Public city bus fleet replacement and assignment decisions considering total costs of carbon emissions and bus fleet ownership

This thesis aims to determine the assignment of an assortment of public city bus types to a given set of routes to minimize the total cost of ownership involving social costs of carbon emissions in addition to the traditional cost elements such as financing, operating, and maintenance. To this end, we use the public transport system of İzmir, Turkey, as our testbed. We formulated a mixed-integer linear program to determine the number of buses of each type to operate on each route and the respective daily cost, which cannot be solved due to its large model size. A simpler version, assuming that a single type of bus is operated on a route, is formulated and solved. Then, its outputs are used as input for a shortest path formulation to determine the replacement and retention decisions over a study period of 12 years. Our results indicate that the currently-owned diesel buses are favored in most of the fairly flat routes whereas electric and hybrid buses are preferred for routes with aggressive road grades. We also investigated the effects of social cost of carbon emissions and the first cost of electricity- and hydrogen-based buses. Our analyses showed that the first cost has a significant effect on the life-cycle costs, assignment and replacement and retention decisions tilting the scale in favor of electric buses.