D.C. Code § 32-1526

No person shall be required to attend as a witness in any proceeding before the Mayor at more than 25 miles of the place of the hearing, unless his lawful mileage and fee for 1 day’s attendance shall be first paid or tendered to him; but the testimony of any witness including that of an interested party may be taken by deposition or interrogatories according to the rules of practice of the Superior Court of the District of Columbia.