To the frustration of thousands of Washington-area drivers, from cabbies stuck on K Street to shoppers headed for Tysons Corner, a midday rush hour is developing that rivals morning and evening commuter traffic.

The region's streets, highways and crosswalks are becoming so choked between 11:30 a.m. and 2 p.m. with utility crews, delivery workers, tourists, taxi drivers, office employees, shoppers and businessmen that in some places, "the midday congestion is worse than during rush hour," said George W. Schoene, the District's chief traffic engineer.

District traffic counters have tallied more vehicles between 11:30 a.m. and 2 p.m. on some streets than during the morning rush hour -- even on a busy stretch of K Street NW between 18th and 19th streets.

Traffic on the 23-mile Virginia section of the Capital Beltway is increasing at a rate of 10 percent a year, officials said, mostly because of an increase in short local trips at midday.

Midday ridership on Metrorail is up at some stations, contributing to crowded trains. Midday and evening passengers accounted for large jumps in riders at Union Station and Pentagon City since commercial centers opened at those stops.

It's not only a weekday problem. In places such as Route 7 through Tysons Corner and Rockville Pike near White Flint Mall, congestion on Saturdays is as bad as on weekdays.

"When we talk about transportation problems, we usually focus on commuting," said transportation expert Alan E. Pisarski of Falls Church. "In many places, though, you could pinpoint 11 a.m. on Saturday as the peak hour of the week."

Nor is the third rush hour seasonal. In the summer, vacationing commuters and students are replaced by so many tourists that there are more cars on District streets during the non-rush hour periods of the summer than at other times of the year.

Besides being aggravating, midday congestion costs drivers and businesses time and money and fouls the region's air.

District cab driver Reza Jafry says he tries to stay away from parts of downtown between 11 a.m. and 2 p.m.

"I was at Connecticut and K the other day at noon and the light changed three times and I just sat there," Jafry said. "If it was a meter, I'd love to sit there. But since we're zoned, I try to avoid that area."

When Ralph Stephens, of Fairfax, and his wife go to Tysons Corner shopping center on weekdays, they have learned to arrive after 10 a.m. and leave by 3 p.m., avoiding the rush hours and heavy noontime traffic.

"We get here and we stay put," Stephens said. "I sit in a corner and read and let my wife shop."

Midday traffic is becoming more of a problem for several reasons. For one thing, the District's parking policies and traffic-light sequences are geared toward speeding the flow of traffic during rush hour.

When parking restrictions come off during the day and the lights return to a slower sequence, at least two lanes on each street are taken up by parked cars. Delivery trucks double park, further narrowing the lanes and impeding traffic.

At 11 a.m. on a recent weekday, delivery trucks filled the north service road along K Street NW between 15th and 16th streets, so trucks began parking in the westbound through lane. Only one westbound lane was unobstructed at the start of the midday rush hour.

Other streets in Northwest Washington are frequently jammed at midday, including sections of 14th Street, Connecticut Avenue, Wisconsin Avenue, 19th Street, Columbia Road, and L and M streets.

Pedestrians have something to do with the problem too. Because many people ignore "Don't Walk" lights, many drivers must wait through several light sequences before turning. There has also been an increase in the number of pedestrians, figures show.

Rush hour commuters have a defined route to and from work, but many midday drivers are running errands or headed toward restaurants and don't always know where they are going or where they will park. Tourists can be spotted sightseeing or holding maps while they drive.

"Your noontime rush is a reflection of more working mothers running errands," said Mary Anne Reynolds of the Virginia Department of Transportation.

Public transportation isn't an option in many places. Midday congestion in downtown Alexandria, Bethesda and along Sunrise Valley Drive in Reston occurs because office workers use their cars for lunchtime personal trips.

Travel near home also is increasing during the day. Since 1985, traffic volume has increased by 30 percent on the Beltway between Braddock Road and Little River Turnpike. Traffic during the morning rush hour rose only 15 percent along that stretch, meaning the increase probably came mostly from midday trips.

Another reason for the third rush hour is that drivers are using more discretion. If the Beltway is crowded in the morning rush hour, drivers wait to take their trips at midday.

Schoene, the District's chief traffic engineer, said the change to a more flexible workday by many employers also contributes to the problem.

"I play tennis with someone who works for a distribution company who works from 10 a.m. to 6 p.m.," Schoene said. "The work force is not just 9 to 5 anymore. Others may use their off time to go shopping during the day."

The third rush hour is not an easy problem to deal with. Transportation planners are accustomed to thinking that if they concentrate on the morning and evening rush hours, the rest of the day will take care of itself. That's no longer true.

One obvious way to unclog roads is to adjust the sequence of traffic lights, and the District's Schoene said he would consider that on more streets.

A trickier solution, especially in the suburbs, is changing development policies to discourage businesses from building along arterial roads, which entice drivers to make frequent stops and turns.

In Northern Virginia, Route 7 from Baileys Crossroads through Tysons Corner to Reston is supposed to be a major route for through traffic, but development has turned the road into a collector and distributor of local traffic.

Ronald F. Kirby, chief transportation planner for the Metropolitan Washington Council of Governments, said the increase in midday travel weakens the argument that mass transit is the solution to the region's troubles.

"People at midday are doing things that you can't use transit for," Kirby said. "You have to provide some increase in road capacity. You can't just say, 'No more roads.' "

Not everyone is convinced that a third rush hour is alarming. After all, said Alexandria traffic engineer Charles Kenyon, "we asked for it" by building businesses to attract people. "You don't want to send anyone away."

Robert S. McGarry, Montgomery County's transportation director, said officials there have planned for congestion along Rockville Pike and in downtown Bethesda and Silver Spring. He argued that heavy traffic is desirable because it's a sign of thriving commercial activity.

"If our dream comes true, on Saturday we'll have congestion in downtown Silver Spring," McGarry said, referring to the planned business expansion there. "Yes, it's frustrating, but there really is no other solution."