Road tolls in the United States will need to increase in coming years to cover a rise in debt, but the slow economic recovery is hampering their ability to do so soon, Moody's Investors Service said in a special report on Tuesday.

Toll roads' combined leverage grew last fiscal year, Moody's found, with debt per roadway mile rising to $18.9 million from $14.3 million the year before.

Tolls also increased to an average of $1.96 per transaction from $1.82 in 2011, according to the rating agency. They will need to continue rising to support the debt burden, Moody's said, adding that drivers may resist paying more during the weak economic recovery.

Looking at 31 states that have a state or regional tolling authority, the National Conference of State Legislatures recently found New Hampshire's Turnpike charges the lowest toll rate of $0.03 per mile and California's South Bay Expressway the highest at $0.35 per mile. Most states charge less than $0.10 per mile.

With the emergence of a national highway system after World War Two, the United States moved away from charging for access to roads. In the last couple of decades, though, state and local governments have embraced toll roads and Moody's expects the sector to keep growing.

"The user-pay model for funding transportation projects is gaining acceptance as traditional tax-supported funding options for infrastructure fall short of needs," Maria Matesanz, the Moody's senior vice president who authored the report, said in a statement.

Moody's median bond rating for U.S. toll roads is A1, with ratings ranging from a high of Aa3 to a low of B1. It has a negative outlook on the sector due to the weak pace of the economic recovery.