From historicaerials.com, the southern hill portion is still in use on their 1968 photo. The next available image is in 1993, where it looks like the northern road is in place. Unfortunately that's a 25 year window, at least that narrows it down a little.
Gentlemen:
Some of us has a question for you about the old Eel River Hwy. 101 Viaduct in Mendocino County, CA. Could you tell us when the new section of Highway was opened and when they closed down the sidehill portion? We would appreciate it very much and would answer some questions from us old timers who drove over this section all of the time.
Thanks for your time in this matter.
Eldon G. Whitehead,
fortuna, CA.
Hello.
I did a little research on the subject viaduct to determine when it was bypassed by the present alignment of Highway 101. Based on a newspaper article in the Ukiah Daily Journal, dated August 17, 1982, the bypass around the old viaduct, sometimes called "the Slab" was scheduled to be completed by August 19, 1982, but was opened to traffic by August 17th that year. I drove on the old viaduct many times while attending Humboldt State University from 1972 to 1974.
Rick Donaldson
Spokane, WA