A comparison of the use of the cell transmission and platoon dispersion models in TRANSYT 13

<title>Abstract</title> The TRANSYT software, developed by the UK's Transport Research Laboratory (TRL), has been used for optimising signal timings in signalised networks for over 40 years. It consists of a deterministic, macroscopic <italic>traffic model</italic> that evaluates the performance index (PI) for any given set of timings <italic>x</italic> and an <italic>optimiser</italic> that seeks to minimise the PI and hence find the optimal timings <italic>x</italic>*. The conventional <italic>platoon dispersion model</italic> in TRANSYT has the well-known weakness of treating queues as if they stacked vertically at the stop-line, and hence does not model ‘blocking back’. However, the latest version, TRANSYT-13, offers an alternative form of traffic model, the <italic>cell transmission model</italic>, which models the spatial extent of queues, and should be more accurate, especially in networks with closely spaced intersections. The paper investigates the sensitivity of the resulting optimal signal timings to the choice of traffic model, through application of TRANSYT-13 to a 6-arm signalised motorway roundabout. The objective is to measure the benefit from using the improved traffic model.