The payphones in real life large cities like this one can dial emergency numbers without needing coin deposits (dial tone-first) and many larger
cities have 9-11 service that can trace emergency calls faster.
When Marty was running towards the payphone at Kilmer and LaGrange, how could've he have missed seeing that sign on the pole so he could tell the operator and call taking police officer exactly where he was when they answered without having to go through the longer motions to try to trace his call ?
Although the ending subway chase was filmed in Chicago, the other exteriors are filmed outside Illinois (Los Angeles and Pasadena, CA.). Detroit, MI., not Chicago, has the street called Detroit Street but doesn't have any subways.
When the couple go to the deserted subway entrance, cars can be seen driving on a street in the background, and surely they would have seen them.