"We haven't gotten the numbers we thought we could," Kevin Gutknecht, a spokesman for the Minnesota Department of Transportation, says of the high-occupancy-vehicle, or HOV, lanes. "People would be sitting dead still, and they would look over and see the HOV lanes empty. So they're thinking, 'Why don't they just open that up?' "

Starting next week, the state plans to do just that. The HOV lanes will become HOT lanes — for high-occupancy toll. Solo drivers on an 11-mile portion of I-394 will be able to drive in "MnPass" lanes formerly reserved for car pools, as long as they're willing to pay electronic tolls that vary dramatically depending on traffic volume.

When traffic is light, the toll might be 25 cents. When traffic's at a standstill, the toll could hit $8. The tolls will vary up to 20 times an hour depending on traffic. Car-poolers, motorcycles and buses can still use the lanes for free.

I-394 will join an 8-mile section of I-15 in San Diego that was the USA's first foray into what traffic planners call "dynamic" toll lanes. The concept is simple: Thousands of drivers who are racing to catch a flight, pick up a child from day care or get to an important meeting on time will pay a premium to escape traffic meltdowns.

The idea is catching on with transportation agencies. At least two other highways — the Riverside Freeway in Southern California and the Katy Freeway in Houston — vary tolls, but the rates are set in advance.

Utah, Colorado, Maryland and Florida are among other states considering HOT lanes. Virginia announced last month that two private companies will build two lanes in each direction on 14 miles of I-495, the Capital Beltway that connects suburbs of Washington, D.C. Construction could begin as soon as next year.

"HOT lanes are ... the best combination," says Anthony Downs, a scholar at the Brookings Institution and author of the 2004 book Still Stuck in Traffic. "If there were no tolls at all, the roads would get very congested and just crawl along. Tolls on all lanes would place a bias against low-income drivers. Americans don't like that. They think it's unfair."

Road funds inadequate

The initial image of HOT lanes was a negative one. They were called "Lexus lanes" that favored affluent drivers. But that image appears to be fading. And the growing acceptance of HOT lanes appears to be a result of several factors:

• The federal gas tax, which pays for upgrades to the nation's aging freeway system, is inadequate. The tax, set at 18.4 cents a gallon since 1993, is not tied to inflation, and Congress is reluctant to raise it. "Our current system of fixing roads is really breaking down," says Kenneth Orski, publisher of the newsletter Innovation Briefs, which reports on roads and transit and supports HOT lanes. "The gas tax is not working."

• The federal gas tax is levied per gallon, and automobile engines are getting more miles to the gallon. That means vehicles are wearing down highways faster than money is being generated to repair or replace them. Converting HOV lanes to HOT lanes raises money, and it's less expensive and more efficient than building new lanes.

• Technological advances make it easier to collect tolls. In systems such as E-ZPass, drivers don't have to stop at toll booths. Equipment in their vehicles communicates with sensors along the highway and deducts the proper toll from their accounts. "You can do it at freeway speed now," says Tim Lomax, a traffic expert at the Texas Transportation Institute at Texas A&M University.

San Diego leads way

The HOT lanes on San Diego County's I-15 opened in the center of the highway in 1998. Drivers pay 50 cents to $4 for a one-way trip on the lanes, known as FasTrak.

"No one had ever done it," says Kim Kawada, the county's original project manager. "People were very cautious. There was a real concern that it would hurt car pools. But we really have not seen that."

Before the HOT lanes opened, about 7,700 car pools a day used the lanes. That was up to 16,000 in January, she says.

Many commuters say they love the HOT lanes. Francie Ford, 58, a real estate agent, says she uses the lanes three or four times a month traveling among her home, her office and downtown San Diego.

"It gives me a little bit of an edge," she says. "You just go be-bopping along instead of sitting in all that traffic. It certainly is more convenient to ride in the FasTrak."

Before retiring in March as an insurance company claims representative, Kathleen Totaro, 58, used the FasTrak lanes three or four times a week. "Oh, I love it," she says. "It just cut my commute in half, even more than that sometimes. I was on the freeway all the time, and it was a parking lot. If you wanted to pay for the FasTrak, you could move a lot faster."

Others are resistant to the HOT lanes. Communities in the Washington metropolitan area, Florida, Georgia and elsewhere have seen angry opposition when HOT lanes have been proposed. One worry is that so many drivers will use the lanes that they'll become as clogged as the rest of the highway.

"Our biggest concern is that you would not be able to charge enough to discourage enough people from using the lanes if the HOV lanes are converted to HOT lanes," says Scott Hirons, chairman of the Committee to Save HOV, a Northern Virginia group founded this year. "There are employers in the D.C. area who would reimburse their employees to use HOT lanes."

Minnesota's Gutknecht says more than 2,100 people have signed up for the I-394 HOT lanes, which are expected to generate up to $2.5 million a year.