Car-pooling requirements on Interstate Rte. 66 are enabling the highway to carry more people during the peak morning period than it could if the rules were lifted, according to a study by the Washington Council of Governments.

The report, which found that about 10,800 persons in car pools, van pools and buses used the restricted lanes of I-66 one recent morning, offers the first tangible evidence that the so-called HOV-4 rules are actually helping commuters travel the new 10-mile roadway in Northern Virginia more efficiently during rush hour. The rules--known as High Occupancy Vehicle-4 in highway parlance--require four or more riders in cars that use the highway inside the Capital Beltway during rush hours.

"We're seeing more people moving in the I-66 corridor at a better level of service, with less congestion, than are seen in roadways with a similar vehicle capacity and no HOV rules ," said Ron Sarros, associate director of transportation planning for the Council of Governments. "It's achieved the objective of efficient movement of traffic. You're getting more for your bucks in terms of moving traffic."

According to the traffic count, which was conducted last month, about 6,200 people traveled inbound on I-66 past the Glebe Road intersection between 7 and 8 a.m. If all those people were traveling in cars at the average rush-hour occupancy of 1.4 persons per vehicle, the COG study reported, the number of cars needed to transport them would have been about 4,400, or more than the road's capacity of 4,000 vehicles per hour.

Such an overloading of the highway would result in bumper-to-bumper traffic at a maximum speed of 30 miles an hour and a high probability that traffic would stop repeatedly, said David Gehr of the Virginia Department of Highways and Transportation.

"I think we have a significant number of people being moved in that corridor, and we still have room to move more people," said Gehr. "If we allowed uncontrolled traffic out there, we would need six to eight lanes to hold them all."

By comparison, a 1981 count of the southbound traffic on George Washington Parkway south of Spout Run during the morning peak hour found about 3,800 vehicles carrying 5,400 occupants in stop-and-go conditions. The parkway has no car-pooling restrictions.

The HOV-4 rules on I-66 have come under frequent criticism over the last few months from commuters who argue that the new $275 million stretch of highway is underused. Virginia Highway Commissioner Harold King has said, however, that the state will not even consider easing the rules until the end of the year, after a road connecting I-66 with the Dulles Access Road has been completed.

A recent highway department study found that the biggest traffic surges on I-66 occur in the hours immediately after the conventional morning and evening rush periods. Highway officials have argued that vehicle counts are not an accurate reflection of the road's usage because they measure only cars, not people.

A vocal group of area politicians has been arguing that three-person car pools should be permitted to use the restricted part of the highway, and Fairfax County Board Chairman John F. Herrity said yesterday he did not believe the COG figures would turn back that effort.

"All I'm saying is that we ought to experiment to maximize the usage of the highway," said Herrity. "Seeing that we're the only area in the whole world with an HOV-4 restriction, where is it written in Holy Writ that that's it?"