As with most things, I think it is a contrived balance between competing benefits and disadvantages. How much slip is too much, how much is too little? One surface cannot provide maximum accessibility for all. For example, a crutch tip would be best used on a surface with the maximum slip resistance possible. I know, I fell several times around my gym pool and locker room years ago when I broke my leg. However, that same resistance to slip would make walking for a person using a walker much more difficult. A certain amount of slip is actually necessary for the act of walking. The person using a wheelchair may want less slip that a person on their feet, but here too, no slip at all would make it difficult to travel and to turn a manual wheelchair. I would bet someone clever like Ed Steinfeld with the IDEA Center has quantified all this. The trick would be to quantify a range that delivered the minimum slip under wet conditions for the mobility device that needs the most resistance, that is still slippery enough for the person walking, being the one who needs the most slip under dry conditions.