Virginia’s HOT lanes, like the ones in the middle of the Capital Beltway through Tysons Corner, allow three-person carpools to ride free. ( Dayna Smith /For The Washington Post)

Dear Dr. Gridlock:

If Virginia was really interested in supporting carpooling instead of having a cash cow, they would change their HOV3 policy to HOV2, like Maryland.

As a former and steady carpooler living in Maryland and working in Virginia, I found it quite easy to travel with one good carpool buddy at work, but it was all but impossible (actually, impossible) to find a third person for our carpool that shared our work hours, home location and place of business.

Let’s hear it for HOV2 and a vast addition to the cadre of carpoolers!

— George J. Leiby, Middletown

Virginia is planning to raise the threshold on carpooling and, at the same time, expand the number of carpoolers.

How could both parts of that work? Let’s look at the record.

“The primary purpose of an HOV lane is to increase the total number of people moved through a congested corridor by offering two kinds of incentives: a savings in travel time and a reliable and predictable travel time,” the Federal Highway Administration says.

The pioneering HOV experience occurred in Virginia during the 1970s, as carpools of at least four people were allowed to share what had been a bus-only lane on the Shirley Highway (I-95/395).

The four-person minimum dropped to three, but most HOV systems use a two-person standard, such as exists today on the Interstate 66 HOV lanes.

As Leiby and many other commuters know from experience, carpooling is difficult, but it’s less difficult to get two people together — and to keep them together — than three people.

Nicholas Ramfos, director of Commuter Connections, a regional program that works on getting more people to share rides, has cited the same practical problems with carpooling that Leiby mentioned: It’s tough to find someone living nearby who shares your workplace, works the same schedule and is going to keep working there for a long time.

Statistics show how difficult this is. Despite the incentives to share rides, carpoolers account for about 7 percent of commuter travel, according to surveys for the D.C. region’s Council of Governments. The figure didn’t budge much between 2001 and the most recent survey in 2013.

Despite concern with the overall trend, something very interesting happened in the I-95/395 HOV system. It became one of the most successful carpool networks in the nation, despite the three-person standard.

Or was it partly because of the three-person rule? The high threshold was one of the factors that fostered the growth of “slugging,” in which commuters clustered at park-and-ride areas in the suburbs to form carpools for the northbound trip to work.

There were other factors, too, including the park-and-ride infrastructure developed for commuter buses as well as for carpools.

Now, Virginia Transportation Secretary Aubrey Layne is hoping to duplicate much of that I-95/395 slugging culture on I-66 through a program that will convert today’s HOV lanes to high-occupancy toll (HOT) lanes.

As with the HOT lanes on I-95/395 and the Capital Beltway, carpoolers will get a free ride, and as with the other systems, many will have to find an extra passenger for the HOV3 trip or board a commuter bus.

That’s a lot of trouble for people to go through, and the alternative, if they want to drive in the I-66 express lanes without meeting the toughened carpool rule, is to pay a toll that’s likely to be pretty steep during rush-hours.

The planners say raising the carpool standard and setting up a variable toll system are necessary to maintain the original inducements that made HOV travel worth the trouble: Commuters will save time and have a trip that’s about the same each day.

Foreseeing that increased congestion on I-66 would diminish the time savings and reliability in the HOV lanes, they had been planning to go to HOV3 anyway. But they’ve now tied it in with the HOT lanes plan.

HOV3 and HOT lanes are supposed to complement each other. The rise and fall of the toll rates are intended to maintain a smooth flow of express traffic. The “cash cow” can be milked to support the carpool and commuter-bus infrastructure. And the chance to escape the tolls in the express lanes creates an incentive to use the carpool and bus options.

It’s tricky for two reasons. By now, one is obvious to you: Combining tolling and free rides in the same lanes is really complicated. Also important is the Leiby issue. Would carpoolers like him be motivated to pick up a third rider, or will HOV3 plus tolling become an incentive to abandon carpooling and return to the regular lanes?

Dr. Gridlock also appears Thursday in Local Living. Comments and questions are welcome and may be used in a column, along with the writer’s name and home community. Write Dr. Gridlock at The Washington Post, 1150 15th St. NW, Washington, D.C. 20071, or e-mail drgridlock@