Updated: November 25, 2020. The spec in this repository is now the most up-to-date version of GTFS-Flex. The code in this repo contains the official proposal for GTFS-Flex v2, a GTFS extension that covers all demand-responsive services for the purposes of discovery in trip planning. All features of this specification proposal are currently produced by Trillium and consumed by OpenTripPlanner version 2. DemandTrans and IBI group expect to produce data in this spec by early 2021.
"Version 1" of the GTFS-Flex specification is utilized by OTP 1.4. You can review the Version 1 specification by reviewing versions of this repository from before October 2020.
GTFS-Flex is a proposed extension to the General Transit Feed Specification. GTFS-Flex adds the capability to model various demand-responsive transportation (DRT) services to GTFS, which currently only models fixed-route public transportation. GTFS-flex is now produced for over 100 transit services, and provides flexible transit trip plans through OpenTripPlanner.
The below shows updated and added files in GTFS-Flex, compared to the current GTFS (original diagram is here).