After the road was closed for a crash investigation, Prospect Mill Road reopened at Elm Street as of 10:45 a.m., according to the Harford County Sheriff's Office.
The single-vehicle crash was reported just after 8 a.m., and when first responders arrived, they found the 74-year-old female driver in cardiac arrest, according to Harford County Sheriff's Office spokesman Eddie Hopkins.
The Bel Air woman was taken to Upper Chesapeake Medical Center, where she was in critical condition Monday morning, Hopkins said.
Witnesses reported seeing the woman's 2009 red Hyundai Elantra drift off Prospect Mill Road, heading southbound, before hitting a tree, according to the sheriff's office.
"Preliminarily, it appears the victim experienced a medical event that caused her to leave the roadway," Hopkins said.
Crash investigators were at the scene as of 9:15 a.m. and planned to be there for 90 minutes, according to Hopkins.