Dynamic network loading (DNL) model is concerned with moving traffic in space and time along road network links in dynamic traffic assignment (DTA) models. DNL models strive to build in traffic realism such as modelling transient queues and spillback to upstream links, yet they need to remain computable. Most models in the literature are skewed towards either realism or computability and thus leave a wide scope for further research in arriving at a balanced model. This research proposes a new DNL model called the Two-regime transmission model (TTM) based on widely accepted first-order traffic flow theory. The TTM aims to be quick and accurate enough for planning purposes, when embedded into the framework of a DTA. The TTM considers the time-dependent density states of network links over two regimes namely, free-flowing and congested regimes, and dynamically models the time-dependent queue length, but without the need to break the link into cells. This article sets out the theoretical background necessary for developing the TTM and it also illustrates the principles with the help of a simple network serving a single OD pair. Although the numerical tests are only preliminary indicators, the TTM has been found to produce promising results, for example producing results that are apparently closer than the cell transmission model (CTM) to predicting the dissipation and formation of a queue in a homogeneous link for the same level of time discretisation. We believe that our work establishes TTM as a candidate worthy of future exploration, especially for representing plausible, first-order traffic dynamics within a dynamic user equilibrium model with a lower number of variables/side-constraints than the CTM.
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