Roundabouts are a widespread form of junction globally, and are particularly common in the UK. This study focuses on the entry capacity at roundabouts under adverse conditions of weather and light in order to determine the extent to which entry capacity is reduced, and the impact that this has on current practical design. So far no specific studies have attempted to quantify this effect. The study was limited to conventional at-grade, normal-size, non-signalised roundabouts in urban areas. Data were collected in 1998 using video capture techniques at four different sites with a total of 13 approaches, comprising 37 entry lanes. The data were transcribed and classified to exclude those minutes of data which did not satisfy the preset traffic criteria, resulting in the extraction and utilisation of 7721 minutes of detailed entry-lane based data. The entry capacities were classified according to whether the conditions were: dry and light; dry and dark; wet and light; and wet and dark. The results identified that wet and dark conditions had a substantial effect on the entry capacity of urban normal-size roundabouts. Entry capacity was found to be reduced by 14% under dry–dark, 17% under wet–light, and 25% under wet–dark conditions. The paper sets out the main practical implications of these findings for design engineers.
The concept of capacity plays a fundamental role within the transportation profession since it is applied in planning, design and operational condition of virtually every road. This paper reports on an in-depth statistical analysis has been carried out for 160 sites across the UK strategic motorway network and 32 sites for trunk roads data to produce a reliable estimation of sustainable lane capacity. Geometric parameters and traffic conditions impacting on capacity have been taken into consideration. The formulation of capacity for motorways and trunk roads has then been produced. The deviation between the link capacities has been found to be significant. It is also found that a number of geometric and locational factors are the explanation to the variations. It is concluded that the average sustainable capacity of a motorway lane is 1780 vehicles/h (2000 passenger car unit per hour with a standard deviation of 130 vehicles/h). The trunk road capacity is evaluated as 1520 vehicles/h with a standard deviation of 170 vehicle/h.
This paper introduces a three layered approach built on robust microsimulation modelling to evaluate the benefits of integrated traffic management (ITM), when combining a MOVA (Microprocessor Optimised Vehicle Actuation) signal with Ramp metering (RM) and access management. The modelling uses additional controller parameters to simulate the coordination of two or more MOVA signal control systems over a relatively simple linkage, by providing an off-set. A case study has been chosen in a motorway environment by using actual data in Birmingham (UK) in response to development pressures. The results were audited through the Highways Agency procedures, were accepted and permission was granted. Signal technologies and advanced microsimulation modelling techniques have been used to manage access at the motorway interface to the local road network. Visual vehicle activated programming in VISSIM (VISVAP) programming techniques have been used to emulate the MOVA traffic signals, RM and the access management. The results show a significant operational benefit. The model predicted that the throughput of the mainline would increase by 9% and the overall average delay per vehicle would reduce by 37%, with the average speeds predicted to increase by 38% following the implementation of the ITM Strategy. A significant reduction in the total stopped time delay of 40.7% is also achieved thereby reducing the flow breakdowns.
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