"360. "Highway" is a way or place of whatever nature, publicly maintained and open to the use of the public for purposes of vehicular travel. Highway includes street."
https://www.dmv.ca.gov/portal/dmv/detail/pubs/vctop/vc/d11/c...
"22400. (a) No person shall drive upon a highway at such a slow speed as to impede or block the normal and reasonable movement of traffic, unless the reduced speed is necessary for safe operation, because of a grade, or in compliance with law."
Yes, shocking. (Also, El Camino is a State Route, I'll let you look that one up.)
My Hanomag A-L 28 is a maximum speed in the papers of 78 km/h. As such I am allowed on the Autobahn, even that I am driving only 50 km/h or sometimes 40 km/h on a hill. Because each hill does slow down a Hanomag considerably.
In most European countries there is a minimum speed requirement when driving on thr motorway (usually between 60 - 80 km/h), and sometimes there are even lane-specific speed limits (e.g. left lane requires at least 110 km/h). All that is only relevant if the traffic actually allows you to go that fast of course.