Towards a Generic Guidance for Modelling Motorway Merge

Towards a Generic Guidance for Modelling Motorway Merge


R Liu, ITS, University of Leeds, UK; G Hyman, Department for Transport, UK


This paper presents a synthetic analysis of the current practice in modelling motorway merge, highlights the inconsistency among government advices, and investigates the use of microsimulation to inform the development of better treatment of merge.


Merging of traffic on motorways has been regarded as a major source of conflict and congestion. As the merging traffic accelerates on the slip road, they are constantly looking for safe gaps to merge onto the motorway. Sometimes, they may merge into smaller gaps to avoid running to the end of the slip road and force the motorway traffic to decelerate, which in turn may cause traffic congestion and possibly flow breakdown. Aware of the approaching merging traffic, many drivers on the mainline carriageway may move to the right (UK driving conditions) and/or creating a larger gap for the merging traffic.

Concerns have been raised in recent years about the adequacy of conventional traffic modelling techniques representing the complex interactions that occur at motorway merges and the advice given by the government agencies on the many

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 motorway traffic.

Recent research effort to improve this situation has tended to focus on the development of microscopic simulation models to explicitly represent the behavioural interactions at merges, notably the effort by US FHWA through its NGSIM (Next Generation Simulation) programme to develop microsimulation algorithms to model the interactive behaviour between the two streams of traffic at merge. In the UK, there are similar efforts in developing explicit microsimulation models of motorway merge.

This paper presents findings from a recent study funded by the UK Department for Transport to improve the current modelling of merges. It takes a fresh look at the general principals in modelling traffic interaction at merges and the methodologies to represent merge performance in simple terms and readily measureable variables. It highlights some major conceptual and methodological differences in the government advices in the UK, US and Germany on the treatment of merges. It further investigates the use of the behaviour-based microsimulation model to improve understanding of the factors involved in the performance at merges and to inform the development of more appropriate treatment of merges in traffic models.


Association for European Transport