"Pricing has the potential to reduce congestion by influencing drivers to carpool, use transit, or drive at off-peak travel times," according to the report.

One measure of improvement cited in the report involves travel time and speed. An evaluation of managed lanes on Interstate 15 in San Diego, for example, shows that drivers in the High Occupancy/Toll lanes save as much as 20 minutes compared to those in the adjacent free lanes during the most congested times.

Similarly, an evaluation of Interstate 95 in Miami -- identified by the Florida Department of Transportation as that state's most heavily congested highway before express lanes opened in 2008 -- indicates that motorists have saved about 14 minutes per trip in the HOT lanes and 11 minutes in the free lanes.

Another measure of progress highlighted in the report concerns an increase in vehicle flow on HOT lanes and sometimes on the adjacent unpriced lanes. The Interstate 394 project in Minneapolis has demonstrated that vehicle throughput grew 9% to 13% in the HOT lanes and 5% in the free lanes.

GAO also reviewed in the report the extent to which drivers have been encouraged to change their trips to travel during off-peak periods. A traffic evaluation of bridges and tunnels into New York City, for example, found that 7% of surveyed drivers said they changed their travel behavior due to variable pricing. That percentage is significant because even small changes in peak demand can have exponential effects on local traffic patterns, according to the Port Authority of New York and New Jersey.

The report notes that not all of the possible impacts of managed lanes have been examined, therefore more evaluations of congestion pricing are necessary.

"New projects under construction and in planning will not only increase the number of roadway miles that use congestion pricing, they will also change the character of pricing in the United States, as some will be operated privately and some will add congestion-priced tolls to previously nontolled roadways," according to the report. "The changing character of congestion pricing and the new challenges it brings make improving the understanding of congestion pricing even more important."

GAO recommends additional study on whether low-income drivers are disproportionately affected by congestion pricing and also if one geographic area is more negatively impacted than others in outcomes such as traffic diversions.

The 60-page report, "Traffic Congestion: Road Pricing Can Help Reduce Congestion, But Equity Concerns May Grow," is available at 1.usa.gov/GAO-Pricing.