The ones people need to worry about will drive at more than 20mph whether or not we may or may not. Putting 20mph signs on roads just increases conflict where it is least needed.... and so on.
This all sounds to me very like entitled drivers' rants.
1. Sadly, that is actually a government-accepted principle and why we cannot get lower speed limits on roads where the average motorist speed is too high without some other measures to reduce that speed.
2. The difference is that speed limits are regulatory signs which (in theory, at least) are enforceable and only imposed after publishing for consultation a justification that can be challenged. This is another way that "cyclists dismount" being information signs is an ugly dodge: they're unenforceable and used willy-nilly in illogical ways.
In the example I showed I'm happy to dismount as it feels safer for everyone.
I should think so because, unless something changed since I left Somerset, you've already passed a "no cycling" regulatory sign to reach that point!
Does it really feel safer? The oncoming dismounted rider completely blocks the full bridge width, so I suspect most people wait because it doesn't feel safer to climb over them! Dismounting seems mainly to bully most oncoming walkers into waiting. Some people are happy to bully but I'm not.
That bridge deck really needs widening or bypassing, but it's in the area of a council that talks a good game but is pretty terrible at cycling (again, unless something changed) and tells disabled cyclists to ride an extra half km to SCAT and Tesco and back instead. The desire is clearly there because otherwise, no-one would put up a wordy non-standard probably-unlawful dismount sign there.