Planning bicycle infrastructure significantly depends on data that provide adequate information. Various studies indicate that GPS data, which have been collected via smartphone application by cyclists themselves, could provide that information. The article presents the results of a recently conducted study that evaluates the usability of such data for bicycle traffic planning in German cities. We used different methods (web-survey, focus group interview, data analysis) to investigate data needs of German municipal traffic planners and oppose it to the information deduced and computed from commercially available data provided by Strava Inc. The article reveals that the provided data are, in general, useful, but there are also serious limitations that must be considered.