This is called the black roads bug (oddly enough ) and it was fixed with the 1.1 patch. Problem is you can't use 1.1 with MTA, so you're stuck with it, you might be able to try older drivers and see if the bug remains.
If you want to play normal SA without the bug, make a duplicate of your SA folder, rename it to something like SA 1.1 , and apply the patch to it. That way your current installation can be kept for MTA with black roads, and your newly modified 1.1 installation can be used for SA without black roads.