of priority rules for the circulating traffic. Since the inception of modern roundabouts, they have enhanced both operational and safety aspects at these locations, even when used to replace large traffic circles/rotaries, proving that circular intersections can still exist in urban transportation networks. Kabul City, the capital of Afghanistan, had more than 30 traffic circles and rotaries where regular congestion is present. Ten of them are recently changed to signalized intersection while the rest remains as either traffic circle or rotary. This report focused on investigating if the option of modern roundabout is a suitable, or even preferred alternative to some of the existing traffic circles and rotaries in Kabul City. The research started with collecting geometric data for existing traffic circles/rotaries, followed by a preliminary analysis where they were scored for modern roundabout conversion suitability, based on initial geometric data of size, location, and shape. An on-line survey and one follow-up interview were conducted with Kabul engineers to characterize existing traffic circles/rotaries and their views on potential conversions to modern roundabouts. Finally, one of the current traffic circles was selected for a more detailed operational analysis via microsimulation, using the tool called PTV VISSIM. Supported by literature, it seems that large traffic circles/rotaries can become a more desired location for modern roundabout conversion and as result as scored higher in terms of suitability in preliminary analysis. In the meantime, the survey conducted with Kabul’s engineer showed either traffic is regulated by a traffic officer or no priority rules are applied in existing traffic circles. It also showed that aggressive driver’s behavior is often noticed due to the lack of strict rules and planning from the authorities. Moreover, the survey showed the skepticism amongst responders in regard to the recent improvements with signalized intersection at traffic circles. Regarding the modern roundabout Kabul engineers were open to the option based on a more detailed data analysis. The Sensitivity analysis with microsimulation tool VISSIM showed that the additional bypass lane further enhances the operation. The results also showed that after certain flow rate thresholds there is dramatic increase in delays and queues. Moreover, when traffic includes higher portion of left turns a modern roundabout becomes more a desirable option. The findings from geometric data, survey, and sensitivity analysis have provided evidence that modern roundabouts should be considered as a potential alternative in Kabul City. However, prior to any implementation, significant data gaps should be closed, so a more detailed analysis can be concluded on their suitability

Anomalous Quality of Service Deterioration at Roundabouts Caused by Rainfall

The paper investigated the extent to which rainfall influences the quality of service delivery at multilane roundabouts using a novel quality of service approach. Quality of service is defined as how well roundabouts operate based on road users and road providers’ perception of service quality. Delay and reserve capacity were used respectively as proxies for road users and road providers’ perception of service quality. The entry and circulating traffic data were recorded continuously for eight weeks under dry, light, moderate, and heavy rainfall weather conditions at each surveyed roundabout, then collated, analysed and compared. Linear regression with dummy variable was used to model the roundabout entry capacity and a corrector factor was added to modify the regression function. The corrector factor considered different entry radii and entry angles of surveyed roundabouts. Multi-criteria quality of service table with travel time as proxy for road users and speed as proxy for road providers’ perception of service delivery was developed from peak traffic data and used to determine the extent of deterioration. The multi-criteria table introduced in the paper is a clear departure from the speed-based criteria used in many studies. The results show a significant increase in time delay and a decrease in reserve capacity relative to rainfall. The paper has concluded that rainfall has an anomalous negative effect on the quality of service at multilane roundabouts. The findings could be used in a variety of ways in traffic management to predict the travel time at roundabouts under rainy conditions and to prescribe speed limits accordingly.</p

Performance Analysis of Roundabouts using Empirical Method for Indian Scenario

With hike in road traffic due to globalization, it has become a necessity to develop a transportation network which could handle the present as well as the future traffic efficiently. To do so, proper designing and analysis of various infrastructures is to be done. One such infrastructure induced in road network to increase the efficiency of intersections by reducing the delay is roundabouts. Presently, as such no robust model is available for Indian traffic conditions to determine the performance of roundabouts. So, an attempt has been made to develop a model to determine performance of roundabouts based on capacity. From the existing global models, it was observed that the geometrics of the roundabout played a crucial role in addressing the roundabout capacity. Thus, with this aspect as foundation, a model for entry capacity was developed for heterogeneity in Indian traffic. For modelling, data from various roundabout with varying geometry and flow properties were selected for including affect due to variations. In all, five geometric elements were observed to have significant impact on capacity. The degree of saturation, determined by taking ratio of the existing mean flow values and estimated capacity was considered as parameter to determine the level of service thresholds using K-means clustering. Most of the entries were found to work at capacity flows, indicating scope for better design aspects. The comparison of developed model with existing models showed that the UK and the Jordan model underestimated the capacity as compared to the developed mode

