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Yep. Autumn, Montgomery and Cahill as well as Santa Clara Ave could/would be impacted by proposed construction. Those are the streets that perhaps 75% of the folks use to get to SAP Center.
There is no virgin land. There may be land that has/can be purchased for redevelopment. (SAP Center sits on land that used to be a car wash and Datsun dealer.) There are some old/empty industrial buildings off Stockton (on the other side of the tracks) that might be able to be developed.
IIRC they are buying up most of the Santa Clara Ave to Park Ave (Guadalupe River to railroad) -- immediately south of SAP Center. (Autumn is one way NB, Montgomery one way SB -- these are the main streets south of SAP Center for folks to ingress/egress.)
There are some parking structures east of Guadalupe SR87. I recall taking 30+ minutes to get out of the structure because of how it's laid out (last parked there more than a decade ago because of that). The flow "in" may be OK, but getting out onto the streets (and/or paying parking fees to exit) is problematic.
A number of high rises on the other side of Guadalupe SR87 have parking structures they open up for games/events (and you're looking at up to 1/2 mile of walking unprotected from any weather -- which is usually mild, but there can be nasty storms).
County: Light rail (limited corridors), bus (limited corridors).
Regionally (Bay Area): CalTrain (but only runs north to SF - last train leaves 15 minutes after game; but not synchronized for non-game events). Amtrak/ACE commuter service (can't get "to" arena for evening, no late "home" runs). (Greyhound station is 1/2 mile from arena)
(Amtrak Coast Starlight SB leaves in the morning, three days a week; NB leaves early evening, three days a week)
BART is "planned" to extend from east SJ to Diridon Station, but may have hit some construction delays.
County bus would be impacted as their transit hub is adjacent to Diridon Station (I don't recall if that area is planned for re-development, but closure/reduction of Cahill, Autumn and Montgomery would impact service).
There is no public transportation realistically for those who live SOUTH of San Jose (Gilroy, Morgan HIll) after games. There are folks who drive 30+ miles north (of SF, beyond CalTrain), east as well and there are no public transportation alternatives.
Ride Share does not have good organization (when I drove it -- three years ago).