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I wouldn't say the signing is always very clear. In my area, for instance, we have some pretty long streets, e.g. Reighton Road E5, and the only sign at the unfiltered end is quite small and not a regular traffic sign, but one with writing on it informing of the fact that the street is filtered. (There's no up-to-date StreetView to show it.) Obviously, that's not an excuse for driving through the filter itself, but the signage is certainly inadequate for preventing driving through camera-enforced filters that are permeable for drivers.
Equally obviously, vandalising traffic signs does not change the underlying traffic order. I think highway authorities have a duty to quickly replace vandalised signs, but there may be a grey area of a short period during which traffic orders whose signs are vandalised may not be enforceable. I don't know how the law works there, though.
amey
Backstop
Oliver Schick
The biggest gripe I have with the LTNs is how poorly signalled at least some are. Dulwich for example.
Of course drivers will oppose LTNs if they get hit with a fine for driving through an LTN that they never saw.