The Rockville City Council, hoping to prevent worsening traffic conditions for thousands of rush-hour commuters, this week approved strict new limits on commercial development along heavily congested Rockville Pike.

The council unanimously approved a zoning amendment that calls for reducing the size of new buildings by cutting the maximum allowable floor space by one-third in commercial developments along the 1.5-mile stretch in Rockville between Twinbrook Parkway and Veirs Mill Road.

About 75 percent of that area is zoned for commercial development.

The "downzoning" will take effect Tuesday, when a nine-month-old building moratorium along Rockville Pike is scheduled to expire.

Council members said the reduction was the first step in the city's effort to ease the pike's traffic congestion, which in recent years has worsened as a result of a surge in office building construction along the six-lane corridor.

"This really is a first step," Rockville Mayor Viola D. Hovsepian said. "The next step is to look at ways that traffic can be improved. . . . We will be looking at all the aspects of the pike."

City planners hope to come up with a comprehensive land-use plan for Rockville Pike during the next year. They also will review several proposed signal and road improvements, including a plan to extend East Jefferson Street to provide an alternate north-south roadway parallel to the pike.

Planners also are considering a proposed "optional method of development," a measure that would allow developers to build to maximum levels if they include certain public amenities, such as devoting spaces for landscaping or works of art.

The "downzoning" was recommended last month by city planners and an advisory group made up of Rockville Pike property owners and nearby residents as a way to lift the moratorium and to protect the pike against future traffic gridlock.

Although renowned for its sprawling retail development, the pike has in recent years become home to several large office buildings -- a change brought about largely by the Metro subway opening and increasing land costs that have forced developers to build more intensely.

The large number of office buildings, and the worsening traffic along the pike, concerned Rockville planners and prompted the council to impose the moratorium.

The city's portion of Rockville Pike carries 2.7 million square feet of commercial space, including 1.7 million square feet of retail space and 568,000 square feet of office space.

Planners predict that office building construction along Rockville Pike eventually will outpace traditional retail development.

By the turn of the century, they predict, the pike will carry as much as 4.5 million square feet of office space, more than twice as much space than is anticipated for retail development.