Re: Sullivan Square
If we end up keeping the underpass in both directions at Sullivan Sq, I've gotten thinking about whether we could have a design somewhat inspired by the
Diverging Diamond Interchange.
My first thought was that the Rutherford underpass would get treated as the major road, and Main St to Mystic Ave / Maffa Way would be the minor street, and then you'd have to figure out how to integrate Cambridge St with that; you'd potentially build a new bridge over the underpass going from Main St to Mystic Ave.
But if the existing bridges turn out to be in good enough shape to keep using them in a reconfigured fashion, or if refurbishing them is cheaper than building new bridges elsewhere, I think we could probably reuse the existing bridges.
In particular, I'm thinking the northernmost overpass next to West St that currently is used as a U turn for northbound traffic to turn southbound could be reversed, and used for traffic coming from Maffa Way and Cambridge St (via Gardner St) that is heading north to the bridge cross the Mystic; on the east side of the overpass that road would have to be reconfigured to point north instead of pointing south. We'd also want a street that I'm going to call New St for now going from the Gardner St / Maffa Way intersection diagonally across what is currently a parking lot to the Alford St / Mystic Ave intersection.
To minimize the amount of traffic at the Alford St / Mystic Ave intersection, we'd have Mystic River bridge to Mystic Ave traffic take Alford St to West St to Beacham St to Mystic Ave, and Mystic River bridge to Cambridge St traffic would go via Alford St, West St, Beacham St, and Gardner St. That should mean that the only traffic reaching the Alford St / Mystic St intersection would be Cambridge St to Mystic River bridge traffic, Maffa Way to Mystic River bridge traffic, and Mystic River bridge to Main St traffic (and perhaps there's some traffic involving businesses on very local streets that would use that intersection as well that doesn't necessarily quite fall into these categories).
The bridge above the underpass that is currently the natural path from Cambridge St to Main St would need to accommodate westbound traffic; while most eastbound traffic could use the overpass next to Mishawum St (that overpass ought to be reversed in this plan), forcing a double crossing of the track for access to the businesses east of the Sullivan Sq traffic circle and north of the track seems like it would be suboptimal, so two way traffic on that overpass which is roughly 250' north of the track would probably be best.
Northbound traffic on Rutherford which takes the surface route instead of the Sullivan underpass, after crossing the track, currently has to turn right onto the traffic circle; that traffic would at least get the option to turn left for a direct path to Cambridge St, and it would probably make sense to make Maffa Way two way from Alford St to Gardner St to handle trips from Main St and from Rutherford in the vicinity of Bunker Hill Community College going to Mystic Ave; at Gardner St that traffic would turn right onto what seems to be an unnamed road that looks like an extension of Gardner St to get over to Mystic Ave.
Gardner St and its extension would need to be reconfigured to be two way.
I think it's probably possible to make most of the lights function with only two phases in this general design. Cambridge St / Gardner St would have one phase for straight through traffic on Cambridge St in both directions, and a second phase for right turns from Gardner to Cambridge St, left turns from Cambridge St to Gardner St, and a crosswalk across Cambridge St to the east of Gardner St. Maffa Way / Cambridge St / Alford St probably needs three phases, one to allow Cambridge St traffic toward Rutherford southbound (and westbound traffic from Main St to Cambridge St), a second to allow Maffa Way traffic toward Rutherford southbound, and a third to allow traffic from the Mystic River bridge to get to Main St / Mishawum St.
There might be a few other details of the street reconfiguration that would be needed that I'm forgetting to mention here that would be needed to make this overall plan flow smoothly.