Free-floating carsharing systems (FFCS) have become a new type of urban sustainable mobility, much more flexible than the previous station-based carsharing but limited by on-street parking availability and managed by municipal administrations. Literature on FFCS until now mostly relies on survey-based methodologies and simulations, and little research on FFCS has been devoted to the scientific analysis of real flows using revealed web-based data. This paper contributes to the existing literature with an analysis of FFCS trips using rental data collected directly from operators' websites, paying special attention to the most frequent trips. The added value of this research is that it provides the first analysis of the more FFCS demanding districts in the city of Madrid. The results showed that the main origin and destinations points were concentrated in low populated and high-income districts that also had good parking availability and connectivity to the public transportation network.Sustainability 2020, 12, 1248 2 of 16 by FFCS on the urban transportation system: What are the main origins and destinations of these trips? Are FCCS systems used instead of alternative public transportation modes or active travel (e.g., cycling, walking)? How should local administrations deal with this new type of mobility? An FFCS has the opportunity to contribute to a low-carbon mobility transition in the case where the vehicles are electric and if the usage does not displace public transport use or active trips.Some authors have pointed out that FFCS quality of service depends on the reliability of finding a car to rent near to the trip origin and the possibility of having a parking place near to the final destination [5,6]. These two variables depend on the number of vehicles provided by the FFCS companies in a city, the number of on-street parking slots available, and local regulation (some councils usually offer privileged access to street parking space for electric cars or less pollutant cars). Moreover, FFCS would not work in a city with parking restrictions and a poor quality or low-dense public transport system, because in the case a user had already rented a car, the web platform would not guarantee finding another car for the trip home or to his/her next activity. Local administrations could regulate on-street parking in a way to promote or deter FFCS systems, acting as "gatekeepers" of this new form of mobility [7]. But before implementing a new regulation context, a greater acknowledge and analysis of FFCS demand and flows is needed. This research approach has been conditioned by the scarcity of data provided directly by FFCS operators and, until now, most of the literature on FFCS relies on survey-based methodologies [8] and simulations [4,9,10]. Revealed web-based data, collected directly from operators' apps, are starting to be used as an alternative to reproduce origin-destination flows when FFCS data are not provided by operators (or as an external way to check data or figures provided by operators). These web-bas...