Carpool lanes -- more trouble than they're worth?

First, let me say that I truly dislike freeway carpool lanes, and I don’t think they are very effective at changing people’s behavior. Unless you are one of the very few exceptions–those who work a straight 8am-5pm non-hourly job with no overtime, have no errands to run before or after work, have no children to pick up or drop off, and live relatively close to a coworker who works the exact same hours–you’re going to have to drive your own car.

However, my own gripes aside, I am looking for a factual answer: Does the carpool lane actually help alleviate traffic congestion (during rush hour), or would it be more efficient to open up that extra lane for all to use? Have there been any studies conducted on this subject? It seems like a big coincidence that, in my city of residence, traffic magically improves at the precise spot the carpool lane becomes a general use lane. I live in Phoenix, if that matters.

The intent isn’t to make traffic better for everyone. The intent is to make it better for those that pollute less, and penalize those that do more. And it does do that.

You are correct that this ends up making traffic worse on average for everyone, i.e. that it decreases the number of people that can can use the freeway per hour.

And I’m with you. Carpool lanes are immoral, and how the greenie weenies ever got enough power to enact them is beyond me. Some day when I’m king I’ll fix everything.

Traffic flow is enormously complex. It is really hard to tell if it would help or not and depends on the actual roads int he area.

A basic ‘rule’ that is not absolute, is you can’t expand a main road to releive traffic much on that road. What you can do is expand the capacity of the main road to eliviate traffic on alternate roads, or expland alternate routes to eliviate traffic on main roads. An alternate route is a route people will take mainly due to congestion on their perfered route.

When you create a HOV lane you are creating an alternate route, this in theory should help traffic on the main road.

Also there is some benifit to single lane roads (most HOV lanes are single lanes) as the flow is helped by not having the frequent lane shifts that happen on multilane roads.

Sometimes I think that congested interstates would move better of they were a series of single lane roads instead of one multilane road.

I would like to Hijack this thread a bit. Why doesn’t Chicago have HOV lanes? Someone told me that because the El trains run between the highways this is roughly the same. True?

Also do the cities get any monies from having HOV lanes?

Also why doesn’t anyone tell people what HOV lanes are? Years ago when I moved to DC I was driving in them (fortunately never caught) before I figured out what HOV meant.

Here in Minnesota, we have had studies on this recently. (And we’ll have more, because there are people in the legislature like vortex, who are anti-carpool-lanes, and they will keep asking for studies until they get one that supports their position.)

But the study here was for a section of freeway with 1 carpool lane and 3 open lanes. To summarize, the results were that the carpool lane carried 50% of the passengers travelling that road, the other lanes carried 50%. Or, the carpool lane carried 50%, the other 3 lanes about 16% each. So the carpool lane was quite clearly carrying more than it’s share of the passengers.

Now people are busy claiming that the number of passengers transported is not the proper measure, but that they should use something else, like taking into account the cargo-carrying capacity of these lanes, and trying to set some rule that x pounds of cargo equals 1 passenger. No doubt if you play around with the definitions like this long enough, you can come up with something to show carpool lanes are not worth the trouble.

It seems to me that one way to guesstimate the answer to this question is to look at the number of cars in the carpool lane, the number of persons per car, and ask if that lane could handle the traffic if all of those people were driving alone. Much of the time the answer is “yes.”

If the highway as a whole could handle just as many people per minute if they were all in their own cars as if there is a carpool lane, then it follows that the carpool lane isn’t really helping things.

Note that some HOV lanes clearly handle lots more people per minute than non-carpool lanes. For example, the bus lane for the Lincoln Tunnel between Weehawken, New Jersey and New York City. It may very well be that the bus-only lane entices more people to take the bus making traffic better for everyone.

For what it’s worth, I agree that carpooling is basically impossible for most professionals nowadays.

t-bonham@scc.net wrote

Cite, please. That flies in the face of every study I’ve ever seen, and also violates what should be obvious to anyone who’s studied queueing theory, and most certainly flies in the face of anyone who’s ever sat in traffic and seen the minimal (relative to the other lanes) traffic in HOV lanes.

kanicbird wrote

Cite, please. This also flies in the face of common logic. You’re saying that when routes A and B exist, that expanding A will give benefit to B, and expanding B will give benefit to A, but that expanding A will not give benfit to A. I don’t buy it.

