D.C. Mun. Regs. tit. 6, r. 6-B4003
For the purposes of this chapter, travel expenses include:
Travel expenses must be reasonable and necessary in order to be paid for or reimbursed by the District government. As used throughout this chapter, an expense is reasonable when it is the least expensive option, taking into consideration government accountability, efficiency, and economy, as well as employee safety.
Subject to the availability of appropriated funds, the District government shall reimburse government travel expenses when approved in accordance with this chapter.
No agency may approve or reimburse travel expenses that could reasonably be perceived to violate the public trust.
If, for personal convenience, an employee travels by an indirect route or interrupts travel by a direct route, the employee shall be responsible for costs that exceed the approved cost of travel by a direct route on an uninterrupted basis.
If, for personal convenience, an employee travels to the destination earlier than the date and time authorized for official travel or departs the travel destination later than the date and time authorized for official travel (such as by extending the employee's stay at the travel destination for the purposes of a personal vacation), the employee shall be responsible for costs that exceed the approved cost of travel for the dates and times authorized for official travel.
D.C. Mun. Regs. tit. 6, r. 6-B4003