The District of Columbia is sending notices to an estimated 500,000 residents of Maryland, Virginia and the District who have failed to pay more than $1 million in parking tickets written by the city since February 1979.

The names of another 1 million motorists who have unpaied parking tickets dating back to 1976 -- more than half of whom live in Virginia or Maryland -- are being turned over to two private collection companies. the fines exceed $10 million.

The two firms, which have been collecting delinquent parking fines for New York City for several years, were hired by the District's Department of Transportation more than a year ago, but only now are getting full computerized lists of the scofflaw motorists, enabling them to launch a major collection effort.

The city notices and letters from the collection agencies make no threats, since Washington at the moment has no power to force nonresidents to pay parking tickets.

They put the motorists on notice, however, that the city knows who they are. "It's just what private industry does with small debts -- notifies the individual his bill is unpaid and puts on a little pressure," says John Brophy, who heads the city's parking enforcement and operations division.

The notices are accompanied by an implied threat that cars will be towed or "booted," their wheels locked, if motorists have accumulated several unpaid parking tickets and their cars are spotted again in Washington by the city's energetic army of traffic enforcers. Washington's civilian tricket writers this summer were rated number one in the nation by the U.S. Department of Transportation. that rating was based in part on their productivity -- more than 100 tickets a day for each of the 100 ticket writers.

Besides the inconvenience of retrieving a booted or impounded car, the city tacks on a $50 towing fee and a $25 booting charge. the city's current booting list has more than 100,000 cars on it.

The District's 22 fulltime booters roam the city with computer print-outs listing the license numbers of cars with four or more outstanding parking tickets. They now also have a high-priority list of what Brophy calls "the heavy hitters," several thousand suburban scofflaws, each with more than 15 unpaid tickets.

The city last year issued more than 1 million parking tickets, impounded 200 cars a day and botted about 100 a day. but booting is on the increase. It averaged 120 cars a day last month and soon will average 150 cars a day "because we're hiring six more booters," says Fred Caponipi, who heads the city's parking operations division.

The notices being sent nonresidents are similar to those going to D.C. residents, notifying the owners of motor vehicles that they owe a parking fine, plus a $5 penalty for failure to have paid within 30 days.

After two or three months, unpaid tickets are turned over to the collection agencies. they send additional letters and then telephone the car's owner if the tickets remain unpaid. District officials said this week that neither collection firm will make harassing calls to the cars' owners in the middle of the night or at work, as some such agencies have done with private debts.

New York City is even more aggressive, says William Inglehart, president of G.C. Services, which has helped collect more than $10 million worth of unpaid New York City parking tickets and is starting to collect on 193,000 unpaid Virginia and District parking tickets. "They impound cars and property and attach salaries" to collect on New York parking tickets, Ingelhart said.

The collection agencies get 32 to 45 percent of the fines collected, depending on the distance and difficulty involved in collection them. Datacom, the New York firm collecting on tickets issued to all out-of-state cars except those registered in Virginia, already has collected $320,000, bringing the city more than $200,000 it previously had written off as uncollectable.

About 40 percent of parking tickets issued in the District are not paid within the required 30 days. Datacom's Washington manager, Sharon Rohrbach, said that of the 865,000 unpaid tickets it has been assigned to collect, 340,000 are against Maryland cars and 525,000 were written on autos registered in virtually every other state in the nation.

The new effort to collect on unpaid traffic tickets has been planned since February 1979, when most Washington parking and minor traffic offenses were decriminalized and the city's Department of Transportation replaced the courts as the enforcing agency. The city's 100 civilian ticket writers -- seven more are now being hired -- are nowwriting more than 10,000 parking tickets a day. their efforts last year not only helped reduce illegal parking, but brought the city $19 million in fines. Police-written tickets brought in another $7 million.

Washington has had little trouble collecting fines for serious traffic offenses, such as speeding. The District and at least 23 states now have an interstate reciprocity agreement, under which motorists given tickets need not appear before a local magistrate and pay immediately. However, their driver's licenses will be suspended by their home states if tickets are not paid within 30 days.

Apparently no states have reciprocity agreements for parking tickets, although District officials have tried unsuccessfully for years to get such an agreement with Virginia and Maryland. Cities within a State have little difficulty collecting from other state residents, however. "A Montgomery County resident cannot get his license or registration newed if he has an unpaid Ocean City Parking ticket," Caponipi said.

Although Virginia and Maryland have refused to join in a reciprocity agreement with the District for parking tickets, the states are now routinely exchanging "information," computerized lists of the names and addresses of motorists whose vehicles have received parking tickets in the other jurisdictions. D.C. courts also exchanged parking ticket information with other states before 1979, but city officials say the exchanges were sporadic and not as well computerized.

One trick still up the District's sleeve, says Brophy, has yet to be tried. Since minor parking offenses were decriminalized, an unpaid ticket has become a civil debt, which the city could go into other states to collect. "We could get more aggresive, like New York," said Brophy. "All we'd have to do is change the law."

The District also could boot and tow cars with only two tickets, which the present law permits. the city now allows a margin for error, and boots and tows only vehicles that have accumulated four or more tickes.