Montgomery County planners are debating whether a proposed bus rapid transit system should be implemented in two phases, or if the project should be done all at once.

The debate started at a Montgomery County Planning Board meeting on Thursday, after Lead Planner Larry Cole said the public was confused about how the plan would be structured.

The 78-mile network would span from Clarksburg to Bethesda on Route 355 and through the Silver Spring area on U.S. 29 as the two main corridors of the plan.

Cole said he heard in meetings with residents a misunderstanding that the system would be built all at once. In reality, the plan is broken into two phases that allow planners to build smaller systems throughout the county and then connect them as time passes.

The first would build lanes along the medians for bus rapid transit vehicles on portions of Route 355 from Clarksburg to Germantown and from White Flint to Bethesda. Vehicles would mix with traffic in areas between those towns.

Phase two would complete the median lanes down Route 355 and would create median lanes on Route 29 from Burtonsville past White Oak.

He said the second phase is much more extensive than the first but creating the system in one phase would be cumbersome. He worried that pushing the second phase too quickly would be difficult in areas on U.S. 29 where houses are close to the road and land-use issues could arise.

Planning Board member Casey Anderson questioned why planners thought doing it in two phases -- which would take longer -- was a better option. Residents are more likely to use the system in highly populated and congested areas near downtown Silver Spring, he said, and he questioned why the county wouldn't implement dedicated lanes for bus rapid transit first in those areas.

"If the ridership is justified, then I think it ought to be clear that we can justify adding two dedicated lanes in those segments," he said.

Cole said the board could modify the plan slightly, allowing future decision-makers to review the project and alter the phasing if they found it possible to add more dedicated lanes in one area.

Board Chairwoman Francoise Carrier said she felt uncomfortable leaving the plan open-ended. The board and the County Council will vote on a plan that has been vetted by planning staff and county residents, and the approved plan shouldn't have dramatic changes in the future, she said.