On today’s Kojo Nnamdi show with Roger Lewis, at 23:14 into the segment a caller complains about bicyclists riding on the sidewalk and says that she’s “pretty sure” it’s illegal in DC. Lewis talked about how it’s often reasonable to ride on the sidewalk, but didn’t address the specific law.
A lot of people think riding on the sidewalk is illegal, but actually it’s not true. You are allowed to ride on sidewalks except in the central business district, which is basically everything between Mass Ave and the Mall.
A map from DDOT: