The Capital Beltway would be widened from eight to 10 lanes in Prince George's County for rush hour car-pool lanes and Metro's Blue, Red and Green Lines would be extended, according to a new study of Maryland's mass transit needs for the next 20 years.

Some of the state Department of Transportation study's recommendations are controversial, such as the Beltway widening, casting doubt on how much of it would be practical.

The cost of making several "quick fixes" such as expanding the use of express buses, building park-and-ride lots and adding restricted car-pool lanes would be about $2 billion over the next five years, but the big-ticket items such as light rail would cost billions more.

There is no money available now, and officials have said that an increase in the gas tax would be necessary before any new transportation projects could be undertaken.

"It's a realistic plan over the long term," said Steve Zentz, Maryland's deputy transportation secretary. "You have to realize the period is 20 years."

The study said that the projected growth in population and employment in the Baltimore-Washington corridor required the state to plot a course for transportation that relies on transit. Otherwise, commuting times in some places will double by the year 2010.

The 18-month study said that the Beltway should be widened to five lanes in each direction from Route 214 to Interstate 495 near the Interstate 95 spur to Baltimore. That would cost $350 million. The extra lane would be restricted during rush hour to car pools, the study said, and one of the existing four lanes between the I-95 spur and the American Legion Bridge also would be for rush hour car pools.

Restricted lanes also would be designated on parts of Interstate 270, the study recommended.

The study also proposed the extension of transit service from the Shady Grove Metro station on the Red Line to Clarksburg in Montgomery County and from the Addison Road station on the Blue Line to Bowie in Prince George's County. When the Green Line is completed to Branch Avenue, transit service could hook up with Waldorf in Charles County. The transit could be rail, express busways or other ways.

New rail or transit lines in seven other places in the state, mostly in the Baltimore area, also were suggested. The study assumed that the trolley linking Silver Spring and Bethesda would be built, though the future of that project may be in doubt if Neal Potter, the Democratic nominee for Montgomery County executive, is elected. Potter has said the project requires more study.

A new cross-county highway connecting I-270 with Route 1 in Prince George's County also is assumed under the study.