The ADA has a simple set of guidelines: Carpet must be securely affixed to the ground, have a stiff or no backing at all, and be no more than 1/2" thick. If the carpet is more than 1/2 thick, there must be a beveled edge for wheels to effortlessly glide over. Recessed entrance grates/mats need to have less than 1/2" between pieces to prevent wheels or any other movement facilitating equipment to get caught.