Flatiron Flyer is an 18-mile (29 km) express bus system between Denver, Aurora, and Boulder, Colorado, traveling along U.S. Route 36.

[7] This new system has been criticized as bus rapid transit creep.The original FasTracks plan approved by voters indicated that center-line BRT stations would be created to avoid buses exiting the highway and sitting at traffic intersections.

[9][10] There are six park-and-rides along U.S. Route 36 from Denver to Boulder, which have been dubbed "stations", that will be served by Flatiron Flyer:[11] These stops have ticket vending machines, and passenger information systems.

Phase 2 of the project extended the HOT lanes from 88th Street to Table Mesa Drive in Boulder through a public–private partnership;[17] it opened on January 3, 2016, with toll collection starting in March of that year.

[23] The completion of the originally-planned route from Westminster to Longmont has been delayed until 2044 due to lower tax revenues and higher costs than expected.

[24][25] RTD introduced "Flatiron Flyer" as its brand for the US 36 Bus Rapid Transit component of FasTracks, serving travelers between Denver and Boulder.

[26][27] As an interim measure until Northwest Rail can be completed, RTD committed to study high-speed bus options in other parts of the corridor as well as the possibility of extending the North Metro Line to Longmont.