“Main-traveled way” is the traveled way of a highway on which through traffic is carried. In the case of a divided highway, the traveled way of each of the separate roadways for traffic in opposite directions is a main-traveled way. Main-traveled way does not include facilities such as frontage roads, ramps, auxiliary lanes, parking areas, or shoulders.
Source:
Section 5216.3, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=BPC§ionNum=5216.3..