California Code, Revenue and Taxation Code - RTC § 8605

“Highway” includes a way or place, of whatever nature, publicly maintained and open to the use of the public for purposes of vehicular travel.

A private road is not a highway for the purposes of this part unless it has been dedicated and accepted by the proper public authorities as a public highway. The mere use of a private road by one or more members of the public, with or without the consent of the owner, does not make it a highway.