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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?

TheEnigmaMachine
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    It reduces congestion for people who are carpooling. – ChrisW Jun 10 '11 at 01:22
  • 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 Jun 10 '11 at 03:01
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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 Jun 10 '11 at 05:08
  • 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 Jun 10 '11 at 09:52
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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. –  Jun 10 '11 at 12:17
  • @jprete not necessarily. Some HOV lanes will diverge from the regular lanes, returning only when exits are approached. This probably makes it easier to construct, since H.O.V. lanes, unlike regular lanes, do not need to be adjacent to the other lanes to allow for lane changes and passing. – Beofett Jun 10 '11 at 12:27
  • @jprete: Yeah, but most of the time when you create HOV lanes, you don't *build* them. You convert an existing lane into a HOV lane. – Lennart Regebro Jun 10 '11 at 14:06
  • I read a story this morning about a guy using a dummy to fool the HOV lane requirements. That led me here, then led me to this article, which I highly recommend: https://5clpp.com/2018/03/01/are-we-there-yet-the-story-of-carpool-lanes-in-southern-california/ Short answer: they work, but not in isolation. –  Jul 14 '19 at 18:39

2 Answers2

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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.

Flimzy
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Lennart Regebro
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  • You've got an obvious point you should correct." Roads are free to use" -- They're not, unless you have some magical vehicles which are free to use. This is changing some as fuel efficiency improves, but roads are very expensive to use. In addition, in places like my state, the gas tax goes to road maintenance and since gallons of gas and road usage vary together, the tax scales with cost (last I checked, a few years ago, we have the best roads in the US). – Russell Steen Jun 10 '11 at 14:52
  • The other serious concern with the "failure of expansions" you cite is that the road improvements took 30+ years (1972 to 2007). Presumably the population of Boston changed during this time, so arguing that "well there's still traffic even though there are more roads" ignores the "yes but there are also more people on the roads" -- so it doesn't look to be a solid point. – Russell Steen Jun 10 '11 at 14:56
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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 Jun 10 '11 at 16:06
  • @Russel: Added reference for the general claim. It is a well known fact that building roads doesn't really help. Quote: "Building more roads or expanding existing ones does not reduce the intensity of peak-hour congestion to any extent". – Lennart Regebro Jun 10 '11 at 16:11
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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 Jun 10 '11 at 16:26
  • @Lennart - It's not a counter argument, it's just a correction to help make your post better. I see your viewpoint that the road is free to use, but it's a simplistic view that ignores all associate costs. The costs of "using the road" include all costs of use, not just the cost of having the road. Businesses don't evaluate cost by just looking at a basic price tag for instance, they also consider labor costs, implementation costs, etc. So when someone determines (for instance) the "cost" of software, a proper accounting includes all costs of support and deployment (ie, use). – Russell Steen Jun 10 '11 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 Jun 10 '11 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 Jun 10 '11 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 Jun 10 '11 at 21:27
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    The answer is quite reasonable for circumstances similar to the example: big city, commute-associated travel in the US (where governments are car friendly). A carpool lane wouldn't help at all with the traffic jams I have around here even on Sunday afternoons, between cities. Those are clearly a case of a grossly underdimensioned highway. There's little commuter traffic at the time, and a high percentage of families. Now that's a case where tolls work even better: social trips are far more price sensitive than commuter traffic. – MSalters Jun 17 '11 at 09:23
  • By today's site standards, we'd hope for actual studies that either support or falsify the claim. Since there's none in this answer, perhaps you'd be willing to add some? This seems especially important considering the answer below gives a study on carpools that exactly contradicts your answer. –  Jul 14 '19 at 15:04
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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

Christian
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B Pitil
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  • So it does what skeptics accuse: makes traffic worse for non-HOV lanes. –  Jul 14 '19 at 15:02