Over at The New York Times's Freakonomics blog, transportation expert Eric Morris suggests an intuitive and economically sound way to fight congestion on American highways: introduce toll-lanes with fees that vary according to the level of congestion. The higher the number of cars on the road, the higher the cost for using the high-speed lane. By adjusting the cost of travel according to traffic conditions, Morris predicts that the average speed of commuters on certain roads could be held at an ideal speed, somewhere between 45 and 55 miles per hour. The basic economic problem with free roads, Morris explains, is that when a certain good (i.e., traveling space on the road) is given away for less than its true value, shortages are inevitable—thus leading to congestion. (This explains why congestion usually returns even after highways are expanded.) By pricing the space at a variable cost, supply will adjust to demand, and congestion will be kept at a manageable level.
One possible objection to this approach is that the burden of any sort of flat travel tax will fall disproportionally on the poor, making the uncongested toll-lanes—or "Lexus lanes," as Morris also refers to them—a privilege of the rich. (On the other hand, Morris quotes Ed Sullivan of the California Polytechnic State University, who notes that California's express lanes on SR91 are used by a variety of income groups: "You don’t have to be rich for your time to be valuable.") Furthermore, our tax-dollars already pay for highway construction and maintenance, so why should drivers pay twice? In response, Morris argues that even those who are unwilling or unable to pay for the toll-lanes will benefit from their existence: Commuters who choose to drive in toll-lanes will divert traffic away from the free-lanes, thus decreasing their congestion as well. Some pay, all benefit.
Another benefit of this approach that Morris doesn't mention is the impact it will have on emissions. By forcing cars to constantly adjust their speed, and thus consume more fuel, congestion significantly exacerbates the problem of auto emissions. If we adjust traveling fees to keep cars at a steady speed, their gas mileage will increase precipitously, and emissions will drop. In this sense, the toll-lane fee would have the additional benefit of acting as an indirect carbon tax.