Capacity and Operational Improvements of Metering Roundabouts in Spain

Abstract

Roundabouts show great advantages due to their geometry and their priority system, both in terms of capacity and road safety, compared to other intersections. However, unbalanced flows may be a problem even at medium demand levels. One single approach may cause queues on the downstream approaches as drivers are not allowed to entry the roundabout. If that downstream approach has the highest flow demand, average delay is highly increased; so, the roundabout can be collapsed. This operational problem is derived from one of roundabouts fundamental properties: movements with greater traffic demand cannot be prioritized, unlike signalized intersections. A few solutions may be found, but the most cost-effective measure is traffic regulation by metering signals. The aim of the research is to analyze capacity improvements and operational performance on roundabouts with metering signals using traffic microsimulation, for Spain local conditions. Metering signals are based on creating gaps in the circulating stream in order to alleviate excessive delays on the main approach of the roundabout. Consequently, they are used only on heavy demand conditions during peak hours. On the metered approach, which causes problems for a main downstream approach (controlling approach), metering signals are implemented. The metered approach is stopped by red signals controlled by a queue detector on the controlling approach. So, the controlling approach is benefited by metering signals as drivers can easily enter the roundabout; and the roundabout overall traffic operation is improved. The research was conducted in 5 stages: roundabouts selection; field study; calibration and validation of a traffic microsimulation model in VISSIM; generation of multiple scenarios; and, analysis of the results. Two roundabouts with known unbalanced flow patterns on Valencia (Spain) were selected; traffic counts were carried out to describe their traffic flow patterns, which resulted on limited capacity at both locations. In order to quantify traffic operation improvement on metered roundabouts, a traffic microsimulation model in VISSIM was elaborated. The model was calibrated and validated using critical gaps, queues, delays, and speeds. Afterwards, a metering signal technique was implemented on VisVAP, based on an ad hoc signal control logic design. Almost 400 combinations of design and control metering signals parameters were required to obtain the optimal model (distances to yield line, red and green time settings) which presented the lowest average delay. It could be concluded that, for the same medium-high circulating flow, a higher demand would be able to enter the roundabout (of up to 80% from the controlled approach) with less average delay values (22% and 56% at each studied roundabout) by installing metering signals.