Nobody beats Washington when it comes to HOV stupidity. Interstate 66 inside the Beltway, between Vienna, VA and the Potomac, is HOV-3--THE ENTIRE HIGHWAY--during rush hour. In the mornings all of the eastbound lanes are HOV-3, and in the afternoons, all the westbound lanes heading back out to the burbs. So if you've got to get into DC from the west in the morning, and you aren't HOV? Surface streets for you--US 50 or US 29, mainly, both of which are stop-and-go stoplight-laden parking lots.
The reason? Parts of I-66 are still only two lanes each way despite running through heavily-built-up areas of Arlington. They won't or can't expand the highway to add capacity, even dedicated carpool lanes, so they make the entire highway a carpool lane. A couple of times when I lived up there--and this was almost twenty years ago--I rode the Metro in from Vienna to DC in the afternoons to run errands in DC, and I'd see the inbound (east) lanes clogged up, but the outbound (west) lanes were almost empty at 4:30 in the afternoon. Because they were HOV only. Twenty years later and they STILL haven't expanded I-66 and it's STILL carpool-only during rush hour. Total insanity.
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It is won't. Adding capacity would diminish use of metro which would add a bunch to its budget deficit. Adding capacity would smash Wash dc real estate prices, and you can't have that.