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

About 300,000 of the notices are going to Maryland residents, most of whom live in Montgomery and Prince George's counties.

The names of another one million motorists -- including 340,000 Maryland residents -- who have unpaid parking tickets dating back to 1976, have been turned over to two private collection companies.The value of those tickets exceeds $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 names and addresses 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 has no power to force nonresidents to pay its parking tickets.

They put 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 a little pressure," said John Brophy, who heads the city's parking endorcement and operations division.

The notice is accompanied by an implied threat: that the car will be towed or "booted" -- the wheels locked -- if it has accumulated several unpaid parking tickets and is spotted again in Washington by the city's energetic army of traffic enforcers. The District's civilian ticket writers last summer were rated number one in the nation by the U.S. Department of Transportation. That rating was based at least in part on their productivity -- more than 100 tickets a day for each of the 100 ticket writers.

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

The District's 22 fulltime booters roam Washington with computer printouts of the license numbers of cars with four or more outstanding parking tickets. But now they 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 then a million parking tickets, impounded 200 cars a day and booted 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. They notify 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 to 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, however, that neither firm will make harassing calls in the middle of the night or to places of work, as some collection agencies have done with private debts.

The collection agencies get 32 to 45 percent of the fines collected, depending on the distance and difficulty involved in collecting 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 the 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 were issued to Maryland residents and 525,000 to residents of virtually every other state in the nation.

G.C. Services, which has helped collect more than $10 million in unpaid New York City parking fines, is starting to collect on 193,000 unpaid District parking tickets and within a month, will be assigned 250,000 to 300,000 unpaid Virginia tickets.

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 are now writing more than 10,000 parking tickets a day. An additional seven ticket writers are being hired. Their efforts last year not only helped reduce illegal parking, but brought the city $19 million in fines. Police 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 interstate reciprocity agreements, under which motorists given tickets need not appear before a local magistrate and pay immediately. Their driver's licenses will be suspended by their home states, however, if tickets are not paid within 30 days.

Apparently no states have reciprocityagreements for parking tickets, although District officials have tried unsuccessfully for years to get such an agreement with the Virginia and Maryland state governments. The two states, however, are now routinely exchanging information -- computerized lists of the names and addresses of motorists whose vehicles have been ticketed 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 aggressive, like New York," said Brophy.

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 tickets.