My earlier suggestion for USBR 66 going south of Los Angeles may have been too hasty. The historic Auto Route 66 stays near the northern edge of the LA basin. So it would probably be understandable to have Rt 66 go thru Pasadena to (or near) Santa Monica. Then the southern route across the LA basin should be labeled Rt 70. My attempts to find local government and local bike club people have not produced any good contacts - yet.
I have some local contacts. Send me an e-mail.