An investigation of flow breakdown and merge capacity on motorways

Recurring congestion in the vicinity of merges on motorways is affecting an increasing proportion of the UK motorway network. In the absence of any clearly definable capacity restriction on the motorway, the mechanisms which lead to instability in traffic flow, and perhaps ultimately to a total breakdown in flow, remain unclear. An understanding of the mechanisms of merging and flow breakdown on motorways could be an important factor in the development of traffic management strategies to relieve the congestion. The study described in this report has attempted to achieve this understanding through a detailed investigation of traffic operations at a number of motorway sites affected by regular commuter peak congestion. This followed a preliminary survey of the national motorway network to provide a general overview of the flow breakdown problem. The main investigation involved the collection and analysis of a very large database of traffic characteristics in an effort to identify the factors contributing to congestion and any inter-relationships that exist. Statistical analyses, using multivariate techniques, have been employed to try to develop predictive relationships between flow breakdown and key traffic parameters. The factors limiting the capacity of merging areas at intersections were similarly investigated. This report describes the traffic survey and data processing techniques used, together with the statistical procedures adopted to analyse the data. The results and main conclusions derived from the study are presented.