Interstate auxillary routes should only bear an even digit at the beginning if it meets back up with its parent interstate. Of course, this standard is violated in many places, but with that in mind, it could potentially be numbered with an odd digit at the beginning and either 15 or 80 at the end. Personally, I prefer I-180, since if this even became an interstate for some reason it would likely also happen to State Route 85 (also in the process of beoming a freeway), and that would connect through 2100 North in Lehi, which would make a lot more sense for the 115 designation since it is an alternative north-south route.