D.C. Mun. Regs. tit. 8, r. 8-B444
Inter-city travel via airplane, bus, or train shall be processed in either of the following ways:
For transportation expenses paid by the traveler under § 444.1(b), each claim shall be accompanied by the passenger coupon or the ticket.
The passenger coupon shall indicate the traveler's name, date(s) of travel, class of travel, origin and destination, and the cost of the transportation.
Expenses related to travel that are of a personal nature, including stop-overs or changes in travel itinerary, shall not be reimbursed.
Taxicab, bus, and limousine fares shall be reimbursed at the actual rate of fare, including tip. In no event shall the tip exceed fifteen percent (15%) of the total fare. Each request for reimbursement of expenses specified in this subsection in excess of ten dollars ($10) shall be accompanied by receipts indicating the date, the origin and destination of the trip, and the fare and tip paid.
The allowable reimbursement for use of a privately owned automobile for official travel shall be at the rate of twenty-one cents (21¢) per mile plus tolls.
Each request for reimbursement of the cost of tolls or parking shall be accompanied by an official receipt regardless of the amount.
The use of rental cars while out of the city on official business shall be approved by the duly authorized official prior to departure.
D.C. Mun. Regs. tit. 8, r. 8-B444