I also wonder which highway we are talking about. The CA 99? Or was there also a connection with the I5, if they even were there at that time age. To answer my own question:
"This north–south central highway (CA 99) became part of US 99 in 1926, as part of the new United States Numbered Highway System developed by the American Association of State Highway Officials (AASHO), though signs were not posted in California until 1928"
She came from south California before moving to the Modesto area. She could have tried to hitchhike back there, but that is pure speculation. I wonder in what city the family lived before they moved to Modesto (area). It's very confusing the whole geographic history in her missing file....things just don't add up?
Maybe it's somewhere up the thread, but what was her address when she went missing?