An Analysis of 2-Person vs. 3+Person carpools in Trip Reduction

With the implementation of the 405 Express Toll Lanes (ETL), the carpool requirement was increased from 2 people to 3 people during peak hours. The intended result was for carpools to add a third person to their carpool. The reality is that did not happen. Instead, 2-person carpools retreated to the general purpose lanes or split up back to solo drivers, some of which use the toll lanes, some use the general purpose lanes, and some now use surface streets. The goal failed horribly. Keep in mind that there were several contributing factors that led to the failure of the new 405ETL (such as requiring a transponder to use the lanes and taking away general purpose (GP) lanes to create the ETL.) This article will focus on the nature of carpools to reveal its contribution to the failure and what steps could be made to reduce vehicle miles driven and the congestion it creates.

Let’s examine this from a human behavior perspective.
Who is most likely to participate in a carpool? Someone who is already in a carpool. This same principle is leveraged by marketing experts toward shoppers all the time. They know the person most likely to buy their product is someone who already bought it before. In other words, the best way to make a 3+ person carpool is from people already in a carpool. So let’s first examine the process of making 3+ person carpools from 2-person carpools.

Take two 2-person carpools. Let’s assume one person from the first carpool lives close enough to join the second carpool,forming a 3 person carpool and a solo driver. How many cars have you taken off the road? NONE. There are still 2 cars, but one is back in the GP lane. In order to reduce the number of cars all 4 people must move to a larger carpool. This would generally be either 3-person or 4-person carpools and the focus will be on these though the same logic holds true for larger carpools/vanpools.

Let’s examine the 4-person scenario first as this would be expected to provide the greatest benefit:

Let’s assume all 4 people can get together in one carpool. Wait. One person drives a small truck and the driver of the other carpool was using a 2-seater. That limits who can drive. And they can’t apply for a vanpool because that requires 5 people.

With so many people, it is not feasible to drive to every house and pick people up. They need a place to meet. How about the park & ride? Wait, the park & ride lot is already at full capacity by the time they would meet. OK, they will leave for work earlier so they can be assured everyone gets a parking spot. And they have to wait around for all 4 people to show up. And by the way, the park & ride is several miles away from everyone, possibly in the wrong direction for some even, so all 4 people are still adding to the traffic getting to the P&R and reducing the overall mileage savings. Once there, they will take up 3 parking spaces that weren’t used before. Since our P&Rs are already full to capacity, we need to increase them, further increasing the cost of carpooling.

OK, so they combined 2 carpools into 1 carpool; they have a meeting place everyone drives to; they get there extra early so everyone can park; they wait around maybe 5 to 10 minutes until all 4 people are there and off they go. They get on 405 and, because the toll lane entry points have been limited, they clog the GP lanes for an extra mile or more before they can enter the toll lanes, as in the case of entering at NE 195th St or at NE 85th St. Or maybe they have to get out of the toll lane 5 miles before their exit because that is the last exit point before their exit, as in going from north of SR527 to 124th St NE. Or maybe they don’t bother with the toll lanes at all because by the time they get to the entry point, they need to exit as in the case of going from SR527 to 195th St. NE or 160th St NE to 124th St NE or NE 70th St to SR520.

So let’s assume they overcome this gauntlet of barriers: they combined 2 carpools into one; found a place to meet; met early enough to get parking; everyone arrives on time; and they drive far enough in the toll lanes to make it worth while. What has this really saved compared to a pair of 2-person carpools? Unless the commute is fairly long, it doesn’t measure up.  In fact, anything less than about 10 miles is likely to be less efficient and take longer than two 2-person carpools.  In fact, 2 person carpools are the foundation of reducing Vehicle Miles Traveled (VMT) for commutes under 10 miles (the distance from Bothell to Bellevue).  And this is why vanpools are generally only effective for long commutes.  Ironically, while most vanpools drive across county lines, vanpool programs are managed separately by each county.

Let’s just examine the impact it has specifically on the traffic on I-405 and ignore all the other considerations for the moment. Assume we start with a very optimistic ratio that half of all people on the road drive in 2-person carpools. To avoid fractions in the ratios, we will start with 8 people: 4 solo drivers and 2 carpools for a total of 6 cars. Now if you could get each and every 2-person carpool to combine into a 4-person carpool, you would cut the number of carpools in half. But that only reduces the cars used from 6 cars to 5 (4 solo drivers and 1 carpool). This is a 17% reduction in the total cars on the road. Combining the 4 people who were already willing and able to carpool into 1 car instead of 2 is barely better than just putting them into two 2-person carpools and that does not consider the mileage driving to the meeting place, the 3 parking spaces needed, the time spent going out of the way to get to the meeting place, or the time spent waiting for everyone to arrive.

Now let’s examine making 3-person carpools from 2-person carpools:
Again, we will start with the very optimistic ratio that half of all people are in carpools. To avoid fractions, we start with 12 people: 6 solo drivers and 6 people in 3 carpools for a total of 9 cars. We will assume All 6 carpoolers live close to each other and work at the same place and work the same hours, so they can combine into two 3-person carpools. And instead of meeting at a park & ride, they find an alternate place to park with ample space, perhaps at the driver’s house so the time to wait for everyone to show up is also reduced. The number of carpools is reduced from 3 cars to 2. When you include the solo drivers, this reduces the total number of cars from 9 to 8. Barely an 11% reduction in cars on the road.

Both of these examples start with an unrealistically optimistic assumption that half of the people on the road are traveling in carpools. During commuter hours this ratio is closer to 35% of all commuters on I-405 as reported by WSDOT. It is also unrealistic to expect to combine 100% of the 2-person carpools into larger carpools. So the actual percentage reduction would be significantly less.

Now let’s examine the efficiencies of forming 2-person carpools:
Following the examples above, let’s start with the scenario of 8 people, 4 solo drivers and 4 people in 2-person carpools for a total of 6 cars. If the solo drivers combine to form 2-person carpools, this reduces the number of cars from 6 to 4. This is a 33% improvement compared to 17% and 11% in the examples above producing 4-person and 3-person carpools from existing carpools. Also consider that this example assumes 50% of people are solo. In reality, over 65% of drivers on I-405 are solo, so the reduction in cars could be even better than 33%. Of course none of these cases will actually get 100% participation, but the efficiency ratios remain proportionately the same. The best and most dramatic improvement is not made by pushing 2-person carpools to 3+ carpools. It is in converting solo drivers into carpools. Furthermore, when moving a solo driver to a carpool, the same number of cars are removed from the road regardless of whether the solo driver joins an existing carpool or pairs up with another solo driver. In every case it reduces the cars on the road one-for-one..

Now let’s examine the other factors that make 2-person carpools so much more effective than 3+ carpools:

The 2-person carpool is most convenient and flexible for the participants. Usually one person will drive to the house of the other on the way to work, while a 3+ person carpool typically requires everyone to drive to a meeting place. Let’s call that the “first mile”. And when it comes to emissions, automotive exhaust systems are designed to not filter the exhaust until the car has warmed up. So a car emits its worst emissions in the first few miles driven. A 2-person carpool is more effective at reducing the number of vehicles driving that “first mile”. 2-person carpools also allow for the use of smaller, more fuel-efficient cars further closing the gap in emissions reduction of a 3-person carpool over a 2-person carpool. And a 2-person carpool is far less likely to use spaces at a park & ride or require people to drive out of their way to join the carpool.

Back to the human behavior aspect of trip reduction.

Keep in mind that most people who can easily be in a carpool already are. The remaining solo drivers will be much harder to get into carpools (or mass transit).
Consider some of the typical reasons that people don’t carpool:
Requires planning in advance.
They can’t find another person traveling to the same place at the same time and returning at the same time.
They are travelling between two points that are not common. For example, Everett to Issaquah.
They have unpredictable or variable times when they need to travel.
Their start or end point varies from day to day.
They drive as a part of their work. (realtor, business salesman, plumber, maid, delivery service, etc.)
They are making an off-schedule trip (dentist appointment, jury duty, business meeting, etc)
They just don’t want to.

Creating 2-person carpools is the low-hanging fruit — easiest to do with the biggest gains. This is demonstrated by the fact that 95% of all cars in the carpool lane were 2-person carpools prior to the ETL.
For this reason, solutions should be pursued to eliminate these obstacles to make it easier to carpool (or use other options like mass transit).
The goal is to reduce the number of cars on the road, not just push them from the express lane into the general purpose lanes and side streets. Moving solo drivers into carpools of any size is the most effective way to do this.
Lacking incentives to form 2-person carpools is counter-productive. Making the incentives start at 3-person carpools sets the bar too high to enable many solo drivers to make the transition from solo to carpool.
Increasing the pain for people via a toll is a tool of coercion, forcing people to choose the least painful option rather than enticing them with a better way to get to travel.
The same examination should be made when promoting other forms of transportation, like buses or biking to work.
It is important to consider that making solo driving harder, without making the alternatives more viable, will NOT have a significant impact on trip reduction. But it WILL have an impact on drivers’ attitudes toward government.

Posted in Uncategorized
7 comments on “An Analysis of 2-Person vs. 3+Person carpools in Trip Reduction
  1. […] More info: We have been posting ongoing blog articles at providing facts and research that prove the assumptions and methods of WSDOT are grossly wrong.  Some of these include: – Truth about federal highway funds and How HOT lanes carry fewer people than HOV lanes – How expensive and inefficient tolls are at collecting revenue – WSDOT Admits Traffic is Worse – An Analysis of 2-Person vs. 3+Person carpools in Trip Reduction […]


  2. Nola Elson says:

    30 years ago, Highway 101, in Marin County, Ca, the main thorough fare to get into San Francisco from the North… also had 3 person HOV lanes. They never worked. The other lanes were always packed and slow and rarely did you see the HOV lanes get used. After years and years of trying to make it work the Dept. of Transportation let the HOV lanes go back to 2 persons. All lanes then got used better. The general purpose lanes lightened the traffic load so people actually saved time getting to and from work. And the HOV lanes actually had reasonable traffic and was used more efficiently. I am appalled that here we are 30 years later, in Washington state, and we are trying these old methods that didn’t work 30 years ago and do not work today. Once again the general lanes are packed and slow and it now takes at least 10 to 20 minutes longer to get where you are going. There are more accidents. And DOT is trying to convince us that this is only happening in certain “bottle neck areas”. Baloney! The Bothell exits have always been a choke point on I 405. At least we used to be able to get onto 405 with in a few minutes. Now it can take 20 minutes just queing to get onto I-405. Why are we trying these old methods that were proven did not work, 30 years ago in California traffic… that was when we even had less cars. This is ridiculous.


  3. […] should be 2+ at all times and there should be no tolling and no need for a transponder.  A 3+ carpool system is less effective than a 2+ carpool system.  Charging a toll does not increase capacity; it decreases participation in carpooling as proven […]


  4. […] Carpool usage in the 405ETL is way below expected, even after giving away 10,000 FlexPasses*. Why are people at WSDOT the only ones surprised by this?  In the Everett Herald article “Fewer registered carpools in I-405 toll lanes than expected” it refers to WSDOT’s quarterly financial report to the WSTC where the #405ETL are showing most people now using the lanes would rather pay the toll than try to get a 3 person carpool.  This is no surprise. The 2 person carpools were driven out of the HOV lane and replaced with those who can – and will – pay.  I reported on the  Texas A&M University research that proved carpools would decline with HOT lanes.  Their research even included SR167.  I also documented the reasons why 3 person carpools fail . […]


  5. […] light of this, I revisit the article I posted on comparing 2 vs 3+ person carpools.  Here is the original article.  Read this for a full explanation of the comparison.  In short, the best way to get someone out […]


  6. […] What’s more is that a 2 person carpool typically does not use any spaces at a park and ride because they meet at one of the rider’s home.  Buses and vanpools have the added cost of still requiring everyone to drive and park somewhere and wait for their ride. That is part of the hidden cost of 3+ modes of transit. […]


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