Aviation Bl: The original plan was to call this road Inglewood-Redondo Rd. It was to extend from the south edge of downtown Los Angeles to west Torrance. The portion from Inglewood to Los Angeles was never built, probably because of the high cost of property acquisition and disruption of existing neighborhoods in southwest Los Angeles. A small segment of this portion exists as Redondo Bl. in Inglewood. This was to be extended in a straight line (a diagonal), running just north of the AT&SF tracks, incorporating a portion of Hyde Park Blvd. It would have continued in a straight line to turn into San Pedro Place at Main St.
The portion through Inglewood was built and connected with Florence Av, and now bears that name. When I first became aware of this street (mid 1950s), the name Florence Av continued to the junction with Arbor Vitae St. In the 1960s the segment between Manchester and Arbor Vitae was renamed Aviation, as it is a north-south segment.
The portion from Century Blvd to Redondo Beach runs along a section line. Originally it was a rural road called Wiseburn St. The plan in the 1930s was to align the street 1/8 mile east, along Judah Av, and Green Ln in Redondo Beach. But instead Wiseburn St was made a highway, extended by Dewey St (the portion between Robinson St-2nd St and Artesia Bl).
Crenshaw Bl: The original Crenshaw was in Los Angeles, in extending it south it linked up with Cypress Av in the South Bay area, but Cedar St in Torrance, fitting in to an alphabetic sequence in Old Torrance. Some maps show Crenshaw extending south to PV Drive South, but I don't believe it ever was an open roadway; perhaps it was graded, but the land proved so unstable that it was never finished.
Pacific Bl: This street was never constructed. It was supposed to continue Pacific Avenue in Long Beach north through Compton, and end in Los Angeles at Central Avenue and Imperial Hwy. There is a small portion in Long Beach, now called Pacific Place, which is merely a continuation of Pacific Avenue to the San Diego Frwy. It was supposed to continue across the L. A. River and run next to the P. E. (now the Blue Line) tracks, then into Compton, where it would run next to the Compton Creek channel. There are a few short stretches in Compton bearing the name Pacific Blvd, but it is not a highway.
Lakewood Bl - Rosemead Bl: This was assembled from other streets with newly-built connections, as state highway #168. It was posted as California route 19. Through Long Beach this street was originally Cerritos Av, heading north toward Downey. The original plan called for a wide street called San Gabriel Blvd. or San Gabriel Parkway, which would have been a parkway with three roadways with the main roadway down the center of a long park.
Through Downey, Lakewood Bl was assembled and included part of Lexington-Gallatin Rd (Lexington was the original name for the settlement of El Monte, and Gallatin was one of the original settlements in Downey, located near the present Downey Av and Florence Av.) The name changes to Rosemead Blvd at Telegraph Rd, the boundary between Downey and Pico Rivera. Through Pico Rivera, Rosemead Blvd partly follows the old Lexington-Gallatin Rd. Interestingly, part of present Paramount Blvd, which crosses Whittier Blvd, was named Lakewood Blvd as late as the 1950s.
North of Pico Rivera, Rosemead Blvd passes through Whittier Narrows, the lowland where the San Gabriel River and Rio Hondo come near each other. Between Garvey Av and the San Bernardino Frwy, what was Bruin Av and Rosemead Blvd were connected, and there is a Rosemead Place between Garvey and the Freeway, which probably was the original Rosemead Blvd.
North of the Freeway to Foothill Blvd, the present Rosemead Blvd seems to have been merely upgraded to state highway standards, at least between the San Bernardino Freeway (I-10) and Longden Av., and again between Duarte Rd and Foothill Blvd. Between Longden and Duarte Rd the street was cut through, because the other streets in that area are at an angle.