A Framework for Optimizing Public Transit Bus Fleet Conversion to Alternative Fuels

Alternative fuel buses (hybrid-electric, battery-electric, compressed natural gas, etc.) have great potential to reduce lifecycle energy use and criteria pollutant emissions from urban and rural transit fleets. However, market penetration of alternative fuel transit buses in the U.S. is currently below 50%. There are a number of barriers to entry that discourages switching from traditional diesel vehicles. Alternative fuel buses have higher capital costs, and the uncertainty associated with fuel savings and potential increases in maintenance costs introduce uncertainty into estimated the time it will take to recover capital investments (whereas diesel vehicle costs are well-known). Discrepancies between the claimed fuel economy and real-world performance under variable and diverse traffic conditions, roadway configurations, and designated routings contribute to economic uncertainty. Some vehicles, such as battery-electric buses and even some hybrid-electric buses, have significant range and performance constraints (maximum speed and acceleration rates under load) that prevent their use on certain routes.

This study is developing an analytical framework to automatically generate an optimal transit agency bus fleet conversion plan based on fleet-specific characteristics. The goal of the framework is to allow agencies to minimize the lifecycle economic costs of replacing current fleet and assigning vehicles to vehicle-amenable routes based upon vehicle performance and route characteristics. Cost changes of adopting alternative fuel vehicles will be evaluated comprehensively and energy use and emissions will be modeled using advanced modeling tools based on real-world second-by-second operations data.

The optimal fleet replacement plan is generated through optimization, including annual fleet purchase/replacement of different vehicle types, vehicle-route assignment, vehicle-depot assignment, and charging station/depot location selection. The framework will be applied to the bus fleet from Metropolitan Atlanta Rapid Transit Authority (MARTA), the largest transit agency in Metro Atlanta. A twenty-year bus replacement plan will be developed under several alternative future scenarios: 1) no changes in routes and fleet operations, 2) increased demand on existing routes, 3) provision of new routes in expanded service areas, and 4) a shift from urban to suburban routes with autonomous electric vehicles covering center city travel.

Dissertation embargoed from publication until August 2020.

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