I believe Mary was struck by a truck, not a car (I could be wrong), while she was riding her bike. Very sad.
It does qualify as an accident, but it’s a bit more complicated. She had a heart attack while riding her bike, then hit her head on the pavement when she fell off the bike. An autopsy showed that she died of severe bleeding in the brain, rather than the heart attack, but if not for the heart attack she wouldn’t have had the accident.
Other singers killed in accidents:
Kirsty MacColl - a boat ran over her while she was swimming with her sons in Mexico
Sandy Denny - fell down the stairs and ultimately died of a cerebral hemorrhage