Think I-85 HOT lanes are expensive? Try Miami or L.A.

Raising or lowering a toll based on the volume of traffic is called congestion pricing. HOT lanes typically make this determination automatically. Georgia's I-85 Express Lanes, for example, run according to an algorithm that weighs several factors, including traffic volume, in setting rates.  Rates may change as often as every five minutes. (Humans may override the system in the event of an accident that affects traffic.)

But most HOT lanes have a peak congestion price - when traffic is so horrible that the toll lane starts looking good at any price. The peak price on Atlanta's HOT lanes - I-85 in DeKalb and Gwinnett counties - recently rose to 90 cents per mile. Now, if you drive the whole way at that highest price, you'd pay $13.95.

Atlanta has a lot more HOT lanes coming: the 30-mile Northwest Corridor project, opening in 2018, is the largest, but there's a new center lane going in on I-75 south of Atlanta, and the I-85 toll lanes are being extended.

$13.95: that's an expensive commute. But even at peak prices, Atlanta's so-called "Lexus lanes" are way cheaper than some in Miami and Los Angeles.

Nick Wood, a researcher at the Texas A&M Transportation Institute, tracks these things and kindly provided a sampling of the costs of HOT lanes. These are peak congestion prices. The chart below shows the maximum price per mile charged by HOT lanes across the country, including the total mileage of the toll lane and the maximum price charged.

*Note that Virginia does not have a cap, and at times has charged more than $1 per mile.