A man was driving a motorcycle south on Bulverde Road near Evans Friday morning when he crashed, accident investigators said.
No other vehicles were involved in the wreck in North Bexar County.
"Several witnesses reported that the vehicle lost control, struck the center median, and the driver was ejected off the bike," said Bexar County Sheriff's Sgt. Mario Cervantes.
Stephen Gonzales, a trauma nurse who works at San Antonio Military Medical Center, was one of the witnesses.
He had just dropped off his son at nearby Johnson High School.
"I happened to look up in my rear-view mirror. I saw a gentleman on a motorcycle hit the end of the curb and began flipping over. I saw maybe four or five flips," Gonzales said. "It didn't look like he was speeding. It just looked like he did not see that median at all. Like, it caught him off guard."
The driver wasn't wearing a helmet, and Gonzales said he could tell right away that he had head trauma.
"You know, something had to be done, so I just took my natural instinct and went and helped someone," he said.
Gonzales helped maintain the man's neck stability and helped apply a collar.
He hopes he started life-saving treatment for injuries that deputies called life-threatening.