Unsignalised Vs Signalised Roundabouts Under Critical Traffic Conditions: a Quantitative Comparison
G Bernetti, M Dall'Acqua, G Longo, University of Trieste, IT
The paper deals with the choice among different alternatives as regard the design of urban multi-junction nodes. As known, the final choice depends on a set of evaluation criteria based on performance indices. Performance could be mainly estimated through the computation of capacity, queues, delays suffered by vehicles and the estimation of the Level of Service. In this context this paper focuses on evaluating the performance of roundabouts with or without signal control.
In the first part of the paper a brief review of common methods used to evaluate the roundabout performance is presented, in order to compare their results in different roundabout layouts and in various traffic situations. In particular, the behaviour of methods is analysed in critical traffic situations, such as in conditions near or over capacity. In these cases, when the degree of saturation exceeds a certain level, the performance of the roundabout worsens rapidly, particularly over short periods of time, as well-known in the scientific literature. In this case queues may form and delay begins to increase exponentially and small changes in volume have large effects on the Level of Service. The results given by the application of different methodologies to not-signalised roundabouts are then discussed.
In such critical situations it is quite interesting to take into consideration the possibility of introducing traffic control devices. Of course it is also necessary to evaluate the performance of this signalised node in the same critical traffic conditions. In other words the aim is to estimate the same indices as before in order to perform a right comparison between the two alternatives. A mesoscopic model is used to analyse the behaviour of this particular signalised multi-junction node. The model describes flows in terms of platoons of vehicles moving from signal to signal. Each platoon is identified by few elements and its behaviour depends on the signal plan and on interactions with other platoons. The model uses geometric constructions based on events modifying the system and it preserves information related to platoons crossing the node. The results allow to estimate queues and delays suffered by different traffic flows.
With reference to some existing multi-junction nodes, the results given by the previous approaches are discussed in order to determine if and when a signalised condition could improve the Level of Service of a not-signalised roundabout. Moreover also some quantitative estimations of these improvements are presented.
Association for European Transport