It has been stated by writers on numerous occasions that Springfield is supposed to be a nonexistant country. That's part of the gag...to make people search for something that doesn't really exist. That's part of why they picked the name Springfield, because there are so many in the US. In the movie they said it bordered 4 states that are no where near each other. In the commentary for one of the DVD episodes a director states that it's in North Takoma which makes sense because you see NT and TA as state abbreviations on licenses and such. So the answer is....it is not based on any of the Springfields in the United states but rather a culmination of any common city found in the US.
Comment
It has been stated by writers on numerous occasions that Springfield is supposed to be a nonexistant country. That's part of the gag...to make people search for something that doesn't really exist. That's part of why they picked the name Springfield, because there are so many in the US. In the movie they said it bordered 4 states that are no where near each other. In the commentary for one of the DVD episodes a director states that it's in North Takoma which makes sense because you see NT and TA as state abbreviations on licenses and such. So the answer is....it is not based on any of the Springfields in the United states but rather a culmination of any common city found in the US.