Investments and innovations in clean buses and trucks are progressing at a rapid pace in the United States and across the world, yet fleet adoption of alternative fuel vehicles has been hampered by the comparatively high costs of the new technologies. Policymakers, manufacturers, and end users have experimented with a number of policy tools to promote clean vehicle adoption, and in several locations these groups have come together to efficiently deploy vehicles through an innovative, flexible tool – the Voucher Incentive Program (VIP).