ISP said a person was shot at the traffic light at the top of the Burr Street exit ramp from westbound Interstate 80/94, near Gary, around 6 p.m.
In an update Wednesday, police said a suspect was taken into custody and official charges are pending.
Responding troopers found the victim around 5:45 p.m. Monday at the nearby Hard Rock Casino, where he had driven his orange Jeep Wrangler to get help.
The victim, whose age was not immediately known, was transported to a local hospital, where he is in stable condition.
Investigators believe the shooting stemmed from a road rage incident that began on westbound I-80 as the vehicles were exiting onto Burr Street.
ISP asked anyone with information to call them at 219-696-6242.
Authorities did not immediately provide further information.