Rockford Police doled out 459 arrests for drunk driving last year, according to a recent study.
Decatur came in second with slightly more than 400 arrests. Chicago is not factored into the evaluation due to its size.
Meanwhile, Illinois State Police saw a significant drop in intoxicated driving arrests.
Rockford Police Chief Dan O'Shea said his department stepped up its efforts after 16 traffic fatalities in 2015.
"There's crashes, crime or complaints, if you will," he said. "Those three things drive how we push our traffic enforcement."