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Globally there have been considerable efforts of decarbonizing the transportation sector, as it has been found to be largely responsible for greenhouse gases and other air pollutants. One strategy to achieving this is the implementation of zero-emission buses in transit fleets. This paper summarizes the characteristics of three zero-emission bus technologies: 1) battery electric buses; 2) fuel cell battery electric buses; and 3) fuel cell plug-in hybrid electric buses. All of these technologies do not produce tailpipe emission and can potentially be emission-free in a well-to-wheel content, depending on the fuel source. This study aims in gathering the needed information for transitioning to zero- emission buses in transit fleets, providing insights from implementations across U.S. Data collection efforts consists of three approaches: a systematic literature review emphasizing on reports released by transit agencies and other relevant organizations, an online survey of several transit agencies that have implemented or are planning to implement zero- emission buses, and interviews with transit agency representatives. Overall, the collected information was used to identify performance measures, cost characteristics, emission savings, and fuel economy, as well as implementation approaches and refueling strategies. A comparison among the three technologies and conventional fuels (diesel, compressed natural gas) suggests that zero-emission buses outperform in fuel economy compared to conventional fleets, but their capital cost is still higher than the cost of a diesel or a compressed natural gas bus. Battery electric buses have been chosen by the majority of transit agencies and present the highest fuel efficiency among the three zero emission technologies. Challenges associated with the implementation of such vehicles and lessons learned are also summarized. Commonly admitted among all agencies is that

for a smooth transition to zero-emission fleet it is important to fully understand the technology and its requirements while starting with a small number of buses should be preferred and eventually increase the size. Further, it is critical for the staff to receive a proper training about the new technology and finally, all the involved stakeholders should maintain a good communication among them that would allow for efficient troubleshooting and information exchange.