The National Institutes of Health plans to bring 3,000 employees to its Bethesda campus in the next 20 years.

Combined with planned development at the Walter Reed National Military Medical Center across the street, the plans pose potential problems for commuters on the already crowded stretch of Rockville Pike during rush hour.

With 20,262 employees, NIH is Montgomery County's largest employer, followed by Walter Reed, which had 11,686 employees in 2011, according to county planning documents.

Over the next 20 years, NIH plans to bring 3,000 employees from satellite locations to its 310-acre Bethesda campus. As a result, the agency plans to build 1.6 million square feet of research space and 775,000 square feet of administrative and support space.

The additional employees are expected to add roughly 433 cars to the road in morning rush hour and 440 in evening rush hour, according to NIH planning documents. With the planned addition of lanes under Base Realignment And Closure, or BRAC, the surrounding intersections will meet standards for the maximum amount of congestion, NIH's Division of Facilities Planning wrote in the documents.

The plans also call for the transformation of existing surface parking lots into "pedestrian malls" and parking garages, to add both green space and available parking.

But Montgomery County planners said adding more parking -- and encouraging more employees to drive to work -- may not be the right approach.

"We are discouraged with the lack of success achieved in reducing the parking ratio at NIH, which continues to be approximately 0.50 spaces per employee (ratio of 1 parking space for every 2 employees)," Montgomery County Planning Department staff wrote in a memo prepared for a discussion of the plans at Thursday's Planning Board meeting.

NIH should provide one space for every three employees, staff wrote, and encourage employees to take advantage of "out-of-the-box commuting options," including coordinating with the military.

Representatives of NIH were unavailable for comment.

Across the street from NIH, the military plans to replace five existing hospital buildings with a five-story building at Walter Reed, allowing the military to convert double-occupancy hospital rooms to single-occupancy rooms. This construction is scheduled to begin in 2013 and take up to five years, according to Environmental Impact Study Project Manager Bill Sadlon.

The military also is planning a 341,000-square-foot research facility and 400-space parking garage to the Uniformed Services University of the Health Sciences campus, though those plans are a little further behind, Sadlon said.

Together these will only add roughly 270 people to the Bethesda location, many moving from leased space in other parts of the county, he said. The military has insisted that the impact on traffic will be minimal.

At a public hearing two weeks ago, residents didn't believe the claims.

"It's like a parking lot [on Rockville Pike]," Bethesda resident Andres Buonanno said.