The woman, who was riding without a helmet, suffered life-threatening head injuries when she rode through a stop sign, struck the center median of Fountaingrove Parkway and was ejected from her bike, Santa Rosa police Sgt. Rich Celli said.
The accident happened at about 12:15 p.m. The woman had been riding south on Round Barn Boulevard when she entered westbound Fountaingrove Parkway and crashed, Celli said.
Police have not been able to talk to the woman, who is a Santa Rosa resident, and her name is not being released because her family has not been contacted, Celli said.
Anyone who saw the accident is asked to call Officer Shaun Spiegle at (707) 543-3636.