The City of Houston has asked the US government to seek a new bilateral agreement with Mexico by the end of 2014, in an attempt to ensure United Airlines and Southwest Airlines eventually can fly the same routes.

The filing with the US Department of Transportation (DOT) comes as United and Southwest jockey for route authorities between Houston and large Mexican cities. The current agreement between the two countries allows only two carriers to serve most routes, with several exceptions, mainly to Mexican beach destinations. In many cases, United essentially holds both authorities from Houston—one for mainline operations and one for an affiliate express carrier. Houston’s filing is directly tied to Mexico City route, on which United has asked to swap regional carriers, subbing SkyWest for ExpressJet. Normally, such a request would be granted without difficulty, but it has become a bigger issue now that Southwest could be interested.

Southwest did not object to United’s Mexico City filing, though it signaled it might in the future. Still, Houston took the opportunity to seek to persuade government regulators to loosen restrictions.  “If this liberalization of the US-Mexico bilateral agreement can be achieved on a timely basis, then the grant of transfer requests such as that of the United carriers in these dockets will become either unnecessary or merely routine, and will not impact the opportunities remaining for other carriers desirous of serving the same city-pair,” Houston officials wrote in a filing. Houston asked that the carriers be given “unlimited third and fourth passenger freedoms,” though it’s not clear whether negotiators will make such liberalization a priority. It is also not known whether an agreement can be reached as soon as this year. 

The filing notes that both United, at George Bush Intercontinental Airport and Southwest, at William P. Hobby Airport, are making considerable terminal upgrades in part to support international flights. 

The US-Mexico treaty was created in 1960. It was last amended in 2005.