I also commute in the DC area which, as glatt indicates, has made heavy use of HOV (high-occupancy vehicle) lanes. Some lanes require 3 occupants per vehicle and some only require 2 occupants.
I do not believe they work. Let's take
395 in Virginia as an example. Highway 395 runs from basically the White House in the center of DC to points south. 395 is broken into 3 sections: a Northbound lane (2-4 lanes) that anyone can use, a Southbound lane (3-4 lanes) that anyone can use and a center, reversable lane (2 lanes) that is HOV northbound from 6am to 9am and southbound from 3pm to 6pm. Outside the HOV hours, anyone can use the HOV lanes which flips direction at noon and midnight.
While HOV may in fact produce the result of "fewer cars on the road" and thereby "prove" that the goal of reducing congestion is met but I would counter that the average commute time across all drivers (HOV and non-HOV) actually increases which more than offsets the "feel good" metric of reduced congestion.
The problem is that not enough people, for whatever reason, use it. So while 10-15% of folks get home 30% faster, the other 85% of drivers' commute time increases by the same proportion resulting in longer commutes, more fuel consumption and more pollution than if the HOV lanes were opened to all drivers and the traffic could adjust between the lanes to achieve the most effective distribution of cars between HOV and non-HOV lanes.
The HOV lanes on 395 are grossly underused (the 3 people per car makes it very difficult to qualify for) and, during either rush hour, the non-HOV lanes are a parking lot and the HOV lanes are full of tumbleweeds. I usually stay late at work to time reaching the HOV entrance at 5:55 pm when the HOV is lifted because no one is on it at that time.
But no government official will speak out against HOV because the money to build HOV lanes is a gift from the Federal government to the states in exchange for enforcing HOV. So the states couldn't care less if it works or not because they need the money to build new roads. So no one at the state level will acknowledge the problem HOV creates lest they risk being cut off from the Federal money.
Now, I only know about the HOV lanes in my experience so there may be cases where it works very well. I just don't think the blanket statement that "it works great in the US" is really true. The government might think it works great but no one is asking the drivers who have to live with traffic decisions made by people who are driven to work in limousines.