If connecting between domestic Terminals 1, 2 & 3, you DON’T have to leave the secured side and take the ATS, you can simply follow the overhead signage to guide you between all domestic terminals. To get between Terminal 5 and Terminals 1, 2 & 3, take the Airport Transit System (ATS).
TTB SERVICE HAS BEEN TEMPORARILY SUSPENEDED - TTB service takes travelers from the domestic terminals (1, 2, & 3) to Terminal 5 without having to exit security or be rescreened in Terminal 5 at the TSA checkpoint. Shuttles to transport passengers between all terminals and remote parking are still operating 24/7, those shuttles pick up on the Upper Level roadway (Ticketing Level).
The International Terminal 5 is not connected to the domestic terminals and is accessible via the Airport Transit System (ATS). If you arrive on a domestic flight and must connect to an international flight departing from Terminal 5, you have two options: 1.) you can take the ATS and pass through the security check point in Terminal 5 or 2.) you can take a Terminal Transfer Bus (TTB) from Terminals 1 and 3 to Terminal 5 and bypass the security check point in Terminal 5.
The Terminal Transfer Bus service is year round with operation hours as follows: 11:00 am - 9:00 pm.
Terminal Transfer Bus (TTB) Terminal 1 and Terminal 3 Pickup Locations
If you arrive at O'Hare on an international flight and have a connecting domestic flight, you will arrive at Terminal 5. You may only use the TTB if you have rechecked your luggage and if you have a boarding pass for your connection. Proceed through Terminal 5 security to Gate M1. If you are arriving internationally and must check-in for your connecting flight, you may not use this service.