In general, tolling involves the imposition of a per-use fee on motorists for a given highway facility. Historically, these fees have generally been flat tolls that may vary by number of axles and distance driven, but not by time of day. Their primary purpose is to generate revenue.
Comprehensive information on tolling activity at the state and local levels is provided in FHWA's Current Toll Road Activity in the U.S. - A Survey and Analysis. This database provides inventory and characterization of recent and ongoing toll development activity from the passage of the Intermodal Surface Transportation Efficiency Act (ISTEA) in December 1991 through December 2008.
Further discussion of tolling as a revenue source from the state and local perspective is provided at AASHTO's Center for Excellence in Project Finance. The Center website also provides a State-by-State Map of state-, locally-, and privately-operated toll roads in the United States.
The IPD Revenue Case Studies provide numerous examples of pricing involving tolling.