A merging model for motorway traffic
Motorway merging has long been regarded as a major source of conflicts and congestion on motorways. Traditional studies of merging behaviour are based on gap acceptance models developed mainly for urban intersections, which tend to oversimplify the very complex dynamic interactive merging behaviour involved. It is believed that this research represents the first comprehensive investigation and modelling of dynamic merging interactions at motorway on-ramps. Emphasis has been given to improving the modelling of merging behaviour and in particular to capture the cooperation between the merging and motorway traffic. This research has developed a feasible integrated microscopic simulation framework to model the interactions among traffic in motorway merging sections. This has been achieved by developing an integrated model (MergeSim) consisting of two sub-models working in tandem: a car-following and a merging model. By assuming different reaction times for different driver states (alert, non-alert and close-following), the new car-following model is shown to be able to capture traffic breakdown, hysteresis, shockwave propagations and close-following situations. The merging model is developed to capture both the acceleration and gap acceptance behaviour of the merging traffic, and the cooperative behaviour of the motorway traffic. The merging model is composed of several sub models: for the traffic in the motorway nearside lane, there is a cooperation model to simulate the cooperative lane-changing and courtesy yielding behaviour and the interactions with the merging traffic; for the merging traffic in the acceleration lane, there are models such as acceleration model, gap selection model, gap acceptance model and a merge model. Sensitivity tests have shown that the integrated model can reasonably replicate all relevant behaviour of individual drivers in merging areas such as normal carfollowing, close-following, cooperative lane-changing, courtesy yielding and gap acceptance. The sensitivity tests on the different merging lengths showed that increased length might reduce merging failures (i. e. the occurrence that the merging driver fails to move into the motorway before reaching the end of the acceleration lane). It can be explained that more merging traffic can successfully take the following gaps with increased merging lengths, which has implications for the geometric configuration of the acceleration lane. The study also established a general calibration and validation framework designed for real-world applications in highway networks using the most readily available traffic surveillance data, the loop detector data. Currently no commonly agreed bench-marking procedure exists (Brockfeld et al., 2005), and this framework has the advantage that the concept and the proposed methodology are suitable for general application to other micro-simulation models using detector data sets. In conclusion, the integrated simulation model (MergeSim) can reliably be used as a tool for further studies and investigations into the effectiveness of techniques related to motorway merging operations.