Every 10 minutes, a blue bus pulls up to the curb just south of the 19th Street NW entrance to the Dupont Circle Metro station. Passengers file off, and a new group steps aboard for the quick ride to Georgetown and Rosslyn. It keeps that schedule most hours of the day and night, all week long.

The service, still undiscovered by many, was launched 16 months ago with an expectation that riders would make 800 trips per day. Today, the total is 4,000 daily trips, making it the fastest-growing bus service in the city.

The service is provided not by the region's public bus system, Metrobus, but by a group of Georgetown businesses. What makes what's become known as the "blue bus" striking is the story behind it and its role in answering the demand for public transportation to Georgetown, the only D.C. neighborhood with a sizable commercial district but no subway station.

The businesses launched the service in late 2001 to help employees get to work in Georgetown, which has 17,000 workers and 3,600 commercial parking spaces. Although Metrobus runs frequent service on P and M streets NW during rush hours, restaurant and hotel workers often must travel late at night or at midday, when buses come less often. In addition, there is no Metrobus service on K Street, a job-rich Georgetown corridor lined with large office buildings and waterfront restaurants.

"We couldn't get [workers] to come here because they couldn't afford the parking," said Ginger Laytham of Clyde's Restaurant Group, one of the directors of the Georgetown Partnership, the neighborhood's business improvement district.

The partnership asked Metro to design a bus service to circulate in Georgetown's commercial district and connect the historic neighborhood with Metro stations in Dupont Circle, Foggy Bottom and Rosslyn.

"They said they had so many requests for such service that there was a long list," Laytham said. "We just thought maybe we should go ahead and try to get it up and running."

The businesses won a federal grant and hired Yellow Transportation to run the service. Capital and operating costs total about $1.8 million a year, split evenly among the grant, the businesses and passengers. Arlington County has volunteered an additional $100,000 annually, recognizing that many riders live or work in Rosslyn.

Frequent service and cheap fares -- 50 cents each way or 25 cents with a Metrorail transfer -- are key to the success of the blue bus, according to riders. Several said they have switched from Metrobus to the blue bus because it runs more often and the drivers are friendlier.

The fleet began with six buses, quickly expanded to eight and now has 10 buses running on two routes: one between Dupont Circle, Georgetown and Rosslyn, the other from the Foggy Bottom Metro station along K Street NW in Georgetown to Wisconsin Avenue and north to R Street NW. Managers said they need two more buses to keep up with demand.

"The ridership is off the charts," Laytham said.

Besides being shocked by the number of people using the service, nearly everyone involved with the service has been surprised by who is using it. They expected the dishwashers and Web designers and bookstore clerks commuting to and from work. They didn't expect the students, tourists, shoppers and residents.

Aboard the M Street blue bus one recent weekday, Hanane Ghiwahe, 23, was riding from her job at Chipotle in Dupont Circle to her apartment in Rosslyn. She used to take the Red Line from Dupont Circle to Metro Center, changing to the Blue Line for Rosslyn -- a roundabout, 40-minute trip that cost $1.10. By blue bus, she spends 20 minutes and 50 cents getting home.

"Now it's better," said Ghiwahe, a recent immigrant from Morocco.

Adam Silverman, 33, boards the bus after classes at Georgetown Business School and takes it home to Dupont Circle. "I could walk but not in this weather," a bundled-up Silverman said.

Herman Cohen, 70, who lives in Georgetown, takes the bus to the movie theater that opened recently on K Street. "Theater parking is $5," Cohen said. "My wife and I take this for 50 cents apiece. This is beautiful."

During peak hours, the buses are full in both directions, a rare phenomenon hailed by planners as a kind of transit state of grace because it means the service is operating at top efficiency. Many transit systems, especially those with a hub-and-spoke design, are empty in one direction when they're full in the other.

Georgetown leaders say the popularity of the blue bus has meant fewer cars jockeying for parking spaces in the congested neighborhood.

But with success comes a price, and growing ridership is beginning to affect performance. The system's minibuses seat 28 passengers who board and exit through one front door. At peak times, unloading and reloading can take five minutes and create delays, said Ken Gray, executive director of the Georgetown Partnership. There is no money to add buses at this point, he said.

The partnership has committed to operating the service for three to five years. At that point, "if we could turn this over to a public agency, and they could maintain the same level of service, we would welcome that discussion," Gray said.

Dan Tangherlini, the District's transportation director, is helping downtown businesses create a circulator bus like the blue bus, which he said should be the model for Metrobus. "We think this Georgetown model is the direction we need to go," he said, "and we're working with Metro to design a new set of service."

The service that planned for riders making 800 trips per day has 4,000 daily trips, including this stop on 19th Street.A Georgetown Metro Connection bus, known around town as part of the "blue bus" service, picks up passengers at 19th Street between Sunderland Place and N Street NW.