From the article:
There are good reasons to believe that the methodology of “Who Pays for Roads?” if anything considerably understates the subsidies to private vehicle operation. It doesn’t examine the hidden subsidies associated with the free public provision of on-street parking, or the costs imposed by nearly universal off-street parking requirements, which drive up the price of commercial and residential development. It also ignores the indirect costs that come to auto and non-auto users alike from the increased travel times and travel distances that result from subsidized auto-oriented sprawl. And it also doesn’t look at how the subsidies for new capacity in some places undermine the viability of older communities.
But couldn't some of these unaccounted for subsidies in the methodology be offset by the productivity gains of having cars? I'm curious what the norm is in the research community are regarding how to measure the productivity gain of automobiles versus the cost of roads.