Cycle campaigning

Yeah, I don't know--the Traffic Signs Manual only says this:

8.1.2. Over time, signs gradually become faded and their retroreflective properties diminish. This will reduce both conspicuity and legibility by day and by night. Signs also become less effective when they are dirty, damaged or displaced as a result of vehicle impact or vandalism. Damaged or dirty signs also discredit the traffic authority and lessen road users’ respect for the signs. A periodic inspection of signs should be made to ensure their early repair and/or replacement when necessary. After-dark inspections should be made of lit and reflectorised signs, or a retroreflectometer may be used to measure the latter

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/771873/traffic-signs-manual-chapter-1.pdf

I'm sure there are clearer rules somewhere, but it's not something I've ever looked into.

What is clear is that, as in the case of the Blackheath filter at South Row, if signs/planters etc. keep getting vandalised, the local authority will most likely tire of having to enforce them.