A witness saw a 51-year-old man lose control of his bicycle for no apparent reason at about 2:05 p.m. near Santa Clara County's Almaden Quicksilver County Park, officials said.
The man was traveling eastbound on Hicks Road, west of Alamitos Road, and not going particularly fast, CHP Officer Peter Van Eckhardt said.
After losing control, the man fell over the top of the bicycle and his head struck the pavement. He was wearing a helmet when the accident happened, Van Eckhardt said.
The incident was reported at about 3:29 p.m. Officials do not know if alcohol or drugs were involved.