You've probably heard of the Washington-Baltimore corridor, that land of future development that will turn the region into a high-technology megalopolis someday.

But you may not have heard of the Rockville-Gaithersburg corridor, for which Montgomery County planners have similar ambitions.

Meet "Rockburg."

It is planned community of sprawling office campuses and surrounding high-density housing that Montgomery County planners would like to see spring forth from the land between Rockville and Gaithersburg. There they would make a home for the kind of "clean," high-tech, office-based industry that just about every other local jurisdiction in the nation would like to attract.

Planners call it the "Shady Grove West Opportunity Area" in the new area master plan the county planning board is developing for the Gaithersburg vicinity--the first land-use revision since 1971.

But officials in Gaithersburg and Rockville and residents of the Shady Grove area are troubled as the plan advances, and what remains, as Gaithersburg Planning Director Jennifer Russel said, are "a lot of unanswered questions."

* Officials in the two cities say this re-zoning represents a fundamental departure from what the county promised to create under the earlier, long-range land plan to keep commerce and employment in "corridor cities" along I-270. They fear that this "third city" will rob Rockville and Gaithersburg of the commercial development for which they have been priming their downtown centers based on the original county plan. Now the county is competing with the cities in trying to attract economic activity, they say.

* Concerns about the area's already overburdened roads have heightened. Improvements, scheduled for years, have been delayed repeatedly, while more development has been approved. Residents and local offcials see two trends in the proposed master plan that worry them: Nearly $75 million in construction funds is set for roads that will attract new development, while more intense residential and office complex development could add more demand on roads in the Shady Grove area before they are widened, worsening hour-long traffic jams at rush-hours.

* The new zoning categories would mark off millions of square feet in development without planning what specifically will happen there. Local officials said they feel this could upset the planning process, giving the county planning board extraordinary control by allowing a blind commitment for uses that would be known only later.

* Residents and officials are critical of the plan's vague areas. The county projects the area will take about 20 years to develop, and city planners said timing mechanisms are lacking, making it unusually difficult to coordinate roads and other services with increases in demand .

But the county sees this area as an opportunity to create an orderly devlopment of corporate office complexes aligned along a "pedestrian spine" with surrounding campuses and, farther away, housing, all of which would be in harmony with transportation systems and with the cities' development, according to Perry Berman, chief planner for northern Montgomery.

"We don't see this as conflicting with the two cities," he said. "We see this as complementary. In no way would this be detracting from what they're planning. . . . We are not creating a city. We are creating a center for a development area."

It would be a different clientel attracted to the Shady Grove area, Berman said.

The zoning categories proposed for the area west of I-270 in some cases are not much denser than was alread designated, he said.

But Jeffrey L. Rubin, chairman of Gaithersburg's Planning Commission, testified to the county planning board in April that "the Shady Grove West Opportunity Area as a high-density core would be inappropriate between the two existing cities."

He noted the county would put 19 million square feet of commercial development and 77,000 employes in a 2.7-square-mile area. In the entire City of Rockville, 14.4 million square feet of commercial space and 30,000 employes are in 11.5 square miles. In Gaithersburg, 24,000 employes are in 6 million square feet of commercial space within an 8.2-square-mile area. He said the development area in Shady Grove would constitute another city, and violate the premises of county long-range plans that go back to the 1960s.

The most controversial zoning classification is the Mixed Use Planned Development, or MXPD. It allows "a more flexible approach" and is designed to "encourage orderly, staged development of large-scale, comprehensively planned multi-use commercial and residential centers," according to the county proposal.

County Planning Board Chairman Norman L. Christeller accused Gaithersburg itself of deviating from early "corridor cities" plans, and moving into a "multinucleated" development pattern. He said the Shady Grove and Airpark areas are basically just more planned nuclei for development.

He and Berman said there would be little competition between the cities and the county's business center in Shady Grove. "They are not competing for the same market. The people who want to have an office along I-270 are going to find something there. Those who need something downtown will go to Rockville," Christeller said.

Gaithersburg Planning Director Russel and others point to the area's projected 20-year "build-out" as an open-ended proposition whose lack of specifics and timing worries them. "It needs to be more firm to direct as for when the facilities will be there to serve development," Russel said.

Berman replied that it provides "flexibility that would allow for change," noting that Montgomery Village was created on a similar approach of zoning stages.

Of more immediate concern is whether the road capacity will carry the people brought in to live and work in the "oportunity area." The stated intention of the county's adequate public facilities ordinance was to allow only development for which services would be available. And swarms of residents in the area have asserted to the county planners that it has not worked.Christeller said people often don't consider the lag between approval of a subdivision and its construction. Roads are often ready by then, he said.