I'd like to contribute some food for thought on SR-259. As we all know, the green wayfinding signage on the SR-259 mainline is silent about which route you are on. The few sign that there are say "TO SR-210" or "TO SR-330". This does not indicate the route you are actually on. However, there are other markings that indicate you are on SR-259.
1. The callboxes are clearly labeled as being on 259.
2. The white identification signage beside all (most?) of the over and under crossings are clearly labeled with the route, county, and postmile.
- Highland Ave. UC is 259, SBD, 00.49 (NB dir)
- 27th Street UC is 259, SBD, 00.81 (NB dir)
- H Street UC is 259, SBD, 01.00 (SB dir)
- 259/210 Sep is 259, SBD, 1.19 (SB dir)
3. The exit numbers correspond with the SR-259 postmiles.
4. The Caltrans TASAS Sequence Listing is the official system of record for all Caltrans state highways. The Sequence Listing lists SR-259 in its entire length from PM L0.0 to PM 1.515. According to the Sequence Listing, SR-259 begins at I-215 just south of the SB offramp to Baseline. The NB direction ends at SR-210 and the SB direction ends at 30th Street.
Hopefully this is helpful.