GTFS-flex exists to bring flexible transit services to trip planners—applications that provide travelers with navigational directions for multiple modes of travel between user-defined points. GTFS data informs a trip planner’s transit directions, but only for fixed-route systems. GTFS-flex adds modeling of demand response, continuous stops, route deviation, and other non-fixed-route services to GTFS. A GTFS-flex-enabled trip planner can read these additional categories of transit data and generate matching itineraries, giving a user more trip options in search results.