For some topics (like parking, hearing assistance, entrances, etc) there are already design guidelines in the accessibility guide for them that define the minimums based on the facility scenarios.
There can also be state/local regulations and then the building codes. They would have to meet the most stringent of the intersection of those requirements.
For areas not defined, now you're at the ADA or state equivalents of accessibility law. For ADA, you're going to be stuck in interpretation land. And it's likely to be driven by notions like "as much as you can offer, without fundamentally altering your business" -- which isn't very helpful. In court you would argue why your capacity is sufficient given the expected normal usages, or the max you can sustain within the reason accommodation language.
This is why I challenged matt's initial proposition of garunteeing on-site guests access - to do so would in effect be saying you have capacity to support that number (with some ratio of expected use)... that would set precedent of what is reasonable for your entity. It would be very hard to exclude people from the services if that capacity is not in actual use at the time.
I don't believe Disney is required to provide "DAS," in and of itself, to anyone. DAS is Disney's own program and may be limited if it fundamentally alters the nature of the services offered. That's what Disney is doing, while still attempting to provide alternative accommodations for everyone who needs them.
The subject matter being raised now is why Disney can't just limit the number of DAS passes based on its own calculations of its business need and then say "sorry" to the next guest, no matter how great the need.
As you stated earlier, the DOJ sets out regulations regarding defined accommodations such as number of accessible seats, entrances, etc. Presumably, they did not pull these numbers out of their collective ***, but based them on information as to what would be reasonable to accommodate the anticipated number of disabled persons in each circumstance. Because it's a government entity, we have given them authority (through a whole number of processes) to make those determinations and impose them on businesses.
The DOJ works through transparent processes, and if they do a bad job (there are numerous disabled persons being denied accommodations in hotels, etc.,) the DOJ takes the heat for it and the business is off the hook for not providing an accessible room to someone who clearly needs it. The regulations may need to be amended, but the business does not have to worry about picking and choosing who gets the accessible rooms as long as they make sure the guests in the rooms asserted they needed it.
It's different where a business is "stuck in interpretation land," yet required to provide accommodations. In that case, arbitrarily denying an accommodation based on its own numbers rather than the need of the individual is going to result in a lawsuit against the business, not a clamor for DOJ amendments. Plus, Disney would have to deal with the optics of denying a severely disabled child simply on the basis that their own numbers convinced them they were on the right side of the law.
If there is a lawsuit, Disney would much rather face a plaintiff who was denied DAS according to their own process and nevertheless claims no other accommodation would work than face a severely disabled child where there is no question regarding need
I'm under no illusion about how many people will read this post.