Starting tomorrow, motorists in Loudoun County can expect that any time, any place, they may encounter a roadblock where the law officers will not be asking for their license or registration card.

Instead, said Loudoun Commonwealth's Attorney William T. Burch, motorists will hear words to this effect:

"Good evening, this is a routine sobriety check. Have you consumed any alcoholic beverages tonight? Sorry for the inconvenience. Good night."

It should take about 30 seconds to a minute for the motorist to be sent on his way, unless the officer suspects the driver is intoxicated. In that case, the driver will be made to pull his car off the road. He will then be observed by police officers and may be asked to take a breath test. If he is suspected of being intoxicated he may be arrested.

In Loudoun County, where the number of automobile accidents has risen steadily in recent years and where this year alone there have been eight alcohol-related fatalities, law enforcement agencies decided the time has come for a sobriety checkpoint program.

Burch's office has coordinated efforts between the Virginia State Police, the Loudoun County Sheriff's Department, the Leesburg Police Department and other town forces to begin a pilot program that is among a handful of similar programs throughout Virginia.

The Fairfax City Police Department began the first such program in Virginia in late December 1981. It has been so effective at curbing drunk driving that the department plans to continue its sporadic sobriety roadblocks.

During their first roadblock on Dec. 30 and 31, 1981, the city police made no arrests, but instead took nearly 60 apparently intoxicated drivers to police headquarters where they were offered coffee, milk and snacks, and time to sober up before being released.

Before their next roadblock, the Fairfax City police announced they would be making arrests, and in March, 1982 they did, about 40 of them.

Since then, said Capt. Donald Taylor of the department, each roadblock (most are during holiday seasons when people tend to drink more heavily) results in about half a dozen arrests.

More importantly, he said, "The number of drunk drivers involved in accidents has declined."

With increasing concern nationwide over drunk driving, the use of sobriety checkpoints, though still controversial, is expected to increase. The Charlottesville City Police Department has run an experimental program with sobriety checkpoints every Friday and Saturday night since late December 1983, and several other Virginia jurisdictions have run sobriety roadblocks. Montgomery County, Md., is among other jurisdictions with similar programs.

Most controversy over the sobriety checkpoints has focused on the legality of police officers stopping drivers to check for sobriety when there has been no indication before the stop that the driver may have been intoxicated.

Among court challenges to police arrests during such roadblocks was a case in Harford, Md., in which the Maryland Court of Appeals in August upheld police action and ruled that the DWI checkpoint operations are a reasonable response to the drunk driving problem as long as the program is carefully planned and carried out.

In Loudoun County last Christmas and New Year's Eve, the state police, sheriff's department and town police joined in a much publicized campaign to curb drunk driving by increasing patrol activity.

Loudoun County's new program differs from the one in Fairfax City. Loudoun officers do not plan to ask motorists for their licenses or registration cards, but merely to look for signs of intoxication. It differs also in the involvement of more than one law enforcement agency.

At each roadblock will be several state troopers, several sheriff's deputies, and if the roadblock is within a town, several town police.

Like the Fairfax program, Loudoun's will run most roadblocks during holiday seasons.

Burch declined to release dates or areas of planned roadblocks "because it defeats your purpose, unless somebody's so drunk he doesn't remember." The roadblocks "could be anywhere in the county," Burch said.