It has been one of this region's most congested highways for more than two decades. Now, this week, Interstate 270 in Montgomery County officially debuts as a 12-lane superhighway of the future, affecting the region's economy and lifestyle into the next century.

There probably won't be another highway like the new I-270 in the Washington area for some time. Certainly not one wider, with six lanes in each direction from just outside the Capital Beltway to Gaithersburg. I-270 introduces something called "collector-distributor lanes," which separate local from through traffic. And I-270 is now a pure transportation corridor, bringing together Metro, commuter rail, buses and automobiles that serve four states and the District.

"You could make a solid case that I-270 is the best-served transportation corridor on the East Coast outside of the New York City area," Maryland highway chief Hal Kassoff said of the 33-mile highway that stretches south from Frederick through Montgomery, Maryland's most populous county.

As a result, commuting habits will change for many drivers of the 160,000 vehicles traveling daily on the highway. Getting up early, coming home late and finding the best back road to avoid I-270 was a common game for years.

"It's great -- just don't tell anyone about it," said Richard L. Strombotne, a federal employee who leaves later for work and has shaved about 15 minutes off his daily commute from Clarksburg to the District.

A ceremony Thursday will mark the completion of the four-year, $220 million highway widening project, but the milestone already was reached on the Columbus Day holiday when crews opened all six lanes in both directions during the evening rush hour for the first time.

Financed mostly with federal money, the I-270 project stretches about 13 miles between Tuckerman Lane in North Bethesda and Route 118 (Darnestown-Germantown Road) in Germantown and includes eight interchanges. The 12-lane section, including the collector-distributor lanes, extends northbound from just south of Montrose Road near Rockville to Route 124 (Quince Orchard Road/Montgomery Village Avenue) in Gaithersburg. Southbound, the 12-lane freeway begins near Interstate 370 and ends near Montrose.

By the end of the 1970s, mammoth traffic jams were common along I-270 -- then four lanes to six lanes through Montgomery -- because of rapid development. When construction to add more lanes began in 1986, drivers faced more aggravation.

"It was so frustrating, especially in the winter when very little work was being done and people were wondering what was going on," said Mary Louise Faunce, of Gaithersburg, who commutes to a job on Capitol Hill. "Now that the pain is over, you kind of forget about it and move on."

The pain is not completely over. Backups remain at the east and west spurs to and from the Capital Beltway, and crews still need to complete the widening near Montrose and Route 118. Isolated delays also occur at other places, but traffic is relatively free-flowing, officials said.

The bottleneck at the spurs happens in part because of the Beltway widening project between the American Legion Bridge and River Road and because there are only two lanes in each direction connecting the wider I-270 with the eight-lane Beltway. In 1988, the Montgomery County Council, led by member Neal Potter, killed plans to build another lane in each direction.

"Mr. Potter can take pride in knowing he precipitated that bottleneck," said County Executive Sidney Kramer, who lost to Potter in the Democratic primary for county executive but is running as a write-in candidate in the Nov. 6 election.

"It shows how little he knows about the situation," Potter replied, adding that state figures show that only the western spur will require an extra lane.

I-270 should be able to handle the growth in traffic through the year 2010, when about 185,000 vehicles are expected to use it daily. That is consistent with Montgomery's staged growth plan in which I-270 will continue to attract new commercial, retail and educational centers while stimulating the development of places such as Germantown, Gaithersburg and Clarksburg.

"The 270 corridor is the backbone of our general plan," said Montgomery Transportation Director Robert McGarry. "At one point, the development got ahead of us, as evident in the daily traffic jams on 270. We're now in balance with the highway and the development that is planned."

Some people don't believe that, and worry that I-270 will fill up again soon.

"The development in this area has been unreal in the last few years," said Rufus Johnson, of Damascus, who takes I-270 into the District to his job at the Justice Department. "It's a strong possibility we may be back to where we were" on I-270.

State and county officials want to avoid that. They will be helped somewhat by the current economic slump, which will slow the pace of development.

Because development is greater between Rockville and Gaithersburg, I-270's planners decided to separate local traffic, which typically was traveling short distances between interchanges, from the longer-distance through traffic.

To do that, four roadways were built between Rockville to Gaithersburg, separated by concrete barriers.

The outside, or far right, roadways are two lanes in each direction used by slower local traffic. The inside roadways are four lanes in each direction for faster through traffic.

Drivers have the option of switching between the local and through lanes, depending on their destination. The local lanes "collect" and "distribute" traffic to either local interchanges or the through lanes, explaining the collector-distributor moniker.

Confusion about using the collector-distributor lanes has led to complaints and accidents. Officials acknowledge the new lanes will require some adjustment, but said they believe they have taken care of the problem with signs.

Another conspicuous sign tells drivers that the inside left lane will be restricted to rush-hour car pools. That won't happen until the highway becomes more crowded, but officials want people to get used to the idea so they won't squawk when the lane is restricted.

A wider I-270 has other benefits beyond getting people to work. Montgomery business leaders are ecstatic about the corridor's ability to move goods and services, and believe the new highway will give them a leg up on their competitors in the region, especially Fairfax, in retaining and attracting companies and workers.

For all the euphoria, there are nagging concerns about I-270's future impact on the region's environment and growth. Will Maryland now be forced to widen I-270 north to Frederick, where much of the highway is only two lanes? And how do you widen a major interstate highway feeding into the Beltway without also expanding the Beltway?

"Give the state credit. That was a humongous job on I-270," said Allen E. Bender, of Rockville, chairman of a citizens group that unsuccessfully tried to stop the I-270 project in court. "But I can't figure out, when all these office buildings are full and I-270 is at capacity, how does a saturated 12-lane highway go into a saturated eight-lane Beltway?"