The Washington metropolitan region is home to more than 3.2 million cars, trucks and other vehicles, or about two per household, according to a new tally. It's a mark of the region's growth and its prosperity -- and a warning of an approaching collision between cars and clean-air requirements.

First, the growth.

Between 1994 and 1998, the region added more than 211,000 registered vehicles, the Greater Washington Research Center reports. That's about equal to the total car and truck traffic over the Woodrow Wilson Bridge in both directions over an entire day.

More than half of these new vehicles showed up in older suburbs: Arlington, Fairfax, Montgomery and Prince George's counties and Alexandria. (With its shrinking population, the District wound up with fewer cars and trucks in 1998 than in 1994.)

The garages and truck lots filled up much faster in the outer suburbs. Vehicle registrations in Loudoun, Prince William, Stafford and Calvert counties increased at double-digit rates over the four-year period; registrations in Frederick and Charles counties increased almost as quickly.

Loudoun County alone added nearly 30,000 vehicles, a 29 percent increase.

It isn't clear how much of this stream of additional vehicles was due to newcomers and how much to greater wealth among existing households that went out and bought second and third cars.

But it is obvious that settlers in the outer suburbs are much more dependent on their cars and trucks than those closer in, where mass transit is a real option.

The District had about one vehicle per household in 1996 and Alexandria and Arlington were both under two vehicles per household. Loudoun, Stafford, Frederick and Calvert counties averaged more than 2 1/2 vehicles per household. (Differences in residents' ages and incomes may explain part of that gap.)

Put that another way, says the research center: "If all the suburbs had the same number of vehicles per household as transit-rich Arlington County, there would have been 643,000 fewer registered vehicles in the area in 1996."

Instead, those 643,000 cars and trucks are out there, adding to an air-pollution problem that may soon confront local government leaders with acutely painful choices, according to Joan Rohlfs, air quality director for the Metropolitan Washington Council of Governments.

As matters stand, the Washington area isn't expected to meet key federal air-pollution standards for the year 2000, Rohlfs said. In that case, the region would have to find significant new ways to reduce pollution from vehicle exhaust, such as even tighter controls on commuting, or risk a possible reduction in federal highway funding. Or it could try to persuade the Environmental Protection Agency to ease the rules -- assuming the courts permitted that to happen.

Alternatively, regional leaders could seek a five-year delay in meeting those standards. But for complex reasons, that would make it very hard to carry out major new highway projects that add to pollution levels -- such as the replacement of the Wilson Bridge, Rohlfs said. A decision to seek the extension could be made next month by the Metropolitan Washington Air Quality Committee.

"Vehicle miles traveled are increasing in the region. We're getting some benefits from technology [cleaner engines and fuels]," Rohlfs said, but at the same time, air-quality requirements will become more stringent.

"The lines will intersect. We have this inevitable conflict in the future," perhaps as early as next year, she added.

CROWDING CARS

The number of cars and trucks in the Washington area increased by 211,174 from 1994 to 1998. Here is the regional breakdown:

Motor vehicles in the Washington area:

Area: District

1998: 230,000*

% Change from 1994: - 7.6

Area: Arlington County

1998: 137,015

% Change from 1994: 7.2

Area: Fairfax County

1998: 751,208

% Change from 1994: 7.0

Area: Loudoun County

1998: 132,006

% Change from 1994: 29.2

Area: Prince William County

1998: 207,020

% Change from 1994: 13.0

Area: Stafford County

1998: 75,222

% Change from 1994: 15.8

Area: City of Alexandria

1998: 113,539

% Change from 1994: 13.6

Area: Montgomery County

1998: 629,888

% Change from 1994: 6.2

Area: Prince George's County

1998: 541,973

% Change from 1994: 4.6

Area: Frederick County

1998: 171,972

% Change from 1994: 9.7

Area: Charles County

1998: 100,960

% Change from 1994: 9.4

Area: Calvert County

1998: 65,956

% Change from 1994: 17.2

Area: Total area

1998: 3,156,759

% Change from 1994: 7.2

* Estimate based on 1997 numbers

SOURCE: Maryland and Virginia state motor vehicle administration; Highway Statistics, Federal Highway Administration, Office of Highway Information Management, varioius years