How can 15 pounds of groceries be packed into a 10-pound grocery bag? Get rid of some of the groceries? Get a bigger bag? Do a better job of packing?

A favorite rhetorical query often voiced by planners and architects, this metaphoric dilemma aptly summarizes smart-growth challenges facing Montgomery County. The groceries are the projected increase of population and employment in the county. The bag is the capacity of the county's existing real estate resources to absorb future growth.

Montgomery County, which is Maryland's most populous political jurisdiction and most potent economic engine, will add 170,000 jobs and need almost 100,000 new housing units by 2030, according to forecasts reported by the Maryland-National Capital Park and Planning Commission.

Demographic trends also point to an increasingly aging population, with less than a quarter of new and existing households expected to include schoolchildren. Consequently, county planners anticipate that as much as 60 percent of future housing growth in Montgomery County will be condominium and rental dwellings rather than single-family homes on individual subdivision lots.

Likewise, the county foresees millions of square feet of additional non-residential development -- places of employment, shopping, schools, recreational and cultural facilities -- being built in the next 25 years.

Meanwhile, the county's boundaries are fixed. But county leaders have essentially made the "bag" smaller by imposing additional capacity constraints. Most important of these is the 1980 creation of the unique Agricultural Reserve.

Encompassing 93,000 acres in the county's northern and western sectors, the Agricultural Reserve includes a third of the county's land area. With density in the sacrosanct reserve limited to one dwelling per 25 acres, new residential and commercial development has stayed within "down-county" and I-270 corridor areas -- Bethesda, Rockville, Gaithersburg, Silver Spring, Wheaton.

Thus, the amount of groceries is growing and the size of the bag is immutable. The only thing left is to do a better job of packing -- and that is precisely what county planners intend to do.

Last Monday, the Maryland-National Capital Park and Planning Commission sponsored a conference to talk about the challenge. To catalyze discussion, the commission distributed a planning framework subtitled "Revitalizing Centers . . . Reshaping Boulevards . . . Creating Great Public Spaces." The subtitle implicitly identifies existing physical problems plaguing the county, offers possible solutions to those problems and suggests strategies for managing future growth.

Topping the all-too-familiar list of county problems are severe traffic congestion and inadequate road capacity. Added to this are lack of road network connectivity and pedestrian-hostile arterials such as Rockville Pike, Georgia Avenue and University Boulevard. Other problems identified include a shortage of affordable housing and the existence of underused, unattractive strip shopping centers scattered throughout the county.

Despite some innovative zoning initiatives, pervasive single-use residential and commercial zoning has produced many isolated enclaves and neighborhoods. Zoning-induced physical segregation not only deters social interaction but also engenders car dependency and guarantees clogged arterials.

The commission's report observes that existing master plans and zoning laws would legally permit much of the projected growth if all county land were built out to maximum allowable density. But the patterns of uses and densities adopted in the 20th century won't lead to the kinds of communities envisioned for the 21st.

So the commission proposes "a new planning paradigm."

Its provisions include:

* Instead of large-area master planning, small-area, neighborhood-based, fine-grain planning would "direct growth inward and upward." There's no choice: Future growth must entail redevelopment of existing properties, and at higher densities. Commercial centers and surface parking lots, covering thousands of county acres, would be especially targeted for revitalization.

* Infill development and redevelopment not only must yield higher densities, but also must include residential, commercial and non-commercial uses to ensure round-the-clock activity. Equally critical is designing animated public spaces, whether plazas or streets. Given pedestrian-oriented activities and ease of pedestrian movement, many people would choose to live near where they work and shop. And if each resident and worker reduced daily automobile trips by only two a day, the aggregate drop in traffic would make a significant dent in highway congestion.

* Transit-oriented development, at Metro stations or along light-rail or bus lines, likewise demands dense, diverse land use. Yet transit still complements an interconnected road network. Because people will continue owning and driving cars, building ample, transit-related parking structures is essential.

* Segments of the county's automobile-dominated arterials should be transformed into tree-lined "boulevards," shared public spaces that are attractive, that accommodate both cars and buses, and that are safe and inviting for bicyclists and pedestrians. Imagine Rockville Pike as a boulevard with tree-shaded sidewalks abutting buildings and storefronts instead of parking lots.

With its ordinances on adequate public facilities and moderately priced housingand with its Agricultural Reserve, Montgomery County has long been viewed by other jurisdictions, regionally and nationally, as a model of enlightened, progressive planning. And pursuing the new "planning paradigm" clearly shows that it intends to maintain its reputation.

Yet little will change unless the county's political leaders, property owners, businesses and voters buy into the new paradigm. This is not an easy sell. Rewriting development rules to radically reshape familiar physical environments requires new thinking and new attitudes, achievable only through sustained public education. Because many jurisdictions face the same dilemma confronting Montgomery County, and because -- despite recent public criticisms of the planning system's management -- it is the model, let's hope the county is successful in helping its residents pack those future groceries into an attractive grocery bag.

Roger K. Lewis is a practicing architect and a professor of architecture at the University of Maryland.