Not much of an SI person, but I couldn't help but notice in my travels on the island and from what I've heard (so not much to base off of, so forgive me) but I couldn't help but notice an obvious gap in service near the center of the island.
https://maps.google.com/maps/ms?msid=213458373195564989412.0004d0846ed6ac303021b&msa=0
In this incarnation of the route, I have combined it with the S56, but it could be an independent route running from the Mall to Port Richmond. Here, I'd straighten the S57 along Jewett, and have this route run using the route depicted, filling a gap around the SI Expwy.