Rush-hour traffic has become less congested since the mid-1970s in the central part of the Washington area, according to data compiled by D.C. transportation officials.

Commuters can drive home faster from downtown at the height of the afternoon rush hour on most major routes leading to the suburbs, the statistics show. Average rush-hour speeds on key arteries have increased by more than 30 percent in the past eight years.

The gains have been attributed largely to a reduction in the number of Metrobuses on city streets in recent years. Buses delay traffic because of their frequent stops. Metro has cut back bus service as the subway system has expanded.

"We are seeing improvements in travel time," said George W. Schoene, D.C. traffic services chief. "If you look downtown today, you don't have all those buses around."

According to the new data, traffic delays have decreased on Massachusetts Avenue NW, East Capitol Street, MacArthur Boulevard, Suitland Parkway, the New York Avenue/Baltimore-Washington Parkway corridor and other routes between the District and suburban Maryland.

Rush-hour driving also was found to have become faster from downtown to some Virginia corridors, such as Shirley Highway, Lee Highway and the George Washington Memorial Parkway.

Overall, the city's survey pointed to quicker trips in 12 of 17 major corridors this year compared to 1977, when a similar study was conducted. Delays increased in only three corridors. In two others, no clear change was evident. Average speeds, including stopping time, climbed from 13 mph in 1977 to 18 mph this year.

The decreased congestion is regarded as especially significant because of recent increases in the number of cars on city streets. Over the same eight-year period, officials said, the number of cars heading into central Washington on the 17 routes during the morning rush hour rose by 12 percent.

In addition to reductions in Metrobuses, officials said, several other factors may help account for the shift, including stiffer rush-hour enforcement of the city's no-parking rules, more flexible working hours for many employes and increased Metro subway ridership.

They also cited improvements in traffic lanes and signals on some D.C. streets and suburban highway construction projects, such as the opening of I-66 between the Theodore Roosevelt Bridge and the Capital Beltway.

The new D.C. statistics appear to confirm a trend long considered likely to have occurred by some local officials. Ronald G. Sarros, a transportation analyst for the Metropolitan Washington Council of Governments, previously pointed to possible reductions in congestion because of decreases in buses on downtown streets, such as K Street NW.

Officials cautioned, however, that the District's survey has not yet undergone a detailed analysis and said the data may be too limited to reflect changes in traffic on specific routes. Nevertheless, officials said, the statistics appear to show an overall improvement in traffic flow.

Despite the reduced congestion in central Washington, traffic delays may have worsened on some suburban highways in rapidly developing areas outside the Beltway, officials noted. The D.C. survey did not include data from outlying areas or cross-county routes.

According to Fairfax and Montgomery county officials, tie-ups appear to have increased in recent years on such heavily used roads as Old Keene Mill Road in the Springfield area, Rtes. 7 and 123 in Tysons Corner and Rockville Pike in Montgomery.

The D.C. survey was based on a series of test drives on the 17 routes, taken last March, by employes of the city's Bureau of Traffic Services. All trips started at 12th and F streets NW at 5:15 p.m. on weekdays. Officials said they sought to avoid unusual weather and incidents that might mar the data.

On several routes, the trips were found to be much faster than in 1977. For example, a driver heading out East Capitol Street went nine miles in 25 minutes this year, compared to 3.2 miles in 1977. On Massachusetts Avenue NW, a 25-minute trip covered 8.8 miles, as against 4.4 miles in 1977.

The three routes on which delays were found to have worsened were 13th Street NW, the Sherman Avenue/New Hampshire Avenue corridor and Rhode Island Avenue.

Schoene said delays apparently increased on 13th Street as a result of a 1980 change in traffic regulations, under which the street was made two-way instead of one-way at rush hour to deter commuters.

The increased congestion on New Hampshire Avenue may have stemmed from a shift in traffic from 13th Street after the restrictions took effect, Schoene said. He said it is unclear why Rhode Island Avenue showed more delays.

No clear change in speeds was evident in the Connecticut Avenue and Arlington Boulevard corridors. Routes on which delays were found to have decreased included 16th Street NW, Pennsylvania Avenue SE, South Capitol Street and the Anacostia Freeway.