36 CFR Part 1191 ABA Accessibility Guidelines, Outdoor Developed Areas - Preamble

1011.2.4 Openings

This section requires openings in the clear ground space such as drainage grates to not allow passage of a sphere more than 1/2 inch in diameter. Where possible, drainage grates should be located outside of the clear ground space. Elongated openings should be placed so that the long dimension is perpendicular, or as close to perpendicular as possible, to the dominant direction of travel.