According to an analysis conducted by the Pew Charitable Trust's Subsidy Scope project at the end of November 2009, road ‘user fees' paid in the United States - including fuel taxes, vehicle registration fees and tolls - make up a decreasing share of the cost of highway construction and maintenance.

The research, based on Federal Highway Administration statistics, concludes that the percentage of revenue coming from road users was 51% in 2007, compared with 61% in 1997 and 71% in 1967. The remaining funds are sourced from general revenues.

Subsidy Scope attributes the trend to inadequate adjustment of user fees to inflation and rising road costs, as well as revenue instability, caused by driving patterns changing in response to fuel prices, and engines becoming more fuel-efficient.

The summary of findings and the dataset developed in the course of the analysis can be found on Subsidy Scope's website.