D.C. Mun. Regs. tit. 24, r. 24-311
The clear sidewalk space shall be measured from the farthest extended portion of the sidewalk cafe frontage to the curb line or the nearest obstruction, whichever is nearest to the sidewalk cafe.
Recesses in the sidewalk cafe frontage shall not be used to satisfy the clear sidewalk space requirements.
The clear sidewalk space shall be measured as parallel to the curb line between the sidewalk cafe and any obstruction, and in line with normal pedestrian traffic.
For the purpose of determining clear sidewalk space, the space between a tree and the building line shall be considered unobstructed if the treebox is protected by a tree grate that is flush with the grade of the sidewalk.
For the purpose of determining clear sidewalk space, trees, streetlight poles, sign poles, fire hydrants, and other objects located on the surface space shall be considered as obstructions.
Sidewalk cafes located at street intersections shall provide corner clearance by providing a ten foot (10 ft.) clear space radial to the corner.
No sidewalk cafes shall be within fifteen feet (15 ft.) of a bus stop, a bus shelter, or a Metrorail station entrance.
D.C. Mun. Regs. tit. 24, r. 24-311