While functional classification maps are a good start to identify what highway classifications roads should be, I would avoid following them one hundred percent of the time. A lot of the times functional classifications are based on previously planned projects that were never completed. For example, Gilbert from La Palma to Orangethorpe was likely classified as a minor arterial because of a planned overpass over I-5.

Lately, the US community has been moving towards forming a coherent network for each classification, meaning that stubs, particularly secondary and above, should be avoided (see https://wiki.openstreetmap.org/wiki/United_States/Highway_classification)

Here's a useful diary entry from another fellow mapper on classifying roads in the LA area: https://www.openstreetmap.org/user/willkmis/diary/399345