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The development of a systematic framework to support the design of transit bus fleets is justified by the significant and long-lasting implications associated with decisions to purchase transit vehicles, as well as by developments in fuel propulsion and battery technologies over the last 2 decades that have increased the options available to transit operators, and, in turn, the complexity of assessing the corresponding tradeoffs. The need to evaluate these tradeoffs is, in part, driven by the emergence of environmental impact mitigation, i.e., emissions reductions, as a critical concern of transit operators and governments around the world. To address these concerns, we present an optimization model to support the design of transit bus fleets while accounting for costs, level-of-service requirements, and environmental impact. Methodologically, the work bridges applications of Economic Input-Output analysis to conduct environmental lifecycle assessment, with seminal work in production economics. We apply the framework to support design of bus fleets consisting of 4 bus types differing in their fuel-propulsion technology: ultra-low sulfur diesel, hybrid diesel-electric, compressed natural gas, and hydrogen fuel-cell. The 4 bus types were assessed in the National Renewable Energy Laboratory transit bus evaluation and demonstration studies conducted over the period 2003-2009. The nominal problem herein is to minimize acquisition, operation and disposal costs. Constraints in the model are used to impose a minimum frequency of service, i.e., headway, and to ensure that route capacity satisfies passenger demand. Environmental impact is considered along the following dimensions: energy consumption, and emissions of greenhouse gasses, particulate matter, and nitrous oxides. Results show that fleet heterogeneity increases in scenarios where demand fluctuates, i.e., peak vs. off-peak. Perhaps even more interesting, we show how the dual/shadow prices provide a (monetary) measure of the tradeoffs among level of service and environmental impact, and discuss how they can be used to obtain robust fleet configurations.