Traffic on the one-month-old section of I-66 is heavier than expected during the day, and rush-hour carpool traffic is close to what Virginia highway officials anticipated when the highway opened Dec. 22.

And while virtually all commuters now are obeying the carpool-only restrictions in the morning rush hours, transportation officials said yesterday that an average of 16 motorists are being arrested daily for attempting to sneak onto the restricted highway in the evening rush hours.

About 40,000 cars a day are now using the new 10-mile extension of I-66 between the Capital Beltway and Washington, a volume of traffic consultants had not expected on the $280 million road until 1984 or 1985.

During the peak morning rush hour, as many as 1,230 carpools have used the highway, far above the 700-to-800 an hour expected in the highway's first month.

Only 388 carpools used I-66 during the peak morning rush hour on the day it opened, according to Thomas Farley, the highway engineer overseeing a computerized traffic control system that will go into operation on I-66 and I-395 this spring.

More precise counts on I-66 traffic are not available because the mechanical counters on the road have not functioned properly, Farley said.

A spokesman for Virginia Highway Commissioner Harold C. King yesterday said King is "pleased at the use being made of I-66 so far, although he knows its unique restrictions will take time to become accustomed to. But with ride-sharing and time, I-66 should be fully used."

By the end of the year state officials have estimated about 3,000 carpools an hour could be using I-66, which would mean relatively heavy traffic during rush hours on the highway itself and traffic congestion at Roosevelt Bridge.

The bridge, which carries I-66, Rte. 50 and George Washington Memorial Parkway traffic across the Potomac, already is at capacity during rush hours.