Highway construction and maintenance projects are being funded more through "non-user" fees, including sales, income and property taxes, rather than user contributions, which cover gas taxes, vehicle registrations and tolls.

This research came out of a study released by Pew's Subsidyscope project.

According to the study, user fees have been decreasing over the last 40 years.

"Our nation's highway system benefits all Americans, even those who are not drivers," said Marcus Peacock, director of Subsidyscope. "However, in recent years, user revenues are paying for a smaller share of the pie. That means the broader population is paying a bigger share."

Over the years, as new technologies have developed and cars have become more fuel efficient, vehicles require less gas. This results in fewer dollars going toward user fees, Pew said. In addition, the cost of maintenance and new road construction has been on the rise.

In the early 1960s, highway user fees once covered more than 70 percent of construction and maintenance costs. In 2007, these fees accounted for 51 percent of the financing of highway construction and maintenance, according to Subsidyscope's analysis of Federal Highway Administration statistics.

The data and analysis were released as part of Subsidyscope's broader look at all federal spending on subsidy programs in the transportation sector, including rail, aviation, highways and maritime. Subsequent analyses will look at other sectors such as energy, healthcare, nonprofits and housing.

More info: www.pewtrusts.org