Did you by any chance unlock the Maxis street & crosswalk tiles, and use any of them to cover a lot?
No. At least, I don't think so.
Have you checked you're other nieghbourhoods to see if the same thing happens?
I have, and it does. In all four - Old Town, Enderby and t'other one. Sometimes, it will happen. Other times, everything seems fine. If I go into a house and back into the neighbourhood, it may go back to normal, depending on how it's feeling.