AT FIRST GLANCE, adding high-occupancy toll lanes to Northern Virginia's congested corridor of Interstates 95 and 395 makes sense. In theory, HOT lanes -- free of charge for drivers with two or more passengers, like the current carpool lanes, but pricey for lone drivers at rush hour -- apply the magic of the market to a sclerotic transportation artery. If the HOT lanes slow, the tolls climb, dampening demand so traffic flows again. The operator of the HOT lanes, a private firm, would have a serious incentive to keep traffic moving: profit. It also would commit to widening and extending the 30 miles of carpool lanes, which now extend from the 14th Street bridge to southern Prince William County, by at least 20 miles to the Fredericksburg area.

So what's not to like? Carpoolers known as "slugs" worry they'd lose drivers who would rather pay a toll than ride with strangers. But if drivers abandoned slugs en masse, they would drive up HOT lane tolls to the point where, feeling the pain, they would be forced back into carpooling. Remember: Tolls could go as high as necessary to thin out driver-only traffic.

A more serious concern is whether the venture is financially viable. The two teams bidding to build and operate the lanes, Clark-Shirley and Fluor-Transurban, estimate that rush-hour tolls for the HOT lanes would range from 10 to 30 cents a mile, depending on congestion, location and time of day. But some independent experts think demand by carpoolers (who, remember, drive for free) will be so heavy that it will require much higher tolls for other drivers to keep HOT traffic moving swiftly.

The most thorough analysis so far, by Ronald F. Kirby, transportation planning director for the Metropolitan Washington Council of Governments, suggests that tolls in some rush-hour chokepoints will have to be as high as $1.60 a mile. A morning drive from the Fairfax County Parkway into the District then would cost more than $13; the southbound evening trip would be even more expensive. For a Prince William County resident working in the District, a round-trip rush-hour commute would cost more than $40.

Are the firms seeking to build and operate the HOT lanes are making realistic revenue projections? In a region that has consistently underestimated the future strain on roadways, that question needs to be answered. So do other questions, such as how to take maximum advantage of bus rapid-transit along I-95/395 and how better to crack down on the staggering number of solo drivers who cheat by using high-occupancy lanes. It is true that HOT lanes are successful in California and elsewhere. But traffic conditions in Northern Virginia are unique, and planners should proceed with caution.