41 CFR 302-5.14 – What transportation expenses will my agency pay?
(a) Your agency will authorize you to travel by any transportation mode(s) (e.g., common carrier or POV) that it determines to be advantageous to the Government. Your agency will pay for your transportation expenses by the authorized mode(s). If you travel by one or more mode(s) other than the one(s) authorized by your agency, your agency will pay your transportation expenses up to the constructive cost of transportation by the authorized mode(s). For trips of less than 250 miles, your agency will authorize travel by POV, unless there are reasons for not using a POV that are acceptable to the agency (e.g., traveler is physically impaired, does not own or lease a POV, has only one POV that is used for family transportation, or the POV is not roadworthy for such a trip). POV mileage reimbursement will be in accordance with § 302-4.300 of this chapter.
(b) Unless the agency performs a written cost comparison that demonstrates cost savings, only common carrier may be authorized for trips with a distance of 250 miles or more.