If you have two lanes and want three lanes, you could build the third as public, or as toll. If you build as public, it comes out of taxes, such as the gas tax. If you don’t have enough public money, perhaps you increase the tax. If you build it as toll, you can bond the construction and pay for it with tolls.

At least in theory, this means the toll lane accomplishes the same total road throughput, but shifts the entire cost of its construction to its users instead of depleting public funds. It then appears regressive, but is arguably progressive.