Montgomery and Prince George's county police plan to greet New Year's partygoers tonight with a series of roadblocks and sobriety tests aimed at nabbingdrunk drivers.

In Montgomery, according to spokesman Phil Caswell, police will expand their existing "sobriety checkpoint" program throughout the county. Until now, the program has operated only in the Wheaton-Glenmont area, where it has netted 90 drunk driving arrests since October.

At each checkpoint, police use road flares to stop drivers and check for signs of drunkenness. Sober drivers are waved on with little delay, but those believed to be drunk are put through a series of intoxication tests that could lead to arrests. Caswell said officers will move the checkpoints to various locations throughout the county during the holiday night.

In Prince George's, police will be trying out a roadblock program for the first time. Roadblocks will be scattered throughout the county, according to county Police Chief John McHale, and will be set up on major thoroughfares as well as side roads.

In the District, police plan to double their normal nighttime traffic patrol for the holiday evening, but no roadblock program is scheduled.

Virginia authorities said they have no special plans for combatting drunk drivers on New Year's Eve.

In both Montgomery County and the District, those who partake of too much holiday cheer can avoid problems with the law by taking advantage of programs that will provide them with free rides home.

Between 7 p.m. tonight and 7 a.m. New Year's Day, people in Montgomery who are too drunk to drive can get a free cab ride home, according to County Executive Charles W. Gilchrist and State's Attorney Andrew Sonner. The service is being paid for by the Montgomery County Chamber of Commerce and a group of automobile dealers who recently organized a campaign against drunk driving. Rides will be provided by the Barwood cab company, one of whose cabdrivers was killed last year in an accident involving a drunk driver.

In the District, the free cab ride program called CareCab is sponsored by the Comprehensive Care Corp., a California-based firm that operates the alcoholic treatment center at Washington's Providence Hospital.

For a ride home from the District to any point inside the Beltway, call CareCab at 269-7777. For a ride from anywhere in Montgomery, call 469-8424.