Bay Road reopened around 1 p.m., according to Menlo Park police. The road had been closed in both directions between Marsh Road and Del Norte Avenue.
Investigators confirmed that two cars were involved: A BMW and a white car that reportedly rear-ended the other vehicle before running into a tree.
The 42-year-old driver of the white car, a Redwood City woman, was critically injured and taken to Stanford Hospital, police said. There were no other injuries.