If you do not travel by the method of transportation required by regulation or authorized by your agency, any additional expenses you incur which exceed the cost of the authorized method of transportation will be borne by you.
Code of Federal Regulations
[FTR Amdt. 70, 63 FR 15955, Apr. 1, 1998, as amended by FTR Amdt. 2010-07, 75 FR 72967, Nov. 29, 2010]