A tractor-trailer overturned onto a car on I-66 near Rte. 50 in Fairfax County during the evening rush hour yesterday after the truck's driver apparently suffered a fatal heart attack, police said.

The car's injured occupants--one of them a pregnant woman--were trapped for about an hour, and traffic was backed up for five miles.

Virginia State Police said the truck was headed east on I-66 when it went out of control, crossed the grassy median and fell atop a west-bound auto about 5:30 p.m. They said the driver, a 59-year-old Gate City, Va., man, whose name was not released pending notification of relatives, was pronounced dead at Commonwealth Hospital.

The car's two occupants--a Manassas woman six months pregnant and a Manassas man--were finally freed by Fairfax County fire and rescue workers, authorities said. They were taken to Fairfax Hospital, where the woman was reported in serious condition and the man in good condition. Their names were not immediately available.

No other vehicles were involved in the accident and no other injuries were reported, police said.

They said westbound lanes were blocked for nearly three hours, but eastbound lanes were not affected. Westbound traffic was backed up for at least five miles for about an hour before being diverted onto routes 123 and 243.