Paul Stilp used to have the nicest Sunday drives on his way home from family visits in Pennsylvania. Those ended when he started running into unexplainable miles-long traffic jams west of Frederick on Interstate 70.

Stilp also used to have an easy-as-can-be drive to his second home on Virginia's Northern Neck. But then he started hitting miles-long traffic jams on Interstate 95 around Fredericksburg.

The constant jams were mystifying. No accidents to be seen, no construction, no merges, no sharp turns or other noticeable explanations.

"You can run into it just for no reason," said Stilp, who lives in Alexandria. Traffic has been backing up around Frederick "the last four or five years, and it just gets steadily worse," he said. "I think it's just local traffic. And it's the same with Fredericksburg. There's just not enough ways around Fredericksburg for local people to get around without getting on the highway."

Stilp's hunch is dead-on, according to traffic experts, who said that drivers in the booming areas at the fringes of the Washington suburbs have turned what are meant to be long-distance freeways into their main streets as they look for ways around overwhelmed local roads.

A generation ago, interstate traffic would slow to a notable degree mainly around major cities. But now, regular jams are appearing all along interstates, especially on the East Coast, largely because of traffic surrounding such growing suburbs as Frederick and Fredericksburg.

Around those communities, I-70 and I-95 are overwhelmed, particularly on weekends, when tens of thousands of locals, heading out to shop or take in a movie or a meal, join the usual weekend traffic on the highways.

"One of the things that has happened, particularly in those two areas, is that they're not well served by major arterials that are not interstates," said Alan E. Pisarski of Falls Church, author of "Commuting in America." Interstates were "never conceived of being designed to serve [as] commuter roads. In effect, what's happening here is that different sets of purposes are conflicting with each other."

That's a pain for the everyday driver and a threat to the movement of goods, Pisarski said.

"There's a lack of reliability in the system and a lack of redundancy in the system," he said. "More and more, it's a clear threat to interstate commerce. I think it's very real."

The recent stories of Frederick and Fredericksburg are as similar as their names: They are both former small towns distinct from the Washington region that have turned into booming mini-cities, surrounded by even faster-growing suburbs.

As recently as a decade ago, they were the kind of place Washingtonians might wander into for a dose of Americana and Civil War history on the way back from the beach or the mountains. Both still maintain plenty of their days-gone-by charm, but they also have added shopping centers, entertainment venues and other attractions that are the staples of a commuter culture.

Frederick County has grown by nearly half since 1990, from about 150,000 to almost 218,000. Growth around Fredericksburg has been even more extreme. In 1990, the population of the city and surrounding Stafford and Spotsylvania counties totaled about 137,000, compared with about 247,000 last year.

They have become the social, cultural and entertainment hubs of their areas, drawing people by the tens of thousands to shops, restaurants and other activities. And both have road systems built for the days well before any of that came to be.

The main traffic problem around Fredericksburg is caused by the Rappahannock River, which separates all those residents from shops and services. There are only two bridges to get across the river -- on I-95 and Route 1. Route 1 would be the logical local alternative, except that there is an intersection on the Stafford County side that is too small for local traffic and has daily backups that rival anything Tysons Corner has to offer.

"Even if you come almost to a stop on 95, you can still make it along 95 quicker than along Route 1," said David E. Ogle, Fredericksburg district administrator for the Virginia Department of Transportation. "As this area has continued to grow, those crossings of the Rappahannock just haven't been able to keep pace with the growth."

Ogle said traffic counts tell him all he needs to know about how the problem has grown in his region. In 1990, 50,000 vehicles a day drove on I-95 south of Fredericksburg, 71,000 in the areas immediately around the city and 77,000 just north of it. That's exactly what he would expect: increasingly more cars on parts of the highway that are closer to heavily populated Northern Virginia.

But by 1998, those counts showed that traffic was bulging around Fredericksburg: There were 72,000 vehicles a day south of town, 122,000 around it and 110,000 north of it. By last year, the numbers had grown again: 104,000 vehicles south of Fredericksburg, 143,000 around it and 122,000 north of it.

"The local traffic using [I-95] as their river crossing is what I would attribute the majority of that difference to," Ogle said.

The Frederick area doesn't have a bridge problem, but it is straining under its own difficulty managing new residents. Many of them converge on Frederick at nights and on weekends, but many also leave for outlet malls and other shopping in Hagerstown, Md., about a 20-mile drive northwest on I-70. The result is a constant flow of locals going back and forth on the interstate, in addition to daily commuters, truck traffic and weekend vacationers.

"Frederick to Hagerstown is pretty much bumper-to-bumper," said Jim Gugel, chief of comprehensive planning for Frederick County. "It's our equivalent of beach traffic on Route 50."

Gugel said drivers are constantly -- and mostly unsuccessfully -- looking for alternatives. "There's a lot of jumping between the interstate or going through town," he said. "There have been times when practically the entire city has been gridlocked."

Although their recent pasts are similar, the futures of Frederick and Fredericksburg are likely to be very different, at least where traffic is concerned.

There are no additional river crossings or expansions planned for the Rappahannock. Ogle said the region's main hope lies in a private proposal to build a toll road between Washington and Massaponax, south of Fredericksburg. That proposal is one of two being considered by the state for toll lanes on I-95, but even if state leaders choose it, there's no guarantee that section of the highway will be built anytime soon.

"Realistically, before you see any actual road built out there, you're talking about 10 years," Ogle said.

Drivers around Frederick might be a bit luckier. Maryland officials are progressing on several fixes on I-70, including a number of interchange upgrades that are expected to ease merges and the highway's intersection with I-270. State officials also said they plan to widen I-70 to three lanes through Frederick but don't yet have the funds to do it.

Transportation experts said no amount of road construction will fix the problem. "It's a fairly common phenomenon in rapidly suburbanizing areas," said Hani S. Mahmassani, director of the Maryland Transportation Initiative at the University of Maryland. He said the only real solution is that the "areas really need to try to control growth."

Paul Stilp, on I-66, also gets caught in traffic on I-70 and I-95.