if there is a large population of rats locally, then there has to be a food source for the rats. All it takes is one homeowner to have a food source available to the rats (pet food, bird food, bad bins, a non-rat-proof home, etc) and it will be rat central. They are capable of breeding at an incredible rate; in theory a pair of rats can increase to a population of 15000 in a year.
Unless the food source is removed or is made so that it is no longer accessible to rats, you will likely always have a rat problem in the neighbourhood.
cheers