The assumption was that the system's maintenance and improvements would be paid for by users: Those who drove on the roads and highways. The fairest way to assess that was through a gasoline tax. Drive more or bigger vehicles, you pay more. Seems rather logical.
Fast-forward a half-century.
The gas tax has been stuck in a time warp. It was last raised in 1993, to 18.4 cents a gallon. Despite the passage of more than 20 years, with both ensuing inflation and an aging system that needs ever-more maintenance, there it has stayed. The Highway Trust Fund has been starved of cash, and is the process of going broke.