What you need to know about Illinois and it's Police.
We all know that police patrol their city limits. Well come to find out after talking to a detective that works for the Fairmont Police Department.
Illinois State Troopers - can respond to any call anywhere in the state. They can also do traffic details in any city, and they may patrol city & county roads. (They are not limited to state highways)
Any other City, County Police Agency - In the state of Illinois as long as an officer is on duty, he/she may do jobs outside of their department. This includes traffic stops in other counties, cities besides their own department