An 18-year-old driver, who might have fallen asleep at the wheel, struck and killed a Johns Hopkins obstetrician-gynecologist who was fixing the chain on his bicycle Saturday morning on the side of the road in Dewey Beach, Del., police said.

Dr. John G. Griffith, 44, of Timonium, an assistant professor at the School of Medicine and director of the hospital's Fibroid Center, died at the scene on Coastal Highway, Delaware State Police said.

He had been vacationing with his family.

According to police, the driver, Barry Boulden of Millsboro, Del., drifted onto the shoulder, hit Griffith with his 2002 Pontiac Grand Am, then swerved and crossed south- and northbound lanes and a grass median before stopping on the side of the road.

Police are investigating.

Griffith, who also held a master's degree in public health, oversaw more than 50 faculty and staff members as director of the Fibroid Center, including reproductive endocrinologists and public health experts, according to a statement from Johns Hopkins University's School of Medicine.

"John was a rising star who has fallen from the sky. ... [He] improved the lives of countless women," said Dr. Edward D. Miller, the school's dean.

rona.marech@baltsun.com

Sun reporter Liz F. Kay contributed to this article.