Photo by derang0.

If the Washington region charged around 15 cents per mile for use of the region’s freeways around rush hour, traffic congestion would virtually disappear, the Federal Highway Administration concluded in a recent report. Cutting the number of cars by 10 to 14 percent would actually reduce delays by 75 to 80 percent. A simple, low congestion charge would do just that.

It’s a well documented fact of traffic that congestion isn’t proportional to the number of cars. Adding ten percent of cars doesn’t just lengthen each trip by ten percent. Roadways have a “tipping point,” where free-flowing traffic suddenly turns into stop-and-go gridlock. Once a road “crashes” in this way, it’s very difficult to speed the cars back up. Some regions have tried to manage this problem with metering lights on ramps, which limit the numbers of cars that can enter. However, drivers chafe at waiting in a long queue to enter a freeway while cars already on the road zip by, creating political pressure to turn off or shorten metering light times.

According to a MWCOG survey, 7.7% of car trips in the morning rush are “discretionary,” meaning drivers could go at other times. Shopping trips or visits to friends, for example, count as discretionary, while trips to work or the airport do not. In the evening peak, that number rises to 10.5%. If we could create an incentive for some of those drivers to take their trips at other times, and for some commuters to shift to transit, carpools, or telecommuting, FHWA believes that we would reduce demand on the freeways below the “tipping point.”

Would a congestion charge reduce car trips enough? Adding peak surcharges onto existing tolls in New York and Florida cut vehicle trips by 7%. Variable tolls in Seattle and on the New Jersey Turnpike reduced it 13-15%. And the more substantial center-city congestion programs in Singapore and London reduced it 20-30%. Most of these programs also increased HOV and transit use as well.

FHWA recommends an initial charge of 15 cents per mile along congested roadways like I-270, I-95, and the Beltway. Carpools of 3 or more people would travel for free, as would buses. Drivers would pay through E-ZPass transponders, avoiding any delays for payment. The major obstacle, as the report points out, is political will.

David Alpert is Founder and President of Greater Greater Washington and Executive Director of DC Sustainable Transportation (DCST). He worked as a Product Manager for Google for six years and has lived in the Boston, San Francisco, and New York metro areas in addition to Washington, DC. He lives with his wife and two children in Dupont Circle. Unless otherwise noted, opinions in his GGWash posts are his and not the official views of GGWash or DCST.