Freezing are "usually" a result of some routing problem in the world.
Freezing is usually the result of cache file problems, especially when that game freeze happens at the same time when the save is reloaded. This is
especially prevalent in Riverview and Barnacle Bay, since these worlds use World Cache files as well. It has little, if anything, to do with sims not being able to get from point A to point B.
Try deleting your cache files. This is safe to do (and should become part of your routine game maintenance anyways), as the game will recreate these files if they are missing. All they really do is store information that allows objects in the town render faster when you move your camera to them.
Follow this path: Documents/Electronic Arts/Sims 3. Once there, find these 4 files and delete them:
CASPartCache.package
scriptCache.package
compositorCache.package
simCompositorCache.package
Once you've done this, open a folder named "WorldCaches." There will be 2 files in here for each world that did not come with the base game (Riverview and Barnacle Bay for certain, but CAW worlds
may create files here too). Delete everything in this folder. It's entirely possible that you don't have a WorldCaches folder. If you don't, all this means is that your computer doesn't require them, or you don't have any towns that generate World Cache files.
From my experience, it is likely that this will fix your issue.