There are a few residential lots in Roaring Heights that have routing issues as far as I know. The empty house between the Shear and Morgan households, empty lot between the Marks and Thebe households and a large empty 60x60 lot near the beach. Those lots have clearly routable area but deemed unroutable. Also the downtown area and the Sixam Field have a confusing routing. Too bad there isn't any fix for this world.

You could just set them to no visitors allowed in edit town. However I'm not convinced you're going to see a ton of difference in RAM usage by doing that.

@Chain_Reaction: I have done this, but at the lots there were some bars and the bartenders went to work, then they went away from the lot, because (I think) it was set to no visitors, but then they came back again, and so on...
So I thought of a more restrictive solution...
And I´ve taken out the MSC-mods and there is a slightly improvement of 100~150 RAM, which helps in the first played Sim days :-]