As someone else suggested, some sort of lottery system may make sense for high demand areas, but I wonder what percentage of available sites that really matters

As someone else suggested, some sort of lottery system may make sense for high demand areas, but I wonder what percentage of available sites that really matters for. I use recreation.gov and the WA state equivalent regularly and am able to get decent reservations pretty easily. During the really high use times in WA we just go to camp sites (Forest Service sites are particularly good for this out here) and see if they have walk up spots available -- all campsites we have visited keep at least some spots available for this. Granted, we have only done this maybe four of five times so far, but we have got a spot every time.