On the Impact of Bus Dwelling on the Macroscopic Fundamental Diagrams

Recent studies show that a well-defined relationship between space-mean flow and density emerges after spatially aggregating the highly scattered plots of flow versus density from individual loop detectors. Such relationships are now commonly referred to as network Macroscopic Fundamental Diagrams (MFDs). When present, MFDs can be used to model traffic dynamics within an urban network by dividing the network into a set of spatially compact homogeneous regions and tracking the average level of congestion in each region. Existing analytical methods to estimate MFD mostly focus on the behavior of a single type of vehicle and do not capture the patterns of mixed traffic (e.g., cars and buses). The existence of buses matters since a bus will block the movements of other vehicles when it dwells at the bus stop. This study proposes an analytical method to estimate the impact of bus dwelling on a network’s MFD based on a network’s geometric features, traffic control strategies, and bus operation parameters, and validates the performance of the proposed method using simulations based on microscopic traffic models. Comparisons of the analytical and simulation results show that the proposed analytical method can generally provide a good estimate of the lower bound and upper bound of the network’s MFD. The results also reveal that the proposed model works well even in scenarios where bus dwell time varies considerably. In addition, investigations of analytically obtained MFDs reveal that bus dwellings will have greater impacts on the MFD of a network with a shorter block length and fewer lanes.