D.C. Mun. Regs. tit. 31, § 606
606.1 A taxicab may be equipped with a locked safe mounted on the floorboard next to the driver, and if so equipped, the taxicab may display a decal on the right rear door or fender consisting of letters not more than three inches (3 in.) stating: 'This driver protected by a locked safe.'
SOURCE: Final Rulemaking published at 37 DCR 3595, 3606 (June 1, 1990).