395 and 66 are two interstates that connect DC to the Virginia suburbs. The HOV lanes on 395 are usually more open and faster than the main highway. 66 is entirely HOV inside the Beltway during rush hour, so if you want to go that way at all, you need passengers.

Virginia just added HOT (passengers or pay) lanes on the Beltway itself; these are usually very clear because you need a transponder to even travel within them legally, and most people haven't bothered to get one. Maybe because the Beltway doesn't actually go into the city, so the utility is low for rush hour.