A Government Accountability Office (GAO) report released this month concludes that congestion pricing has helped reduce traffic delays in several metropolitan areas where it has been tried, the American Association of State Highway & Transportation Officials (AASHTO) reported.

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

One measure of improvement cited in the report involves travel time and speed. An evaluation of managed lanes on I-15 in San Diego, for example, shows that drivers in the high-occupancy toll (HOT) lanes save as much as 20 minutes compared with those in the adjacent free lanes during the most congested times.

Similarly, an evaluation of I-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 I-394 project in Minneapolis has demonstrated that vehicle throughput grew 9-13% 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.

The GAO reported, "Some HOT lane projects also added new lanes, and studies did not distinguish the extent to which performance improvements were due to added lanes or pricing. In addition, although the number of cars using HOT lanes has risen, there were fewer people in those cars because of an increase in the proportion of toll-paying solo drivers or a decrease in carpools."

GAO recommends additional study on whether low-income drivers or some geographic areas are disproportionately affected by congestion pricing: “Potential concerns include income equity (whether low-income drivers are disproportionately affected by congestion pricing) and geographic equity (whether one geographic area is more negatively affected than another, such as when traffic diversion occurs).”

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.