I thought this comment from local historian Derek Whaley was interesting enough to post. Also check out his AWESOME website: http://www.santacruztrains.com/p/santa-cruz-rr.html
Derek Whaley Highway 9 has never officially been called Big Trees Highway. In fact, it hasn’t even been called Highway 9 for very long. After it was bought by the county, it was either simply the “county road” or “San Lorenzo Drive”, which was split between “East San Lorenzo Drive” (now Graham Hill Road), and “West San Lorenzo Drive” (now Highway 9 between Felton and Santa Cruz). In Felton it was Baldwin Ave. In Boulder Creek, Main Street and, later, Central Ave. The road was only rerouted through Ben Lomond in the mid-1930s. I don’t know precisely when the highway was bought by the state, but it was originally a hodgepodge of local, county, and state roads that were all merged together and renamed Highway 9. Big Trees Highway was used by some marketers as a fanciful advertising gimmick, but it had no basis in official legal names.