Total Cost for Interstate System subsidizes sprawl?

[Parkguy]

This blog entry from Richard Florida links to research that shows the total cumulative cost for the U.S. highway system is nearing $700 Billion. In addition, user fees [[gas taxes, primarily) as a percentage of total funds have dropped by about a third over the last 40 years, from about 73% to just over 50%. The rest of the costs are met from selling bonds and from non-user sources-- which I assume means general fund taxes. These figures are all based on constant 2005 dollars.

The sudden rise in total costs beginning about 1999-- perhaps driven by demand caused by ultra-cheap gas prices in the 90's?