Would it be fair to say that the random, occasional bumper-to-bumper traffic you'd see on Prospect either north our southbound is caused by overflow/issues on 93?
Prospect really isn't a crosstown Artery. That traffic is mostly local, although the bunching is infamously exacerbated by lousy signal timing in the Union Sq. vicinity...which is a tertiary outflow problem of the parkway. Some of the
current McGrath improvements are going to help with that, because the staggered Washington signals around the parkway ramps were so wretchedly timed that they routinely backed the street up right into Union. Somerville Ave. + Washington have some of the most bugfuck-unexplainable signal timings in all of Metro Boston, and absolutely none of the lipstick-on-a-pig things the DOT has done to-date have substantially targeted that festering problem...until now. You'll probably see some corresponding improvement around Union as an echo effect from the signal retimings...but beyond that the southern and western feeders into the Square are largely a separate ecosystem.
The Northern Expressway was the last of the major Boston highways to open...not being fully tied-into the Artery until 1973. From the 50's to the '73 the MA 28 mainline
was the load-bearing highway into Downtown from points due north, until completion of the Medford-Artery gap transferred that function to 93. Drivers would bail onto 28/Fellsway at the Spot Pond exit when the Northern's construction stalled out there in the early-60's...and eventually to Mystic Ave. when the expressway advanced another couple miles. Completion of the planned highway
should've allowed them to consider grounding the McGrath 45 years ago when its substandard Wartime construction first would've started showing its age, because the volumes should've transferred over to 93 upon its completion. Unfortunately the extreme-substandard Artery was simply never equipped to absorb all of the traffic from the much higher-capacity Northern, and the cancellation of the Inner Belt splitting off there right before the Route 1 merge took away the primary means of defusing the capacity mismatch. So from literally Day 1 the decks opened, the backups onto the decks spanned for miles...and lasted hours beyond the regular rush period. Average length of the AM backups put it to the Mystic ramps, and on a bad day it would go all the way to Spot Pond. PM was somewhat better, but not better enough because the Somerville decks were always in active process of disaster-triaging the outflow of the Artery's backups...and were perpetually constipated as a result.
Because 93 never did its intended thing, the volumes never left McGrath like they should've in the early-70's. Average backup queue was so bad you still had a faster trip bailing at Mystic Ave. (or even Spot Pond) and doing 28 the rest of the way in. Wellington Circle, Leverett Circle, Storrow Drive, Memorial Drive, and so on paid additional induced demand price as a result. On the way home the Artery ramps were such an unbelievable clusterfuck that shortcutting was the norm. This vicious cycle of overload didn't stop until the CA/T bores and connections all opened throughout '03-04. The reduction on McGrath was swift and immediate, because finally 93 had enough matching capacity through Downtown to take the decks and staying on the highway became, extremely more often than not, the fastest way.
I lived in East Somerville right off Pearl St. from '02-04 and witnessed the changeover up close on my own daily walking commute. You'd see McGrath completely locked stone-solid every afternoon between the Medford St. and Broadway lights (some days stretching all the way to Mystic Ave.), and the frontage roads between Medford and Somerville Ave. all backed up because of choked merge queues before the Medford lights. Almost from the day the SB tunnel opened in '03 those backups largely disappeared. No longer spanned multiple signals. Looked, comparatively, like School Vacation week traffic every week compared to what was the norm immediately beforehand. NB's opening in '04 and completion of the last disrupted links knocked another peg out from under it. You almost never had jams spanning the Medford-Pearl signal pair anymore...rarely ever had backed-up merge queues between Washington-Medford anymore. And the Viaduct itself more often than not was a 60 MPH drag strip at 9:00am and 4:00pm between the Medford lights and the Twin City lights. The qualitative effects were that dramatic just as a daily passerby. Those qualitative effects were quickly backed up by new MassHighway traffic counts showing the magnitude of the load reduction on hard-data spreadsheets. And the loads haven't crept back in any way/shape/form in the 15+ years since.
The cause for the overload was *extremely* singular. Extremely. Like...I get at least the sentiment of *some* of the highway teardown fears, like Casey Overpass @ Arborway. Because the data said that there was significant induced demand that would go away with a traffic calming, but said data was abstract enough for buying into that concerned citizens could be excused for asking "But what if you're wrong?" There's no such abstraction here. It was clear as day where the traffic was coming from, where it was going, and where it was going to vanish to if the source problem got licked. Somerville writ-large has never really concern-trolled the whataboutism of dieting McGrath--or the Sullivan Sq. overpass + Rutherford Ave. for that matter, since those problems were equally fed by the 93 breakage and equally vanished in '03-04--because of how duh-obvious the traffic dynamics were. And building up the neighborhoods was never ever going to induce a new growth spike strictly on local traffic that would backfill any of the slack capacity. Too small-potatoes compared to everyone from Stoneham to Concord, NH who used to be using 28 as the daily shortcut to spray
everywhere across the CBD. Any increased replacement volumes drawn by strictly local destinations are going to be diffused from all directions...not pushed through a singular "everywhere due-north to everywhere CBD" funnel like the shortcutters of old.