High level discussion for design of using ACL to retrieve Data

We currently use a system with 4 tables (division, region, district,
location).

The data for the most part has a field named location id.

The users get a level and location id (if they are district it would
represent a district id etc).

I have been asked to make this more flexible, for example if the user needs
data for two locations but they are not in the same district.

One thing I have had to add was the ability to have a user have access to a
location and then also have access to a related location (main and auto
center have different location number, but if the user has access to the
main location he can also see auto center data).

I did this with a xref table and a union, but it seem pretty slow.

So adding even more flexibity like multiple districts, locs etc (was
thinking of trying to do some kind of grouping that would encompass our
current plan) has been a problem I have thought about a lot, but I have not
figured out a way that will give fast access.

I could do groups of access rights and do unions with distinct to get data,
but I fear that would be really slow.

Any one have ideas on this subject?

Thanks in advance.

Joel Fradkin