Planned expansions at the National Institutes of Health and Walter Reed National Military Medical Center will add thousands of employees to the facilities, and Montgomery County planners want to keep many of the new workers off Bethesda’s congested roads.

NIH, which is Montgomery’s largest employer, is planning to add 3,000 workers to the 20,262 already at its main campus along Rockville Pike, just inside the Capital Beltway and north of downtown Bethesda. That expansion would occur over the next 20 years.

Meanwhile, across Rockville Pike, Montgomery’s second largest employer, Walter Reed, is planning its own expansion. Under that plan, an additional 925 employees would join the base over the next decade, increasing the staff to 12,611 from the current 11,686, according to a planning staff report.

“The trick is really how they’ll both manage their traffic,” said Cherian Eapen, the transportation planner who reviewed both growth proposals.

Montgomery’s planning board is scheduled to vote Thursday on its staff’s recommendations as part of providing input to the federal planning process. But the county has no regulatory control over the growth plans, which will be reviewed by the National Capital Planning Commission.

Eapen said that both federal facilities will need to limit the number of employees driving alone by encouraging more to walk, ride bikes, take shuttles or use transit. Planners recommended designating a kiss-and-ride area along Rockville Pike and Jones Bridge Road and implementing a bike-share program that could be used by Walter Reed, NIH and nearby Suburban Hospital.

The addition of 3,000 employees to the NIH campus would be significant, particularly because parking there is relatively plentiful, which encourages people to drive alone, planners said. They wrote that they were “discouraged by the lack of success” in reducing on-campus parking. The National Capital Planning Commission approved an NIH growth plan update in 2003 on the condition that NIH reduce its parking ratio to one space for every three employees, planners said. But with more than 10,000 spaces, NIH still has one space for every two workers.

The area surrounding both facilities is a bottleneck for traffic between the District and Rockville, Kensington and other Maryland suburbs. The Bethesda area has already seen significant increases in traffic congestion since September, when the military closed Walter Reed Army Medical Center in the District and moved its facilities to the campus of the former Bethesda Naval Medical Center. That consolidation was part of the Pentagon’s base-realignment process.