Where to find guidance on spacing between pickets and horizontal bars on a fence
ASTM-14 (speaks to fence requirements and standards for residential outdoor swimming pools, multifamily residences, playgrounds, ball fields, sports facilities, and education facilities.
ASTM F-1908 fencing standard for residential pools (originally published in 1998 and renewed in 2006. ASTM F-2049 fencing standard also limits the fence vertical members to 1.75 inches apart if the horizontal members are less than 45 inches apart. This standard states that it was developed based on the head breadth and chest depth of a 13 to 18 month old child.
These standards should be well known to anyone staying informed about developments in the fence industry.
ASTM F-1908 was developed in part, using information from playground equipment and product standards which addressed spacing between 3.5 to 9 inches. This spacing is viewed as a well established child head entrapment danger by United States Consumer Product Safety Commision (CPSC). This is also backed by the American Association of Pediatrics, National Center for Injury Prevention, and American Medical Association.