According to friends whose families were lifelong Santa Cruz residents, in the late '50's the city was pushing for a signed route that would expedite access to the beach from both SSR 1 and SSR 17 after 1 was moved a bit inland onto its current expressway/freeway alignment and away from downtown. Since Ocean Street, the original surface alignment of SSR 17, was directly served from both routes, it was considered the logical place to start the loop. The Division of Highways commissioned LRN 287 but tried to keep it as short as possible, not wanting to assume any more city street maintenance than was absolutely necessary -- thus returning it to CA 1 at Chestnut street, where the then-new CA 1 expressway segued onto city streets. Santa Cruz wanted the loop to "wiggle" all the way down to the boardwalk, then return to CA 1 via Bay Street, a bit to the west. After the number was changed to CA 100 after '64, the DOH proposed a simpler loop -- down Ocean, over Broadway, and back up Bay, since they didn't want to deal with the streets along the boardwalk where active SP tracks featured daily street-running freights to the Davenport cement plant up the coast. The city and DOH could never reach a final agreement, so the only active portion of CA 100 to exist were the Ocean Street ramps from the 1/17 interchange; back circa 1969, while on a visit to the area, I actually saw a CA 100 milepost at the end of the SB offramp (don't remember the precise mileage figure, but it reflected the fact that the ramp was about a quarter-mile long). It was definitely gone by the early 1990's. But given that Caltrans is presently more into shedding city streets than adding them to its inventory, CA 100 -- probably still on the books to placate local interests -- will never see the light of day.