Modelling Motorway Merge: The Current Practice in the UK and Towards Establishing General Principles

AbstractMotorway merging has been regarded as a major source of conflicts and congestion on motorways, but has long been recognised as an area in which modelling has been relatively weak. The current traffic models represent the traffic operations at a merge using gap-acceptance approach, with the merging traffic giving-way to traffic on the mainline carriageway and imposing little or no delay to the mainline traffic. The results tend to underestimate the capacity of the merge, overestimate delays to the merging traffic, and underestimate delays and interruptions to the mainline traffic. This paper investigates the current practice in modelling motorway merge in the UK and highlights some major conceptual and methodological differences to those in the US and Germany. Studies based on empirical observations and a microsimulation modelling were carried out to improve understanding of the factors involved in the performance at merge. The results demonstrate the sensitivity of model predictions – and perhaps policy decisions – to the behavioural assumptions made in traffic models. The paper provides simple, practical recommendations to improve the current practice on modelling merge by the consideration of peak flow profile in the estimation of merge capacity, realistic local and variable pcu values for heavy vehicles, and a more confined merge influence area than currently advised. It calls for further empirical and behavioural-based microsimulation analysis to inform the development of more appropriated treatment of merge in traffic models for improved predictions and better investment decisions