The challenge with restricting left turns in much of those corridors is the street "grid" (using the term very loosely) does not accommodate a right, right, right alternative for the left turn. It becomes a "you can't get there from here" exercise. Each left turn would have to be examined carefully for rational alternatives.
A rough list of intersections I think you could reasonably eliminate:
In commercial or business areas, or even in dense apartment building areas, it would work. However, in residential areas like North Cambridge there would be objections from residents to the increased traffic on their residential side streets.
In the case of North Cambridge, or really Mass Ave from Harvard to Arlington center, I'd argue that the bigger obstacle is the fact that streets don't line up across Mass Ave. When they do it's usually not two streets you'd really want to make into a thru way. The exception to this is Cameron - that left off Mass Ave recently got turned into a jughandle via Cedar & Harvey during phase 1 of the Mass Ave bike lane project.
The jug handle turn off Mass Ave EB to Cedar/Harvey/Cameron is in a commercial block, so it works politically.
A lot of left turns are already restricted - I'd be interested in finding ways of making more of the existing left turns to become indirect/semi-direct turns instead. Even if they are, as others rightly point out, put a bit more traffic on semi or residential streets.
A couple of more wild + crazy pitches would be at the intersections are the most annoying to drive thru and also really awkward when you walk or bike through them because of all the time spent waiting for the left turns on the higher priority through route.
The Lechmere Sq. re-streetscaping taking the third travel lane for a separated bike lane and widened sidewalk is a really good start, but they really need to extend that much further north to Twin City/Squires Bridge. At least knock out everything within the City of Cambridge so the whole drag is set up to interface with a torn-down McGrath on the Somerville side of the bridge whenever they get around to that. I can't find any MassDOT info about whether the ongoing Squires rehab project is actually going to touch the surface level to do something about trading a lane to widen the beyond-tiny sidewalks. If they were that would be good enough impetus to treat the rest of the East Cambridge blocks now.
This section is now two lanes headed toward the 'vull and two lanes splaying out into a third / right turn pocket headed toward the 'bridge.
oh! Where did these graphics come from?
The first is from the McGrath Grounding presentation from February specifically relating to Squires Bridge scope, it was basically a filler slide. The second is from 1 McGrath's 2023 application to the city, it's listed under mobility plans.
To an extent, MassDOT has been; in 2012 they owned 9572 lane miles of roadway in the state. In 2022 they controlled 9279 lane miles. That said, MA added over 800 lane miles of all roadway in that same time period, but almost all of that growth was in what FHWA considers urban local roads - something likely inevitable in any condition, and especially with new development areas building out roadways.
I'd go ahead and say basically none; the chart I attached is all lane miles in the state by roadway type, regardless of owner (HM60). I'd pulled the MassDOT mileage using the data from a different chart only showing state agency owned mileage (M81).
That concept has been the holy grail of a bunch of us here on archBoston for seemingly decades. A 2015 study was done by MassDOT to add on/off ramps to the Mass Pike at Charlesgate and elsewhere to make it into a viable replacement for Storrow Drive. Here's one ramp addition concept out of that study which I especially like:
