Perversely, the Interstate 405 toll lanes may turn out to be a nail in the coffin for that old urban environmentalist standby, the carpool.

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The $10 designer toll lanes on Interstate 405 are fast becoming a symbol for Seattle rich. But they also may be a sign of something that hasn’t gotten as much attention: the death of carpooling.

It’s only anecdotal evidence right now. But commuters who regularly drive the corridor between Bellevue and Lynnwood say the new express lanes seem to be stuffed mostly with toll-paying solo drivers.

“Yes, but what happened to all the carpoolers?” wrote one reader in response to my column last week about how people incredibly were flocking to pay the pricey $10 tolls.

“It used to be all carpools in the HOV lane,” she said. “Now when I go through there I feel like the only one.”

That would be perverse, if true. The old high-occupancy vehicle system on I-405 was so well-used it started to become as congested as the regular lanes. In those days, the only cars in the lane were carpoolers, as well as some buses.

The idea under the new system was to add a second HOV lane plus charge a toll on solo drivers. Carpooling would be free but faster and more reliable — so in theory as desirable as ever. The big caveat is the state upped the minimum number of people required in the car from two to three during peak commute hours.

David Hablewitz, a computer-infrastructure architect who lives in Bothell, recently decided to do his own analysis of how the new toll lanes are affecting carpooling.

Hablewitz stood on the footbridge at 100th Street in Kirkland last Tuesday and simply tallied the occupancy of the cars passing beneath. Out of 120 cars that rush-hour morning, only four had someone in the passenger seat.

“That’s a small sample,” he acknowledged, “but it comes out to 97 percent solo drivers. These are supposed to be our carpool lanes!”

State figures reflect the same trend. According to data released by the state Department of Transportation, about 75 percent of weekday drivers in the express lanes in November didn’t have carpool status that allowed them to travel free.

In other cities that converted their HOV systems to high-occupancy toll (HOT) lanes, the carpoolers sometimes mysteriously vanished. In Atlanta there was an average loss of 2,500 carpools per day — about 20 percent of the total. Carpoolers stopped using the new HOT lane almost entirely — even though it was free to them. Many shifted to the general lanes. Nobody knows why.

“Overall, we see a pattern of substantial decreases in carpools on many of the HOT lanes,” concluded researchers at Texas A&M who looked at projects in eight cities, including the one in Atlanta.

In Los Angeles, when they converted HOV lanes on Interstate 110 to HOT lanes, carpooling plunged by 20,000 cars per day, a 37 percent decrease. A transportation group there blamed the drop on the fact that the new lanes required carpoolers to go through the hassle of getting a digital-transponder pass in their cars (as do the lanes here).

The Texas A&M researchers also suggested that the siren call of the HOT lanes may cause some carpool groups to “break up” (that’s actually the term they used). Suddenly you can go in the fast lanes without enduring all the relationship and logistical hassles of carpooling.

The state says it will take time for carpoolers to adapt. Also, the premium on keeping the lanes moving fast should be a big boon to buses, which are just supersized carpools.

In any case, old-fashioned carpooling was already on the decline because of low gas prices and all sorts of changes in how we live and work. These toll lanes may speed it along.

“I absolutely think this is the beginning of the end of carpooling around here,” says Hablewitz, who is involved in the petition group “Stop 405 Tolls,” which has more than 26,000 signatures. He said his motivation for the petition is environmentalism. Watching what were formally HOV lanes fill with solo cars, here in the Emerald City, was galling.

“We’re Seattle, we’re supposed to love carpooling,” he said. ”Now we’re going to be ones to kill it off?”

Information in this article, originally published Dec. 18, 2015, was corrected Dec. 19, 2019. A previous version of this story incorrectly said 75 percent of November weekday drivers in the express lanes drove alone. But the state says that figure could include cars with one or two people because at peak times carpool status requires at least three people.