the last few times i've come over the bridge there have been police officers preventing left turns into those lots. I used to take the 14th street bridge to Maine Ave. but I've found the 14th St. bridge is usually gridlock whereas the 295 route flows much faster, even during rush hour.
Gotcha - they don't let you turn make left turns off of South Cap in the vicinity of the stadium (such as into the lots by the cement plant and the new soccer stadium), but the lot I'm referring to is much further north (the one under the SE/SW Freeway, just past where the McDonald's - RIP - used to be) and it's not an issue there.
And I also used to go the Maine Ave. route but that's hit-or-miss. I get on the 395 express lane bridge in Arlington right by the Pentagon and that usually flows better than the mainline (and it's not HOV coming into the city during the evening rush). The express lanes run in each direction (not one direction and reversible like most of their length) from the Pentagon over the river.