2010 ADA Standards for Accessible Design Pocket Guide
705.1.2 Dome Spacing.
Truncated domes in a detectable warning surface shall have a center-to-center spacing of 1.6 inches (41 mm) minimum and 2.4 inches (61 mm) maximum, and a base-to-base spacing of 0.65 inch (17 mm) minimum, measured between the most adjacent domes on a square grid. [S.H. 4.29.2]