Cite, please to back your assertion that lane changes decrease potential flow. Queuing theory (and again common logic) dictates that when a clog occurs, the ability for others to get around it (by changing lanes) increases the flow.

And to those who say “you’ve got a big mouth, Bill H.; where are your sources?”: congratulations; I don’t have any on hand. But I have read non-biased studies that dispute the reports above, and I’m fairly knowledgable about the math behind this.

HOV lanes reward carpoolers with faster traffic. While traditional carpooling might be difficult for most professionals, the system of slug lines as practiced in DC makes things a lot easier.

As I recall from college (I studied Operations Research), giving drivers the opportunity to change lanes can sometimes make everyone worse off.

One example of this is a stretch of highway where there is an extra lane for a short stretch. In heavy traffic, traffic frequently jams up around such an area.

Fascinating! I had no idea something like this exists. I particularly like the “no talking” rule. :slight_smile:

This kinda came up on UseNet misc.tranport.road a while back - one problem with dedicated HOV single lanes is that people may not want to drive in them if they can’t switch back and forth - this is because invariably a ‘left lane bandit’ (aka: Blithering oblivious idiot who putzes along in the fast lane) gets in the HOV lane and creeps along, and the traffic behind the LLB is stuck (fuming away) while general lane traffic moves along…

[QUOTE]
*Originally posted by Bill H. *
Cite, please. This also flies in the face of common logic. You’re saying that when routes A and B exist, that expanding A will give benefit to B, and expanding B will give benefit to A, but that expanding A will not give benfit to A. I don’t buy it.
Cite, please to back your assertion that lane changes decrease potential flow. Queuing theory (and again common logic) dictates that when a clog occurs, the ability for others to get around it (by changing lanes) increases the flow.
/QUOTE]

In regards to the above, it’s a fairly well accepted principle of transportation planning called latent demand. Generally speaking, for any road with significant congestion, there’s some percentage of trips that aren’t taken because of that congestion. Hence if you increase capacity, you also increase the trips being taken. This may not be automatic, but any improvement will be relatively short lived, as previously diverted trips discover the new route and fill whatever capacity is created. This may be delayed somewhat by building road capacity far beyond demand, but such excess capacity will soon draw additional development, and you’re right back to where you’ve started with little to no improvement in travel time.

This is a general condition that does not hold in all cases. There are many conditions (for instance bottlenecks) where an improvement could have a very real impact by eliminating a specific impdiment.

With regards to carpool lanes, I’d be rather skeptical of the 50% figure as well. Most cars in the lane will have the minimum necessary number of people in the car, and for that one lane to cary 50% of the people on a four lane road, it’d have to carry more than its fair share of vehicles which is empirically and theoretically dubious.

The real purpose of carpool lanes is to reduce the number of single-passenger cars by providing an incentive to carpoolers. Traffic is a very dynamic system, and one with many externalities. Given that we can’t ever provide enough capacity (see latent demand), the goal is to divert some traffic off the road. One way to do that is to get more people into each car. Thus, the argument goes, encouraging people to carpool serves a societeal good as it helps change people’s habits, and forces single-passenger cars to bear some of the real costs they create.

In reality, most car pool lanes have proven to be politically unpopular, practically useless and difficult to enforce. I’m not aware of any area that is currently building more carpool lanes, and many are taking them away.

On a related note, carpooling also brings up the matter of trust. I used to carpool until my fellow driver irresponsibly drove us into the highway ditch. I stopped carpooling after that.

No cite, but some anecdotal evidence.

What happened here (LI, NY) is that a particular rush-hour highway was very crowded so they closed it off to add capacity. People crowded up the alternate routes in the interrim.

When the work was finished, the main highway reopened and - voila- it was just as crowded as before; because people hear that the capacity is expanded and they flock to the “wider” road; new “regular” users show up, along with the original users!

