LlamaMama,
This isn't really an answer ... but once I accidentally placed a lot without street access / being adjacent to a street. It was actually in a really horrible spot, more than half the lot was horribly sloped.
The standard residential items (mailbox and trashcan) were placed on the lot.
Just to test, I saved the game and moved a Sim family onto the lot, and everything "seemed" to work fine. Carpools would show up like 2 blocks over, but my Sims just did that "fade out" when they reached the border of the lot, only to pop into the carpool. I purchased a parking place and a car, and it worked fine as well, again just doing that "fade" right onto the street. Sims could run on and off the lot, but any vehicles (or a Sim using a vehicle) would just "fade" from the lot a few blocks into the car / moped / bike.
It wasn't pretty, for sure ... and I couldn't edit anything in "Town Mode" because there weren't any lots "connecting" the lot to the street.
I suspect if you had the World Editor installed (and knew what you were doing) you might be able to edit the town and create paths and what-not, but that's all I can think of. I've not messed with the World Editor enough to know (it's not available for the Macintosh, and I've not tried installing all my Sims3 stuff on a BootCamp partition).
-- Happytime