Page 29 of this report shows test results of the Dallman power plant in Springfield, Illinois.
This is the potential number of people living outside the city of Springfield, within driving distance, who don't pay property taxes or other taxes limited to the residents of the city of Springfield, with a population limited to 115,715 (2016).
Many of these people probably travel to Springfield regularly to do business, shop, eat, catch a flight at the airport or entertain themselves.
Since Springfield is the state capital and the county seat, there are potentially many more people who regularly visit Springfield. See Sheet1.