The effect of OD trip dispersion versus concentration in express service design
In public transit systems with high demand levels, the use of express bus services that serve only a subset of stops along certain routes would seem to be a promising alternative given the benefits they offer to both users and operators. In actual practice, express services in systems such as Transmilenio (Bogota, Colombia), Transantiago (Santiago, Chile), and Metro Rapid (Los Angeles, CA) have proven to be highly appealing. This raises the question about when express services are a reasonable option. Previous work has focused on how some characteristics of the demand structure of a corridor affects the benefits that express services can yield, showing that the load profile shape and the average trip length are crucial. This work presents some evidence that also the dispersion of the demand among different OD pairs (keeping the load profile and the total number of trips constant) affects the potential benefits of express services. As expected the more concentrated the demand into few OD pairs, the more cost savings that can be obtained. To answer this question we developed a methodology to generate OD matrices that share all relevant attributes but differ in variability among OD flows. Thirteen matrices were generated and their optimal sets of bus services with their respective frequencies were obtained. Using the coefficient of variation as a measure of matrix variability, we confirmed that more demand variability (i.e. more flow concentration) gives room to more express services and lower social costs.
Design of limited-stop services for an urban bus corridor with capacity constraints
In high-demand bus networks, limited-stop services promise benefits for both users and operators, and have proven their attractiveness in systems such as Transmilenio (Bogota, Colombia) and Transantiago (Santiago, Chile). The design of these services involves defining their itinerary, frequency and vehicle size, yet despite the importance of these factors for the network’s efficiency, no published works appear to provide the tools for designing high-frequency unscheduled services on an urban bus corridor, minimizing social costs.
This paper presents an optimization approach that minimizes these costs in terms of wait time, in-vehicle travel time and operator cost. Various optimization models are formulated that can accommodate the operating characteristics of a bus corridor, given an origin–destination trip matrix and a set of services that are a priori attractive. The models then determine which of these services should be offered at what frequencies and with which type of vehicles. A case study in which the model is applied to a real-world case of a bus corridor in the city of Santiago, Chile, is presented and the results are analyzed. Finally, the model is used on two different demand scenarios establishing which type of services tend to be good candidates on each case and providing preliminary insights on the impact of some key parameters.
