Beginning Monday, for the first time in 27 years, the four-mile stretch of hilly 13th Street NW between Logan Circle and Georgia Avenue will carry traffic both north and south at all times.

To the cheers of many 13th Street residents and the jeers of many Maryland commuters, the four-lane street will no longer become one-way during weekday morning and evening rush hours.

January 7 was selected for the traffic change because that is the day Metro is scheduled to increase the frequency of rush hour Red Line service to and from Silver Spring.

Thus, said D.C. Department of Transportation officials, who studied the change for four years before announcing it seven months ago, Marylanders who now commute by automobile will have a more convenient alternative than ever.

To encourage the use of Metro, DOT mailed a free farecard good for one ride to more than 10,000 potential riders living in Takoma Park, Silver Spring, Wheaton and Kensington.

The commuters were identified by DOT employees, who stood along 13th Street during two evening rush hours and dictated the license plate numbers of passing cars into tape recorders. The numbers were later traced and the owners identified. More than $10,000 in federal and District funds paid for the project.

"We've been surprised at how many people -- commuters as well as residents -- have said it's time this happened," said DOT director Douglas N. Schneider Jr., of the Jan. 7 changeover. "We're expecting this to be very successful."

Similar reversions to two-way traffic at all times are being studied for Reno Road NW, Independence Avenue SE, and Constitution Avenue NE, Schneider said. Like 13th Street, those streets run through residential areas where houses sit close to the sidewalk, and are heavily traveled during rush hours by Maryland car commuters.

Schneider said the change in roles for 13th Street marks the first time a U.S. city has deliberately curtailed an arterial rush hour street because mass transit had been built roughly parallel to it.

But DOT backed off an earlier plan to set aside two of 13th Street's four lanes for parking at all times.

Under the compromise plan that goes into effect Monday, two lanes of traffic will flow southbound during morning rush hour, and one lane northbound. The easternmost lane will be set aside for parking.

During evening rush hour, two northbound lanes and one southbound will be open to traffic. The westernmost lane will be set aside for parking.

At non-rush hours, both the easternmost and westernmost lanes will be parking lanes, and traffic will move along one lane in each direction.

The affected section of 13th Street extends from N Street at Logan Circle north to Fort Stevens Drive NW. At that point, 13th Street feeds into Piney Branch Road, which continues northeast for three blocks to Georgia Avenue.

Since 1953, traffic on 13th Street and the short stretch of Piney Branch Road linking it to Georgia Avenue has moved exclusively southbound from 7 to 9:30 a.m., and exclusively northbound from 4 to 6:30 p.m. Lights are synchronized during the rush hours to keep traffic moving steadily at 30 miles an hour.

According to DOT figures, at the peak of each rush hour, about 3,000 cars an hour use 13th Street, compared with 2,700 on 16th Street and 2,100 on Georgia Avenue, both of which run parallel to 13th Street.

According to Schneider, one casualty of 13th Street's new configuration will be the lane that has been set aside during rush hours for bicycles only. "With only two lanes in each direction, those'll have to go," Schneider said. "

But according to both residents and commuters, little will be lost. Very few motorist paid attention to bikes-only signs during rush hours, and even fewer bicyclists took advantage of them, residents and commuters said.

A far more troublesome problem for 13th Street residents has been parking, and only slight improvement awaits under the new DOT plan.

At present, cars have to be moved off 13th Street before 7 a.m. to escape ticketing and towing. Under the new system, only half the parked cars will have to be moved in the morning, but the other half will have to be moved in the evening. It will remain impossible to park legally on 13th Street for more than 12 1/2 weekday hours at a time.

In addition, Schneider pledged that his office will be "particularly attentive" to ticketing and towing along 13th Street once the new plan goes into effect.