Myungseob Kim

Conventional, Flexible, and Variable-Type Bus Services

To provide efficient public transportation services in areas with high demand variability over time, it may be desirable to switch vehicles between conventional services (with fixed routes and schedules) during peak periods and flexible route services during low-demand periods. This option is called variable-type services. Conventional, flexible, and variable-type service alternatives optimized for various conditions are compared to explore when variable-type bus services might be preferable to purely conventional or flexible service. The optimization models used for purely conventional or flexible service are adapted from previous studies. These models are integrated into a new model for optimizing variable-type bus service. The results of sensitivity analyses show how demand variability over time and other factors affect the relative effectiveness of conventional, flexible, and variable-type bus services.

Comparison of Vertical Alignments for Rail Transit

AbstractIn this paper two vertical alignment profiles (labeled dipped and undipped) connecting rail transit stations with different elevations are compared. A deterministic simulation model is developed and used to analyze train motion and energy use for three different vertical alignments. To minimize the cost of rail transit operation for both directions, cruising speed is optimized as a decision variable while train operations are simulated on the proposed vertical alignments. The numerical analysis and sensitivity studies show that the developed deterministic simulation model is useful for comparing vertical alignments, and that the proposed alignment concepts can significantly reduce travel time, energy use, brake wear, operating cost, and total cost compared with the baseline alignment profile.

Simulation-Based Rail Transit Optimization Model

A proposed bilevel optimization process determines the depth of a dipped vertical alignment between rail transit stations as well as the cruising speed for each direction. This model also considers how regenerative braking may affect vertical alignment decisions. The optimized depth and directional cruising speeds were jointly obtained in a numerical example. A sensitivity analysis showed that regenerative braking reduced the total cost but did not significantly change the optimized depth of the dipped vertical alignment. The developed model is more realistic and useful than previous ones because it allows unequal station elevations, jointly optimizes decision variables, and considers regenerative braking.

Switching service types for multi-region bus systems

ABSTRACT Conventional fixed-route bus services are generally preferred to flexible-route services at high demand densities, and vice versa. This paper formulates the problem of integrating conventional and flexible services that connect a main terminal to multiple local regions over multiple time periods. The system’s vehicle size, route spacing (for conventional services), service area (for flexible services), headways and fleet sizes are jointly optimized to minimize the sum of supplier costs and user costs. The route spacing for conventional bus services and service area for flexible bus services are also optimized for each region. The proposed solution method, which uses a genetic algorithm and analytic optimization, finds good solutions quickly. Numerical examples and sensitivity analyses confirm that the single fleet variable-type bus service may outperform either the single fleet conventional bus service or the single fleet flexible bus service when demand densities vary substantially among regions and time periods.