Determinants of schedule-based transit performances in the city of Bologna

In this paper, the results of a research regarding the transit network of the city of Bologna are presented in order to highlight those factors affecting the reliability of a transit service at high frequency. In literature, several authors relate bus service reliability to the variability of travel time performance (Polus, 1978; Sterman and Schofer, 1976); others with the adherence to schedules (Bates et al., 2001), others to the maintenance of the designed service frequency (Janos and Furth, 2002), others with the waiting time at the bus stops (Bowman and Turnquist, 1981), and others to the impossibility of getting on the bus because of capacity issues (Chapman, 1976). Anyway, the reliability of a transit service is a key element both for users - real and potential ones - and providers. Low service reliability implies the reduction of the service quality perceived by users causing an increase of the amount of time passengers must wait at a bus stop as well as the comfort reduction due to vehicles’ overcrowding (Bates et al., 2001; Rietvield et al., 2001; TCRP, 1999; Prioni and Hensher, 2000; Welding, 1957; Turnquist, 1978). From providers perspective, low service reliability means a reduction of the veh-km delivered and an inefficient utilization of the fleet: some buses travels full of passengers whereas others are empty. This implies the need of additional vehicles to be provided, thus increasing the operating costs. Moreover, as reliability decrease, public transport demand will decrease too thus engendering a lower income for the service provider. The reliability is strictly related to the bunching phenomenon. Bus bunching refers to a group of two or more transit vehicles scheduled to be evenly spaced along the same route, which run in the same location at the same time. This occurs when at least one of the vehicles is unable to keep its schedule because of the interaction with other vehicles of the traffic steam, presence of signalized intersections along the route, demand increase, etc., which implies a bus to be early/late with respect to the planned schedule. Theoretically, the driver should try to adjust the bus speed in relation to the gap with the preceding bus; although it represents one of the most important hypothesis of bunching simulation models, this procedure, which requires driver knows the position of the preceding vehicles, is not usually adopted (Nagatani, 2001). This research provides a general analysis based on reliability, identifying and discussing its determinants. The relative importance of each determinant is analyzed through a survey conducted along two of the main bus routes of the Bologna transit network. This analysis could allow providers identifying appropriate strategies, preventive or corrective ones, in order to improve the service reliability.