A Haymarket, Va., man and his 12-year-old daughter were killed Saturday night in a two-car collission on Rte. 15 north of Rte. 234 in Prince William County, county police reported.

Michael R. Cosgrove, 37, and his daughter Mary, of Alvey Drive in Haymarket, were believed to have died instantly in the crash, police said. The accident occurred on about 1 1/4 miles north of Rte. 234, police said.

Occupants of the second car were not immediately identified. Police said they suffered only minor injuries. Cause of the accident was under investigation.

The deaths brought the traffic fatality toll for the Labor Day weekend in Virginia to 16 Monday afternoon. State police had predicted that the toll would reach 20 fo the weekend.

In Maryland, eight persons were reported killed on the state's highways.

Among those killed in Maryland and Virginia were:

An unidentified tractor-trailer driver, who killed Saturday when his rig smashed into a parked car on, the Capital Beltway near Largo.

James Singleton, 35, of the District of Columbia, who lost control of his motorcycle and was thrown 75 feet into roadside shrubbery in southwest Anne Arundel County.

David Johnson, 37, of Gaithersburg, who was killed Friday when his car went out of control and struck a guardrail in Montgomery County.

Steven Williams, 18, of Falls Church, who was dead on arrival at Peninsula General Hospital in Salisbury, Md., after a two-car collision Friday.

Richard MacMillan 49, of Manassas, who was killed shortly after 11 p.m. Saturday when his car ran off Rte. 215 in Prince William County near Manassas and overturned.