Carpooling is not the most popular idea among Snohomish County commuters. If the experience of other states is any indication, tolling on I-405 this fall could draw down that rate even further.

Express toll lanes will replace the high-occupancy vehicle lanes on I-405 from Lynnwood to Bellevue starting Sept. 27.

Studies show that carpooling often goes down after tolling. With the incentive to buddy up taken away or diminished, more people choose to drive solo.

Georgia experienced at least an initial slowdown due largely to raising its carpool minimum, from two people to three, when it switched to HOT lanes on I-85 near Atlanta in 2012. Researchers found that the new rule “may have dis-incentivized carpooling,” with far fewer carpools and many of the carpools that remained choosing to travel in the general purpose lanes.

On I-405, the requirement to register as a carpool also could stymie efforts to have commuters buddy up.

The I-95 express lanes in Miami made a similar switch and early on saw fewer three-person carpools overall, as well as fewer two-person carpools using the toll lanes, according to a recent Texas A&M survey.

There were exceptions to the trend.

Projects that added capacity in addition to tolls saw carpooling hold more steady or go up, the survey found. (Sound familiar?) And that’s something we’re getting south of Highway 522, where I-405 was widened to include two express toll lanes.

The added lane was in response to congestion in the carpool lane. From Bothell to Bellevue, the HOV lane often is as clogged as the general purpose lanes.

If the express toll lanes get too clogged again during a commute, they’ll switch back to “HOV only.”

So why not just switch to a three-person carpool requirement and skip all this tolling business?

In Texas, where a lot of transportation research originates, the conclusion was that it doesn’t work. Past experience in that state showed the switch from two-person to three-person carpool lanes resulted in a 65 percent drop in carpool lane use.

And while our state fares better than others, carpooling nationwide has fallen sharply out of favor since its oil embargo-fueled heyday of the 1970s. Nationally, carpooling is down by half since 1980, with less than 10 percent of drivers traveling by carpool.

In 2015, soaring gas prices might still be the best bet to get folks to carpool, according to a Puget Sound Regional Council survey.

We’ve seen it on Highway 167, which has a high-occupancy toll lane. Carpooling and transit use went up there with the cost of gas. When gas prices fell, more folks drove alone and paid the toll to speed up.

But gas prices continue to fall this summer, even as our state’s gas tax is ratcheting up. Washington already was No. 5 for highest average price at the pump, AAA says.

What will fall bring? Traffic and tolls, for sure. We’ll see about the rest.

Need a carpool?

To date, 11,000 people have requested free Flex Passes through a WSDOT carpool incentive program on RideshareOnline.com. Free passes run out soon; requests are due by the end of August. The website also helps folks find carpool partners and sign up for vanpools.

More than one-third of the 130 Community Transit vanpools travel on I-405 and may have room for more. In addition, 10 vans are available for new vanpools. Find out more by calling 888-814-1300.

Have a question? Email me at streetsmarts@heraldnet.com. Please include your first and last name and city of residence. Look for updates on our Street Smarts blog at www.heraldnet.com/streetsmarts.