a boolean field that might actually be useful?: walk-in / no walk-in

Walk-in would be anything you can, well, walk in at. No walk-in (or whatever it would be called) would cover a bunch of different situations, including shelters that have lottery systems, places by referral only, clinics that require an appointment, etc [and does this last category even belong in NeedNow??].

The place where this gets sticky (because of course there is one) is places that mostly offer appointment-based services but also have specific walk-in hours.