For Una May Anderson, life in pastoral Elkridge, Md., meant unlocked doors and trusting her neighbors.

Then the break-ins began.

Last month, thieves broke a window in the rear of Anderson's century-old shingle house in southeastern Howard County and stole a color television set. Three weeks later, Anderson's teen-age son returned home from school to discover the front door unlocked and another TV set taken.

"Maybe it would be better if I got farther out," said Anderson, whose house is about 25 miles northeast of Washington near a fast-growing town-house development. "I'm not sure I feel safe here anymore."

Many residents of Washington's developing outer suburbs are finding that while urbanization means congested roads, clusters of new houses and soaring school enrollment, it also may mean more robberies, burglaries and drug raids.

In the past year, reported crime has soared 10.5 percent in Howard County and a record 20 percent in neighboring Anne Arundel County, contrasted with a 4.2 percent increase in the District of Columbia and a 6.3 percent rise nationally.

It is a time of painful change in the counties, grouped like a horseshoe just outside the heavily urbanized Washington area, which include Prince William and Loudoun in Virginia and Frederick, Howard and Anne Arundel in Maryland.

Frederick County, just northwest of Montgomery County, has become an interstate exit for drug traffickers. Authorities in Prince William County, southwest of Washington in Virginia, say they are girding for a surge in crime.

Residents of communities far beyond the Capital Beltway once thought they were fairly immune from crime, but criminologists say that where development and affluence go, so do criminals, who have become a lot more mobile than they were 10 to 20 years ago.

As rolling rural landscapes become more urban, experts say, the old, close-knit communities in which neighbors borrowed a cup of milk or gossiped over coffee are now becoming populated with people who do not look out for each other.

Additionally, new and improved highways make access to communities 30 or 40 miles outside Washington quicker and easier for criminals from other areas. Frederick County, for example, is battling a major drug problem in part because of the convergence there of major roads, including I-270 and I-70, which easily connect it to Washington and Baltimore.

Drugs have brought a new element to some rural areas, where drug traffickers are producing and distributing cocaine, marijuana, PCP and heroin. Subsequently, drug-related crimes such as robberies and burglaries have increased, police say.

Although crime -- particularly against property -- in these places is far from that experienced in the District or other suburbs such as Prince George's County, it is still a new and alarming part of growth and affluence.

"It's easier to get into the suburbs, and the targets are more luscious," said Robert Figlio, an associate professor of criminology at the University of Pennsylvania. "That information is not wasted on criminals."

While the majority of new suburbanites are law abiding, vandals and burglars are among those setting down roots, police said. "You don't know if the young man with the television set is moving into the house or stealing it," said sociologist Richard P. Taub, a professor of public policy and social studies at the University of Chicago. Anderson, for example, said she knows few of her new town house neighbors.

Although crime has not shot up as dramatically in some outlying suburbs as it has in Howard County, police in those areas still are preparing to battle criminals, setting up special task forces, increasing police budgets and hiring more officers.

Police in areas such as Frederick and Prince William said they are looking at the experience of Howard County, whose growth has been one of the fastest in Maryland, for clues to what they can expect.

In Howard County, where the population has grown 35 percent in the past seven years, auto theft, burglary, robbery and vandalism have risen sharply: 474 auto theft cases were reported last year, compared with 292 the previous year. Robberies with force or the threat of force rose from 47 to 67 in 1986, and burglaries increased from 814 to 1,020.

Arrests for the sale and possession of heroin increased from nine in the first six months of 1986 to 34 for the same period this year. PCP-related arrests rose from 17 last year to 29 during the first six months of the year.

Howard County is part of a major corridor for drug traffickers operating between Florida and Baltimore and Washington, police said. These drug dealers use the rural roads of the county to transport drugs between the two cities, police said.

Howard's proximity to the District and Baltimore is a key factor in the county's crime picture, said Howard Police Chief Frederick Chaney. The county is within minutes of downtown Baltimore and borders Prince George's and Montgomery counties.

Businesses along highways such as Rte. 40 and Rte. 1, which carry traffic between Howard County and Baltimore, have had a rash of robberies and two slayings recently, he said. About 40 percent of the arrests in Howard are of people who live outside the county, Chaney said.

A lifelong Elkridge resident, Anderson, 35, said she remembers a time when neighbors knew each other and often left their doors unlocked, unafraid of crime. But Elkridge, an old lumber mill town along I-95, is changing from a farming community of prime grazing lands and handmade roadside signs offering fresh eggs and hay.

Dirt-filled construction trucks rumble up and down the two-lane road adjacent to Anderson's two-story house, new working on condominium and town house developments. Since 1980, Elkridge's population has nearly doubled, from 7,222 to 13,450.

In Frederick County, once known primarily for its dairy farms, many of the newcomers are affluent, white-collar professionals working for major electronic companies and high-technology firms along I-270.

But as Frederick's population has grown 17.3 percent in the past seven years, burglary, robbery, auto theft, vandalism and drug arrests have risen dramatically, according to Sheriff Robert Snyder.

Snyder said drug arrests have increased "by a high percentage" in the past few years. However, no figures were available, he said, because the county is in the midst of computerizing its crime statistics.

Frederick police have formed a task force with state and local authorities to combat increased drug trafficking, particularly in marijuana, cocaine and PCP. Police blame the increase on the county's easy access to the District along I-270, the Capital Beltway and other roads.

So far this year, Frederick has confiscated about $200,000 in property and personal belongings during drug raids, Snyder said. It may be a drop in the bucket for more urbanized areas, but it is alarming to people in Frederick.

Growth, by itself, however, does not breed crime, some sociologists say. For example, Prince William County, a patchwork of hilly farmlands wedged between I-95 and I-66, is being transformed into a bedroom community of young, well-educated and affluent families.

Along with pickup trucks and farm tractors, the county's roads increasingly are traveled by Audis, Volvos and Mercedes-Benzes.

Prince William Police Chief George Owens notes the presence of what he calls "a lot of drugs countywide." But, Owens said, the county's crime rate has stayed relatively low, in part because of the large number of high-income professionals who have moved there and the state's law-and-order reputation.

But Owens said he does not expect that to last long. "The day is coming when it'll bottom out and start up," he said. "We just don't know when."

In response to higher crime rates, police departments in rapidly growing suburban areas react in similar ways. Last year, Howard County approved a $1.4 million increase in the police department budget, from $10.8 million to $12.2 million. Police also strengthened the crime prevention program, added a foot patrol unit for more visibility and created a street drug unit.

Frederick County hired eight additional police officers this year, the largest single-year increase in recent history, Snyder said. Prince William, with a police force of 248 officers, increased the department's budget from $8.4 million to $10.8 million last year.

Meanwhile, Anderson is waiting for police to catch the thieves who stole her television sets and changed her life. "It's a scary thought," she said of the increased crime. "I guess I'll have to lock myself in my own home."