Abstract:Carsharing in the United States has been a focus of study and research for more then a decade. Its proponents argue that carshare adoption will reduce VMT in cities and can serve as a travel demand management policy. However, no comprehensive forecasting of carsharing impact has been done and most of the evidence is based on evaluation studies of new carshare projects. This paper focuses on the gap between the available data and knowledge and the required data and knowledge. The study demonstrates how the data and knowledge gap contributes to optimism bias about the expected impact of carsharing as a TDM policy.