A DC Metro bus. A new study says that new express bus lanes in Montgomery County would not attract enough riders to be practical. (Luz Lazo/The Washington Post)

A 160-mile system of express bus lanes, recommended last year by a Montgomery County task force to help ease some of the nation’s worst traffic, is not practical because it would not attract sufficient ridership, a new study concludes.

The Institute for Transportation and Development Policy, a New York firm commissioned by the county, said the MD 355/Rockville Pike is the only corridor where demand for service and plans for new construction justify bus rapid transit. Redevelopment underway in the White Flint section of North Bethesda is expected to create 3,000 housing units to replace traditional shopping malls and parking lots over the next decade.

The task force, appointed in 2011 by County Executive Isiah Leggett (D), recommended bus rapid transit as a less expensive and more quickly constructed alternative to light rail. The group envisioned a system that crisscrossed the county, with an initial phase that included Viers Mill Road from Rockville to Wheaton, Georgia Avenue from Glenmont to Olney, and along Route 29/Colesville Road.

But development on these routes is not dense enough to generate the necessary ridership, according to the study, which was completed in December but is now circulating among county officials. Although traffic can be challenging in spots, the travel time drivers would save by taking the bus was only about 10 minutes.

In other words, congestion will have to worsen before drivers ride the bus.

“In Montgomery County, most people can park their cars directly in front of their homes and also very close to their place of work,” the study said. “By contrast, most people would need to walk some distance to the nearest bus stop and wait for a bus with a frequency of greater than 10 to 15 minutes. . . . For the vast majority of drivers, congestion delay will have to worsen to the point where the average car trip is another 10 minutes slower than it currently is before many passengers will switch.”

Leggett said Monday that the county did not intend to build out the whole system in one piece. A first phase of bus rapid transit would probably include no more than MD 355, the Corridor Cities Transitway — between Shady Grove and Clarksburg — and possibly Route 29.

“You build it one leg at a time,” Leggett said. As a practical matter, he added, it all remains in the planning stage because there is no money for construction. The first phase would cost about $2 billion.

“There’s a lot that’s still up in the air. We can’t afford any of this right now,” Leggett said.

Bus service is part of a broad debate over transportation funding that is reaching a critical stage. Montgomery officials say that unless the state increases revenue for transportation, major projects such as bus rapid transit and the long-discussed and studied Purple Line light rail, are likely to remain in planning limbo.

County officials said the bus rapid transit report — financed by a grant from the Rockefeller Foundation — used ridership figures from large Latin American and U.S. cities that may not provide valid comparison.

They also said that the findings reflect a chicken-and-egg difference in philosophy about long-term planning. Consultants said there is not enough development for the bus service. The county contends that the kind of economic development that would generate bus ridership will not take place until mobility improves.

“Our view is that the change is coming and requires these sorts of transportation links, otherwise the traffic congestion issues will make sure that the economic development doesn’t work,” said Patrick Lacefield, Leggett’s spokesman.

Council member Marc Elrich (D-At Large), a member of the transit task force, said a more robust ridership would emerge if bus service was better.

“If you put something in that actually makes sense for people to use, more people will use it,” he said.