Dive summary:

• Analysts at the College of William and Mary have put data behind the perception that rising fuel efficiency cuts down more than visits to the pump – cutting resources available to fund highway construction and maintenance as vehicles get along with less fuel.
• The report from the school's Program in Public Policy stated that not indexing the federal fuel tax to inflation 20 years ago, when it was last raised, left $64.4 billion the table rather than in the Highway Trust Fund, the primary source for federal road spending.
• The number-crunchers also concluded that not changing now to an indexed tax or finding another system will forgo $365.5 billion in the coming two decades.

From the article:

"HTF revenues are inadequate to support today's road and bridge spending levels, which are already well below what's needed to maintain the interstate system's performance," said Christian Klein, vice president of government affairs for Associated Equipment Distributors (AED), which sponsored the research. ...