Think of it this way: you walk into Harvard station at 8:00am bound for Haymarket, North Station, or Science Park. Which is the easier trip?
-- Elbow-to-elbow on the Red Line 4 stops to Park, then a long platform dwell to the stairs, then waiting for a trolley that doesn't loop at GC, then that elbow-to-elbow trip 2-4 stops on Green?
-- Grabbing an outbound seat on an empty Red train 1 stop to Porter, grabbing an empty seat on a Green Line train starting its run, 4-6 stops to your destination.
Arguably, passenger loads could even make that more hassle-free to GC too.
Now, look at the growth on the Alewife end of Red and consider these constituencies:
-- Exploding development at Alewife and perpetual overfull parking lots.
-- Lots of riders streaming into Alewife for bus routes.
-- Arlington residents who take an unpleasant 77 or 79 trip inbound to catch Red.
-- The poor saps downwind at Harvard and Central whose trains are getting fuller earlier because of all the Alewife-end growth.
How much easier is it going to be for all of these people to divert loads for:
-- 77 riders who transfer at Porter who can now get to the downtown Green Line with 1 transfer and 1 fewer transfer than before, and get a seat to boot. East Arlington riders will even stay on Green to Boylston, Arlington, Copley, and Prudential/Back Bay rather than do the straighter 2-transfer route because of this convenience.
-- Alewife boarders who'd rather transfer 2 stops inbound instead of going all the way to Park in order to avoid the crowds who'd normally be looming over their seat after Porter.
-- Fatigued bus riders who've already had a long trip to get to Alewife who will gladly take the less crowded transfer or 1 fewer transfer to get to anywhere on Green from Lechmere-Prudential/BBY.
-- Reverse commutes from downtown to the employment centers at Alewife. Red may be pretty easy in the contraflow direction, but the Park St. transfer is still horrifying at rush. Many will gladly stay on an E train and plant their butts on a seat right at Park rather than fight through the crowds streaming up the stairs.
-- ALL OF THE ABOVE when Red is borked with delays. The alt route and sending more run-as-directeds up to Porter is big relief and big system redundancy. And way better than hoping you get lucky with a Fitchburg Line schedule for the freebie between NS and Porter. Subway riders do not typically have the Fitchburg departures/arrivals memorized, so if Red's stranded it usually takes an in-station announcement at Porter and a crowd squinting at the Fitchburg schedule board in the lobby to know if they've got a non-bus or non-wait diversion.
I think the load-dispersing motivations of the extension are underreported. Maybe rightfully so because it is such a key neighborhood builder and that's the main selling point. But you can see why this is one of the few extensions other than Red-Blue and Seaport-Back Bay that both sharply increases the ridership and adds all-new riders while providing substantial radial circulation relief. And since it's much less expensive than those two very invasive subway digs desperation for downtown relief can easily rocket it to the top of the list as something low-hanging fruit that they can grab-and-go without too much financial pain and put through a study-design-build process faster than a subway dig. Something I am sure STEP is ready to promote.
