This Master System Plan Map for the Los Angeles County Highway System shows highways "built" (in blue-grey) and "not built" (in orange). Most of the non-built projects have been cancelled as a result of opposition to the plans by activist communities and residents who recognized that simply more freeways is not the answer to traffic congestion. The SR-710 Freeway remains among the few active unbuilt idea.
This image is used with permission of the editor of the Summer 2014 WPRA (Western Pasadena Residents' Association) newsletter, Chuck Hudson. Thank you, Chuck. Much appreciated. The above is the caption that was underneath the image.
My opinion based on my last two years acquiring knowledge about, and actively opposing, the 710 Freeway extension: it's all about creating a "shortcut" for cargo traffic from the two ports and CalTrans/Metro does give a hang about traffic congestion, one way or the other.