I wonder if it had something to do with the I-70 project.
There is an "End 15" sign as Main Street goes under the I-55/64 mainline. I am going to make that the "I-55" endpoint - well, midway between the two one-ways approaching Broadway - and it will not have a direct I-55 counterpart because the exit would require an extension down ramps down to Exit 1A. That is to say, a traveler coming into Illinois would end their I-55 count at 1A and pick up IL 15 without the onramp mileage counting. Does that sound like a good solution?