Modeling pedestrian safety at roundabouts

This study proposes a method for using a human participant in a field experiment to model pedestrian safety at roundabouts in the United States. Studies show that roundabouts are safer for vehicles, but are inconclusive as to whether pedestrians are at greater risk at roundabouts than at signalized intersections. Recent simulations, including virtual reality, can model pedestrian vehicle interaction, but the proposed technique could use real-world data to calibrate these models. Eight hours of video was made to gather data at a signalized intersection and a roundabout. A physical simulation was used to assess the pedestrian’s cross/don’t cross decision. Standard walking pace was simulated at 3.5 feet per second and a disabled pedestrian at half that pace. This study focused on factors such as signalization, approach streams, exit vs. entrance lanes, pace and direction to provide a realistic picture of the cross vs. don’t cross decision. Data showed that slow pedestrians had a significantly higher rate of don’t cross decisions at the roundabout. Roundabouts are thought to be safer for pedestrians than signalized intersections due to a lower number of conflict points, but the confusing multiple streams of roundabout traffic converging on exit lanes and the frames of approaching traffic at roundabout entrances may mean that another concept may be needed to fully capture pedestrian risks. The data on ‘relevant traffic’ showed that pedestrians had to be attentive to almost six times as many approach streams of traffic in the roundabout as in the signalized intersection. The value of this study is four-fold: 1) Future studies could revisit the conflict point at the core of Traffic Conflict Analysis and consider conflict streams as well; 2) Future studies could consider the cross/don’t cross decision as an important data point with which to evaluate the safety of roundabout crossings; 3) Slow pedestrians fared worse in their ability to cross at the roundabout than at the signalized intersection; 4) The human participant in a field experiment method can be a valuable source of data for calibrating pedestrian safety simulation systems

Modeling pedestrian safety at roundabouts

This study proposes a method for using a human participant in a field experiment to model pedestrian safety at roundabouts in the United States. Studies show that roundabouts are safer for vehicles, but are inconclusive as to whether pedestrians are at greater risk at roundabouts than at signalized intersections. Recent simulations, including virtual reality, can model pedestrian vehicle interaction, but the proposed technique could use real-world data to calibrate these models. Eight hours of video was made to gather data at a signalized intersection and a roundabout. A physical simulation was used to assess the pedestrian’s cross/don’t cross decision. Standard walking pace was simulated at 3.5 feet per second and a disabled pedestrian at half that pace. This study focused on factors such as signalization, approach streams, exit vs. entrance lanes, pace and direction to provide a realistic picture of the cross vs. don’t cross decision. Data showed that slow pedestrians had a significantly higher rate of don’t cross decisions at the roundabout. Roundabouts are thought to be safer for pedestrians than signalized intersections due to a lower number of conflict points, but the confusing multiple streams of roundabout traffic converging on exit lanes and the frames of approaching traffic at roundabout entrances may mean that another concept may be needed to fully capture pedestrian risks. The data on ‘relevant traffic’ showed that pedestrians had to be attentive to almost six times as many approach streams of traffic in the roundabout as in the signalized intersection. The value of this study is four-fold: 1) Future studies could revisit the conflict point at the core of Traffic Conflict Analysis and consider conflict streams as well; 2) Future studies could consider the cross/don’t cross decision as an important data point with which to evaluate the safety of roundabout crossings; 3) Slow pedestrians fared worse in their ability to cross at the roundabout than at the signalized intersection; 4) The human participant in a field experiment method can be a valuable source of data for calibrating pedestrian safety simulation systems