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Many cities have added carpool lanes to their roads, in hopes of encouraging people to carpool and reduce traffic. The idea is that, not only does the road now have an extra lane, there will also be less cars on it as well.

I've had friends claim, however, that they are actually less effective than an extra lane. They claim less people drive in the lanes, so they aren't actually helping.

So, is there any evidence, either way, that carpool lanes reduce traffic congestion more than a normal, extra lane would?

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    It reduces congestion for people who are carpooling.
    – ChrisW
    Commented Jun 10, 2011 at 1:22
  • With most H.O.V. lanes (which are less restrictive than carpool lanes because they allow all types of High Occupancy Vehicles including motorcycles) it seems that there isn't enough usage, even during busy times, to justify the restriction. When you get to benefit directly from them by using them, though, it sure can be a nice luxury. It definitely is beneficial to buses (and other such forms of massive transit), which may encourage more people to use these alternatives due to the time-advantage during peak travel times.
    – Randolf Richardson
    Commented Jun 10, 2011 at 3:01
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    In the vein of @ChrisW's response, even if the carpool lane does not reduce congestion much, if it incentivises people to carpool, I'd say it has a net positive effect.
    – Fake Name
    Commented Jun 10, 2011 at 5:08
  • A normal extra lane costs a xxxxload of money, and there may not even be space for it. It also tends to only add congestion at other places, such as where that extra lane stops. So you are not only comparing apples and oranges, the answer is "Yes it does". Better public transport also helps.
    – Lennart Regebro
    Commented Jun 10, 2011 at 9:52
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    @Lennart: An HOV lane is a normal lane but with restrictions, so there is no apples-and-oranges comparison here, especially in regards to cost.
    – user1770
    Commented Jun 10, 2011 at 12:17

2 Answers 2

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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.

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    @Russel Steen: No, sorry. The road is free to use. That your vehicle isn't is another issue. You are incorrect here. The road improvements did not take 30+ years. It took 30+ years from the start of planning to the finish of the project, but the project took less than that (but still over ten years). But the planning doesn't plan for todays traffic. It plans for tomorrows traffic, and this is shown by the improvements actually getting rid of congestion where they improved. The point is that when you do that congestion just moves elsewhere. So sorry, your counter-arguments are incorrect.
    – Lennart Regebro
    Commented Jun 10, 2011 at 16:06
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    +1 for being an answer from an actual expert, plus references. This is a complete and correct answer, even if it's a wildly unpopular one.
    – Ernie
    Commented Jun 10, 2011 at 16:26
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    I really like this answer but it would be even better with a little more detail on the carpool lanes at the end. It address the general congestion issue well and it approaches the theory of carpool lanes but... then it ends. :) A few references for carpool lanes effectively reducing traffic would be cool. (Where did it work? Where didn't it?)
    – MrHen
    Commented Jun 10, 2011 at 17:11
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    @Russel: You have associate costs for everything. The point is still that the road is free to use, and therefore it will be susceptible to overuse as every single free resource will be. This is an uncontroversial issue. See: Tragedy of the commons.
    – Lennart Regebro
    Commented Jun 10, 2011 at 21:26
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    @MrHen: I agree, more references on studies of carpool lanes would be good. I'll see if I can dig some up, but I won't promise anything. :-)
    – Lennart Regebro
    Commented Jun 10, 2011 at 21:27
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Here is a study made on the effectiveness of HOV Lanes.

Essentially, the study says that: 1. Speed of traffic in a pool lane is reduced by 20% compared to off peak hours. 2. It increases congestion in other lanes, and reduces the speed in those lanes. 3. People who have long term pooling arrangements are not influenced by the time saving factor

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  • So it does what skeptics accuse: makes traffic worse for non-HOV lanes.
    – user11643
    Commented Jul 14, 2019 at 15:02

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