How about put the bench there, and then if people are using it to the point the disabled and elderly can't use it, add the sign. I get what your saying comparing it to handicapped parking, but IMO it's a false equivalent. I've personally never seen a parking lot that was handicapped parking exclusively and people park there cars and can leave them there for hours as opposed to sitting on a bench for ~5-10 minutes at a bench and since you are physically present, you can get up of someone needs the spot more.