Reliable source for this answer: Me. I've been involved in traffic and public transportation politics for years (although I stopped ten years ago). I'll try to find other reliable sources too if I can and have time. :-)
If you have a congestion problem there are a bunch of different things you can do about it. The thing that is most natural: Add an extra lane or a whole new road, is at best a short-term solution. In most cases what it does is only move the congestion from one place to another.
If you for example have small highly trafficked road into a city, making a highway there will only cause congestion once the highway reaches the city and you can't have a highway. Boston famously tried to solve this by simply tearing down parts of it's center and building a highway straight through Boston, finished in 1959. However, it was quickly evident that it didn't help, since all these cars still needed to get off into town, and there was limited space for off-ramps. ref Already 1972 plans started for burying the whole elevated roadway in tunnels instead, which happened in the 1990's and finished in 2007. And congestion is not gone either. It's right now 10:17AM, after the morning rush is over, and many parts of Boston traffic (viewable here) is congested, with MA-9 east running on an average of 10mph.
That is not to say that you shouldn't build roads. People will need to get into the city, you need to move goods etc. But it is not a very good way to get rid of congestion. This is to some extent an effect of supply and demand. Roads are free to use, and therefore there is nothing to regulate demand, so people will over use it. That suggests other solutions, like congestion charges. Simply speaking, you'll have to pay tolls to use roads that are highly trafficked, therefore giving an incentive to those who don't have to go in rush-hour to go earlier or later. Stockholm did this with success.
You can also go for straight toll booths, but that also requires you to have good options to go around the city.
Another option is to extend public transportation. If it's faster to take the train into the city than to sit in the traffic jams, people will take the train.
And one option is, of course, to make carpool lanes, or similar. Do they work? Yes, at least sometimes. They work by encouraging people to carpool, which lessens the amount of traffic. This means, most importantly, that you are not just moving the congestion from one place to another, you are actually reducing traffic.
But just as congestion charges it has to be applied wisely to be effective, and in some cases a carpool lane can probably create congestion where there was no congestion before by using up one of the existing lanes. It needs to be evaluated on a case-by-case basis.