Constructing spatiotemporal speed contour diagrams: using rectangular or non-rectangular parallelogram cells?

A spatiotemporal speed contour (SSC, or time-space traffic) diagram that exhibits traffic dynamics in time and space is of importance in transportation research and applications. This paper empirically investigates the feasibility of using non-rectangular parallelogram (nRP) cells to construct the SSC diagram, given complete trajectory and probe vehicle data. Compared with the traditional way of using rectangular parallelogram (RP) cells, using nRP cells further considers the direction of backward-moving wave. The trajectory dataset provided by Next Generation Simulation is employed as the data source, and travel time is taken as a metric to measure the quality of the constructed SSC diagram. Various comparisons demonstrate that using nRP cells outperforms using RP cells, in particular when the traffic is congested and the cells are relatively large. Therefore, we recommend using nRP cells to construct the SSC diagram in future, which is as simple as the traditional way but more accurate particularly in terms of estimating travel time.