After a successful launch with TriMet, Google Transit offered their trip planner service for free to any agency that formatted and maintained their data in the GTFS format. In 2006, five more agencies were added. Google Transit’s success continued as more and more agencies wanted access to a free trip planner, and were willing to put their data into the GTFS format to get it.
Since its creation in 2005, GTFS has become the most popularly-used data format to describe fixed-route transit services in the world. Many agencies have decided to share their GTFS data openly with the public, while others choose to restrict access only to select partners (e.g., Google Maps). As of
March 2012, there were are an estimated 261 transit agencies worldwide, including 227 transit agencies in the U.S., that share their GTFS data openly with the general public<ref>Front Seat Management, LLC. "City-Go-Round." Accessed March 1, 2012 from http://www.citygoround.org/agencies/</ref>. As of August 20, 2015, Google lists approximately 5900 agencies around the world for Google Transit coverage<ref>Google, Inc. "Transit – Google Maps." Accessed August 20, 2015 from http://maps.google.com/landing/transit/cities/index.html</ref>, however, some of this transit information is probably derived from sources other than GTFS.
Even though many transit agencies created GTFS feeds with the primary purpose of benefiting from the free Google Transit trip planner, application developers, often not affiliated with the agency or Google, quickly realized that they could also create many new types of services based on the same GTFS transit data.