California Code, Public Resources Code - PRC § 5022.5

There shall be two categories of places of historical significance:  the registered historical landmark and the registered point of historical interest.  The location of the point of historical interest shall be designated by a sign indicating “Point of Historical Interest” with an appropriate direction, which sign shall be erected and maintained by the Department of Transportation, as to state highways, or the county authorities or city authorities, as to streets or highways under their jurisdictions.  A local historical group or organization may raise a marker or plaque at a registered point of historical interest.  Nothing herein shall require the signing of such points where parking is not available or where such signing would cause a traffic safety hazard or would interfere with the normal flow of traffic.