Browsing by Author "Silva, Ana Bastos"
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- Capacity of normal and turbo-roundabouts: comparative analysisPublication . Vasconcelos, Luís; Silva, Ana Bastos; Seco, ÁlvaroWhile researchers agree as to the safety benefits of turbo roundabouts, their improvements in terms of capacity and delay remain open to discussion. This is mostly because previous research is based on capacity models that do not fully describe the complex interactions between the traffic streams on multilane roundabouts. This paper proposes a procedure to calculate capacity based on gap-acceptance theory. It addresses the limitations mentioned by accounting for usually disregarded effects such as the dynamic choice of the entry lane and unequal allocation of traffic in the circulatory lanes. Capacities were calculated for a wide range of demand scenarios and it has been shown that only under demand scenarios that are very specific and uncommon in real world networks, associated with very high percentages of rightturning entry traffic, can a standard turbo roundabout be expected to provide more capacity than the equivalent two-lane roundabout. It has also been shown that two lane roundabouts can normally be expected to provide capacities 20 to 30% above those of comparable turbo roundabouts.
- Comparison of procedures to estimate critical headways at roundaboutsPublication . Vasconcelos, Luís; Seco, Álvaro; Silva, Ana BastosThe capacity analysis of roundabouts in Portugal is mostly done using the UK regression method. Due to its empirical and non-explanatory nature, this method has some limitations, particularly for studying innovative layouts, which has recently motivated research in Portugal into the use of capacity methods based on gap-acceptance theory. This paper describes the results of a related project: the estimation of critical headways and follow-up times at Portuguese roundabouts. For this study, gap-acceptance data were collected at six roundabouts, in two cities, and used to estimate the parameters at each entry, for the left and right entry lanes independently. Several estimation methods were used (Siegloch, Raff, Wu, Maximum Likelihood and Logit). The results have revealed important specificities of the methods that have significant effects on the results and therefore on the capacity estimate exercises. The comparison of the estimates with reference values from several countries indicates significant differences, suggesting the existence of relevant driving style differences, which implies that locally calibrated, country-specific, parameters are required for capacity calculations.
- Das rotundas convencionais às turborotundasPublication . Silva, Ana Bastos; Vasconcelos, Luís; Santos, SílviaO conceito de turborotunda surgiu recentemente, na Holanda, como solução alternativa à rotunda normal com múltiplas vias, com o objetivo de resolver os problemas de segurança relacionados com entrecruzamentos no anel de circulação. Embora sejam poucos os estudos científicos que abordam esta matéria, a experiência estrangeira mostra que esta solução assegura elevados níveis de desempenho geral. Nesse contexto, o presente artigo apresenta o conceito de turborotunda e as suas potencialidades comparativamente à rotunda normal, quer em termos de segurança quer de capacidade. Complementarmente, são apresentadas algumas regras básicas de apoio ao seu dimensionamento geométrico e à estimação de capacidades.
- Estimating the Parameters of Cowan’s M3 Headway Distribution for Roundabout Capacity AnalysesPublication . Vasconcelos, Luís; Silva, Ana Bastos; Seco, Álvaro; Silva, João PedroCapacity models based on the gap acceptance theory are widely used in unsignalised intersections and roundabout capacity analysis. These models are based on the statistical distribution of major vehicle headways. In this field, Cowan’s M3 distribution is usually recognized as the most adequate, but the estimation of its parameters is not trivial. In this paper, the main estimation methods are reviewed and a new method (Simultaneous Numerical Estimation – SNE) is proposed. The SNE method was used to develop a calibrated relation between parameters, using field data collected in Portuguese roads and roundabouts. It was determined that the new formula improves capacity estimates, either in one-lane or in two-lane roundabouts. The paper also addresses the importance of each input variable and parameter in the resulting capacity model, through a sensitivity analyses.
- Microssimulação aplicada aos estudos de acessibilidadePublication . Silva, Ana Bastos; Vasconcelos, Luís; Correia, Gonçalo Homem; Santos, SílviaA necessidade de salvaguardar um bom nível de funcionamento do sistema global de transportes constitui um dos maiores desafios que se coloca às entidades gestoras e ao qual apenas os modelos de simulação podem responder. Este artigo procura evidenciar as potencialidades do uso da microssimulação, recorrendo a um estudo de caso real que envolve uma zona de expansão urbana em Coimbra. Apresenta-se a metodologia de trabalho adotada, o desenvolvimento do modelo, os métodos de calibração e validação do modelo e os principais resultados da aplicação.
- Novos modelos de estimação de capacidades em rotundasPublication . Vasconcelos, Luís; Santos, Sílvia; Silva, Ana Bastos; Seco, ÁlvaroEste artigo apresenta uma análise comparativa de modelos de capacidades aplicados a rotundas, suportada por uma base de dados real. O estudo demonstra que os modelos convencionais – TRL e Siegloch – são incapazes de produzir estimativas fiáveis em toda a gama de variação dos fluxos conflituantes. Por outro lado, o modelo de Hagring, associado a um novo método de parametrização dos intervalos nas correntes prioritárias, permite diferenciar as condições de circulação nas diferentes vias de entrada e de circulação e revela-se capaz de estimar com fiabilidade a capacidade de soluções sujeitas a repartições desequilibradas do tráfego direcional.
- Turboroundabouts: Assessment of Intersection Capacity, Safety, and EmissionsPublication . Vasconcelos, Luís; Silva, Ana Bastos; Seco, Álvaro; Fernandes, Paulo; Coelho, MargaridaA “turboroundabout” is a variation of the conventional multilane roundabout in which spiral road markings and raised lane dividers force drivers to follow a specific path according to their intended destination. This geometry eliminates weaving and cut-in conflicts by guiding drivers continuously from entry to exit. Turboroundabouts were conceived with the main aim of improving safety, but their practical benefits are relatively unknown. Likewise, the few existing studies on turboroundabouts do not allow definitive conclusions to be drawn about the delay and emissions performance characteristics of turboroundabouts; further research is needed. This research focused on the use of appropriate modeling methodologies to understand the effects of turboroundabouts on capacity, safety, and emissions in comparison with the effects of conventional singlelane and double-lane roundabouts. The results indicate that turboroundabouts have capacity levels comparable to those of two-lane roundabouts but are less robust concerning the directional split of the entry traffic; turboroundabouts lead to fewer traffic conflicts, but the traffic conflicts that do occur are more severe. The results also show that the implementation of turboroundabouts provides no advantages for emissions when the main concerns are carbon dioxide and oxides of nitrogen.
- Validation of the Surrogate Safety Assessment Model for Assessment of Intersection SafetyPublication . Vasconcelos, Luís; Neto, Luís; Seco, Álvaro; Silva, Ana BastosThe surrogate safety assessment model (SSAM) is a software application that reads trajectory files generated by microscopic simulation programs and calculates surrogate measures of safety. This approach eliminates the subjectivity associated with the conventional conflict analysis technique and allows assessment of the safety of a facility under a controlled environment before accidents occur. The specific goal of this research was to validate SSAM as a tool for accident prediction at urban intersections. Two methods were used for validation. The first method compared the simulated number of conflicts from the use of SSAM and the predicted number of injury accidents from analytic models in three reference intersection layouts (four-leg priority intersection, four-leg staggered intersection, and single-lane roundabout). The second method compared SSAM results with conflicts observed on site in four real intersections: two priority ones and two roundabouts. The results indicate that, despite some limitations related to the nature of current traffic microsimulation models, SSAM analysis is an extremely promising approach to assessing the safety of new facilities or innovative layouts.