HOUSTON -- Alaska State Troopers say a 78-year-old Willow woman died as she drove on the Parks Highway at Houston in the Matanuska-Susitna Borough.

Troopers say Betty Lou Tilghman lost consciousness because of an unknown medical condition shortly after 6 p.m. Tuesday as she drove north at Mile 60.2 with her granddaughter.

Tilghman's car crossed the southbound lane and came to rest on a guardrail. An ambulance rushed her to Mat-Su Regional Medical Center, where she was pronounced dead.

The death was first reported by KTUU-TV.

Tilghman's granddaughter was wearing a seat belt and was not injured.