Severe congestion has spread to vast stretches of Washington area highway where only three years ago traffic moved freely, drastically slowing commuters on roads such as I-95, I-270 and the Capital Beltway, according to a regional study released yesterday.

The analysis by the Metropolitan Washington Council of Governments, based on extensive aerial photography, represents the most comprehensive overview yet of the region's congestion and confirms what many weary commuters already know: Traffic has grown considerably worse. Since COG's last study in 1996, daily backups have developed on dozens more miles of highway, aggravating traffic that was already ranked the second-worst in the country behind the Los Angeles area.

The swelling congestion on the Beltway is perhaps the most dramatic illustration. Morning commuters had previously encountered severe tie-ups on the Maryland portion of the outer loop between Interstate 95 and Georgia Avenue. But now that bumper-to-bumper traffic stretches 13 miles farther, all the way across the American Legion Bridge to the Dulles Toll Road.

"It has gotten quite a bit worse over quite a short period of time," said Ronald F. Kirby, COG's director of transportation planning. "It was bad only up there in Maryland. Now it's getting bad in Virginia."

The finding that the region's congestion is now spreading like an epidemic comes only weeks before the Virginia and Maryland legislatures are expected to tackle proposals for raising billions of dollars to expand roads and transit.

But the COG analysis offers a sobering lesson, because it shows traffic slowdowns materializing even on roads that recently have been widened, such as Interstate 270. Three years ago, motorists confronted severe morning traffic on I-270 from Falls Road to the Beltway. Now the backups begin nine miles to the north, as far back as Germantown Road.

"I noticed it's gotten heavier and bottlenecks in both directions . . . at both rush hours," said Sari Hurdle, of Bethesda. "It really is a pain in the neck."

Serious new congestion also has materialized on Route 50 in Prince George's County, snaring morning motorists along a stretch between Route 301 and Church Road where traffic was previously heavy but largely flowing.

And evening traffic headed south on I-95 in Virginia that moved relatively freely only three years ago is now regularly backed up from the Springfield interchange for more than 10 miles to the Occoquan River. The opening of car-pool lanes earlier this decade had significantly eased I-95 traffic from 1993 to 1996, according to COG's studies, but rapid development in southern Fairfax and Prince William counties has devoured most of that short-lived relief.

"New road capacity helps, but growth is outrunning our ability to add capacity," Kirby said.

Mariadel Silva, of Dale City, who travels daily along I-95 to her job at Reagan National Airport, said: "The traffic has been increasing dramatically. The commute used to be very easy."

And Linda High, who said she has seen the traffic along her commute from Dumfries to Washington get "steadily worse," attributed the growing backups to the crop of new affordable housing as far away as Fredericksburg: "Everybody's moving south."

Kirby suggested that the experience of I-95 offers a cautionary tale for those now enjoying the improved traffic on the Dulles Toll Road, where car-pool lanes opened late last year. In offering a glimmer of good news, the study showed that adding the car-pool lanes on the Toll Road eased the congestion on the regular travel lanes, eliminating severe congestion that had plagued most of the highway in 1996.

The rapid spread of congestion reflects the fact that so many of the region's roads are carrying heavy volumes of traffic. Under these conditions, it takes only a modest increase in cars to provoke a dramatic slowdown. And the study's findings do not bode well for the future: Many highways that are still flowing freely are approaching the breaking point.

"Congestion is a lot worse. I drive through it every day, and I hear about it every day from my constituents," said Montgomery County Executive Douglas M. Duncan (D). "The lesson is if we don't do something, we're going to drown in all the traffic."

The findings--reached by photographing 300 miles of highway from aircraft and then calculating the density of cars, which reveals their speed--may offer the most rigorous overview yet of the region's congestion. But, as with previous traffic studies, the COG report yielded different interpretations.

Robert T. Grow, transportation director for the Greater Washington Board of Trade, said the study's conclusions underscored the need for an extensive network of new roads. "It's not unexpected, because we are not addressing our transportation needs quickly enough. We just haven't been fast enough to get off the block," he said.

But Stewart Schwartz, of the Coalition for Smarter Growth, said the study showed that widening roads fails to provide relief. "It illustrates the need for expanded transit and improved access to job centers near the Beltway," he said.

Staff writers Steven Gray and Christina A. Samuels contributed to this report.

CAPTION: MORNING CONGESTION (This graphic was not available)

CAPTION: EVENING CONGESTION (This graphic was not available)