Many law-abiding Virginia motorists apparently find it hard to believe, but state officials say it's all right--for some people anyway--to drive their vehicles with expired inspection stickers.

State officials are allowing the exceptions in certain cases as Virginia changes from a twice-a-year inspection system to an annual one. The changeover has caused considerable consternation among motorists who are not sure when they should take their cars in for inspection.

"It's not confusion so much as disbelief," said Lt. Jerry Conner, the Virginia State Police's assistant safety officer. "They can't really believe we will let them drive with an expired sticker."

In the past, Virginia motorists have had to have their cars inspected twice a year. The safety inspections, which were required every six months, cost $4 each. Since Jan. 1 motorists in Northern Virginia have also had to bring their cars in for an annual emissions inspection at a cost of $3.50.

Now the goal is to combine both the vehicle and emission inspections in one annual visit. The annual inspection costs $7. The cost of the emissions test--which is required of 1975-or-newer cars-- remains unchanged.

The problem for the state in the transition to the new system was this: If motorists were required to have their cars inspected when the stickers expired, virtually everyone would come in during the last six months of next year.

Conner said the extension of the validity of certain inspection stickers will ensure that about half of the state's estimated 4 million motor vehicles undergo safety inspections during the period from January to June and the other half from July to December.

"We had to come up with some sort of system to spread these inspections over a 12-month period," Conner said. "This way was the only way we could do it."

Here is how it works: If the car's current tags expire between July 1, 1982, and Dec. 31, 1982, the safety inspection will be due by the expiration of the sticker now on the windshield. But if the tags expire between Jan. 1, 1983, and June 30, 1983, the safety inspection currently on the windshield will be extended until the tags expire.

Suppose, for example, that the current safety sticker expires at the end of July 1982 and the tags expire in August 1982. The deadline for a new annual safety sticker will be July 1982, state police said. But if the tags on that same car do not expire until February 1983, the safety sticker will be valid until then, seven months beyond the sticker's current expiration date of July 1982.

After June 30, 1983, Virginia will have completed the transition and motorists will be expected to have their cars inspected by the date shown on their windshield stickers, Conner said.

Virginia officials said that the move to once-a-year safety inspections is part of the national trend toward deregulation.

District of Columbia motorists already have the once-a-year inspection program. In Maryland an inspection is required only when a used car changes hands. Attempts to require periodic inspections have been defeated repeatedly in the state legislature because of the cost of the program and the absence of statistics.