Lesson:
A bigger “pipe” means more water, not more room for the existing water to flow.

The alternate routes are “emptier” than they were before the roadwork on the main highway. To this day the situation persists; people don’t learn.

One of the more STUPID experiments in “HOV” lanes…in Boston, on the SE Expressway. What the “taffic engineers” (idiots) dreamed up, was a HOV lane, beginning in Braintree (at the Rt 128/Expressway split), and continuing up untill South Boston. It makes absolutely NO SENSE, because it merely takes one lane away, and dumps the same traffic back into the existing road. Brilliant idea! It results in MORE congestion, and more accidents. Yet, this idiotic experiemnt has been going on for years!

In New Jersey, they changed the HOV lanes on I-287 and I-80 back to normal lanes a few years ago.

A PDF study on why

I think you’re talking about 394. 394 has a bad habit of jumping from 2 lanes to three, and most of the time the 3rd lane eventually becomes an exit only lane. So, I think 394 is really a two-lane (each direction) with an optional lane for carpool, especially between 169 and Minneapolis.

A study did say that the lane carried half the rush hour traffic. However, that is likely because busses are allowed on it. If busses were forced to run on the main part (and some already do), the regular lanes would carry most of the traffic. So what?

The study that was to be performed was to open the carpool lane to general traffic for a few weeks to see the result. That seems fairly common-sense to me – why would someone oppose trying to test the effects of HOV lanes? If things are especially bad, the study could be ended early. (Sadly, I believe the federal DOT put the brakes on this experiment.)

MN-dot seems very slow to change, and it isn’t very receptive to change or experimentation, even when its construction is based off of theory. Ususally the legislature has to step in and force them to try something, even if it is temporary. IIRC, they resisted experiments in changing the freeway entrance lights for some time before trying it a couple years ago.

There are plans to extend car pool lanes in the Bay Area. Gary Richards, the traffic columnist for the San Jose Mercury News, covers this all the time. Studies have shown in all but one case car pool lanes do carry more traffic than single occupancy lanes. The ones one the roads I drive are certainly crowded enough.

Around here car pool lanes can be used by anyone at 9 am and 7 pm. If I am late, having the car pool lane open up on 237 at 9 am does not help traffic at all. This is also true when the lanes end - however this blockage may be due to a merge from an entrance ramp nearby.

I don’t carpool, alas, but it seems that lots of people around here do, even more over bridges that are free to carpoolers. The enforcement is reasonably good, also. I don’t seem many apparent cheaters.

When I have time I’ll try to see if there are studies on the traffic sites. The Murky News is pay for old material.

And if we restricted the HOV lanes to red cars, again, the other lanes would carry most of the traffic. Or if we only allowed blonds to drive in the HOV lanes.

This is just the kind of thing the fools at the legislature do. Play around with the ‘counting rules’ in studies until they get one to come up with the results they want.

Yes, because MN-DOT (Dept. opf Trans.) had the figures to show how many more deaths & injuries there would be on the roads if the freeway entrance lights were turned off. And they seem to have been right. Notice how the highway death rate is up in Minnesota this year?

You’re wrong on two counts (or at least I think you are on the first–not quite sure what your point was). HOV lanes take the “high population” vehicles (those with more passengers) out of the normal lanes and put them in a carpool lane. Therefore studies showing that they carry more passengers does not prove that they are worth-while, merely that you have segregated “high population” vehicles into one lane. The arguement against getting rid of HOV lanes should not be based on how well they segregate traffic, but on if HOV lanes actually increase bus usage/carpooling, and if so, if it is worth having underutilized HOV lanes and overcrowded standard lanes to get the extra carpooling/bus usage.

I see nothing foolish about wanting to test the assumptions that were made when constructing an HOV lane. The only thing I see that’s foolish is massaging the same statistical numbers to justify NOT testing the original assumptions.

The lights aren’t turned off. The timing on most of them has been changed. How do you know that the increase in highway accidents is attributable to the changes?

It sounds like what you are promoting is that what the DOT says should be taken as gospel, not questioned or tested, even if the system were installed decades ago and all else has not remained constant.