Two that I encounter frequently in California are El Camino Real on US Route 101/several state highways and Redwood Highway US Route 101/US Route 199 north of San Francisco to Grants Pass, OR.
(https://live.staticflickr.com/65535/48801537138_0f8f122a81_4k.jpg) (https://flic.kr/p/2hmqCLJ)ECRa (https://flic.kr/p/2hmqCLJ) by Max Rockatansky (https://www.flickr.com/photos/151828809@N08/), on Flickr
(https://live.staticflickr.com/65535/50485708751_b6965e5df7_4k.jpg) (https://flic.kr/p/2jVfsjk)199USc (https://flic.kr/p/2jVfsjk) by Max Rockatansky (https://www.flickr.com/photos/151828809@N08/), on Flickr
Pertaining to El Camino Real it was probably the earliest example of a Californian Auto Trail having been established back in 1904 with the formation of the El Camino Real Association. The Redwood Highway was typically associated with Legislative Route Number 1 in California which became US Route 101 and US Route 199 from San Francisco Bay to the Oregon State Line. Interestingly pertaining to the Redwood Highway it seems that Caltrans and ODOT both sign it.
I know there are numerous examples of historic Auto Trails (like the Lincoln Highway) which are signed retroactively and sometimes include State Maintained Highways. What other examples are out there of Auto Trails which have continuously been signed on State Maintained Highways?