In this context "backs up" isn't meaning "to move in reverse" or "backwards" but rather is talking about traffic congestion. If cars are stopped in traffic from Buckingham Fountain to McCormick Place you should take different roads. IE "Traffic is backed up from Buckingham Fountain to McCormick place", or "Cars are backed up from x to y". The idea is that they hit a spot where they have to stop, and all of the cars behind that stopping point are "backed up". Like when your sink or toilet gets full and starts overflowing.

"It" is the traffic. I know which text you are reading. ;) When traffic becomes slow or stuck, there's usually a point where this problem begins. Maybe it's a slow set of traffic lights, or even an accident. Moving away from that point, we say the traffic "backs up". If the problem starts at an intersection and it causes a traffic jam for one kilometre, we say, "the traffic's backed up for one kilometre."

Hi Bulat, "it" refers to the traffic. "backs up" means stops and blocks something from moving. So it refers to a traffic jam. Hope this helps. Cheers, Lance

In this sentence "backs up" refers to the traffic I think. So in this sentence, there is a lot of traffic between Buckingham Fountain to McCormick Place.