The Capital Beltway, the Washington area's most heavily used highway, faces severe congestion and an increasing number of accidents as a result of long-term suburban growth, a regional study group said yesterday in a report calling for "urgent" action to relieve traffic problems.

At rush hour, commuters now travel at speeds of only 15 to 25 miles an hour on some parts of the Beltway, according to the study by the Metropolitan Washington Council of Governments. Within 20 years, the report said, similar congestion is expected to spread to nearly half of the 66-mile Beltway.

"We're approaching gridlock," Peter F. O'Malley, a Prince George's County lawyer and president of the Greater Washington Board of Trade, said at a news conference held to release the study. "The cost to the work force and employers of these lost hours is staggering. Our quality of life is threatened."

Fairfax County Supervisor Nancy K. Falck, president of the Council of Governments, called for immediate steps to improve traffic safety on the Beltway. "For every day that passes, we can expect six accidents on the Beltway, at least one involving a tractor-trailer," she said, citing COG data.

The study was drawn up in preparation for a regional conference next month sponsored by COG and the board of trade that will attempt to devise measures to ease congestion and improve safety. Options cited in the study range from highway construction to tighter enforcement of traffic rules.

When the Beltway opened in 1964, the study said, it was expected to serve mainly interstate traffic. But as suburban development expanded, the highway increasingly attracted local commuters. Now, the report said, local cars and trucks account for nearly two-thirds of all trips on the Beltway.

The study attributed the mounting congestion largely to growth in population and employment centers in the suburbs, along with increasing auto ownership. It described the Beltway as the Washington area's "Main Street."

In 1981, the study said, only one part of the Beltway -- the Montgomery County section between the I-270 and Connecticut Avenue interchanges -- was heavily congested. But in the next few years, officials said, traffic increased markedly because of economic expansion and a leveling of gasoline prices.

By 1984, the report said, severe congestion had extended over 15 to 20 miles of the Beltway, including all Montgomery County sections, a Fairfax stretch near the Cabin John Bridge and portions near the Woodrow Wilson Bridge. Rush hour speeds in these areas averaged 15 to 25 mph, the report said.

By the year 2005, the study said, up to 30 miles of the Beltway will face similar rush hour backups, and most other sections will be at least moderately congested with speeds averaging 30 mph. The speed limit on most of the Beltway is 55 mph.

Some officials said these levels of congestion may be reached sooner, possibly as early as 1990. According to the report, tie-ups will increase even if other planned highways, such as the proposed Inter-County Connector in suburban Maryland and the Springfield Bypass in Fairfax, are completed.

Most sections of the Beltway now average 120,000 to 150,000 vehicles a day, the report said. By 2005, it added, these totals will increase by 20,000 to 40,000 vehicles. About 2,400 accidents occur on the Beltway annually, including more than 400 involving tractor-trailers.

In 1984, 14 fatal accidents were reported, three of which involved tractor-trailers. Last week, Maryland and Virginia officials announced extensions of rules banning trucks from the left lane on parts of the Beltway in response to widespread concern among drivers and local officials about safety.

The report urged officials to consider a wide range of options, including building more highways, adding lanes on parts of the Beltway, widening exit ramps, installing electronic equipment to monitor accidents, instituting a controversial system of signal lights at entrance ramps, and tightening traffic regulations and truck inspections.