As several have noted, span wire signals are generally only used in California for the purposes of maintaining an existing signal system during a construction project.
The Carmel Valley Road/SR-1 intersection is one of two that I know of in California that has a permanent span-wire installation. The other is at the end of the SR-4 freeway at Fresno Avenue in Stockton; that signal will probably not be around for many more years as there are plans to extend the freeway westward. There are thousands of signals in this state (6,000 alone on the state highway system) and there is, of course, the possibility that I may have missed some others.
I am aware of at least two other permanent span-wire configurations that have since been removed.
There was a span-wire installation at the junction of Monterey Highway and Cochrane Road in Morgan Hill. US-101 used to end at Monterey Highway in south San Jose (near the present day interchange at Blossom Hill Road) and followed the Monterey Highway alignment to Cochrane Road, where it turned east to connect with the northern end of the Morgan Hill bypass freeway. The remainder of the Morgan Hill bypass was completed in 1984, eliminating the need to travel on Monterey Highway; however, I believe this span wire signal remained in place for a number of years after the freeway was completed.
I am also aware of a span-wire installation that was used at the intersection of El Camino Avenue and Rio Linda Blvd here in Sacramento. This installation was removed in the early 1980s. I suspect it dated back to the days when North Sacramento was its own city.
(Please note that I work for Caltrans but do not officially speak for them on this board)