D.C. Mun. Regs. tit. 31, r. 31-820
Whenever a taxicab is proceeding to take on a passenger in answer to a telephone call or previous appointment, or is engaged by the hour for the carriage of passengers or making an emergency delivery of a parcel or package, the operator shall display a sign reading "On Call."
The "On Call" sign shall not be displayed unless the operator is engaged in the manner outlined in § 820.1. Whenever the "On Call" sign is displayed, the operator shall immediately enter on the manifest the time at which he and she went on call and the destination.
Whenever a taxicab operator ceases to be for hire and is proceeding to a place without intending to take on passengers, the operator shall display a sign reading "Off Duty" or "Out of Service," whichever is appropriate under these regulations, and turn off the cruising light. The operator shall enter on the manifest "Out of Service" or "Off Duty," and the time and location; provided, that the "Off Duty" sign shall not be displayed during rush hours between 7:00 a.m. and 9:30 a.m., and 4:00 p.m. and 6:30 p.m.
All "On Call," "Off Duty," and "Out of Service" signs shall be displayed at the bottom of the right half of the windshield so as not to obstruct the operator's vision. These signs shall be uniform in size and lettering, three inches by fifteen inches (3 in. x 15 in.) with letters two inches (2 in.) in height; provided, that the signs may be of a manufactured type and lighted from within, and may be smaller in dimension than three inches by fifteen inches (3 in. x 15 in.) when they have been approved by the Commission.
"On Call," "Off Duty," and "Out of Service" signs, when not in use, shall be placed and secured in the taxicab vehicle so they are not readily accessible to the operator while the taxicab vehicle is available for hire.
D.C. Mun. Regs. tit. 31, r. 31-820