Following from the Tower Bridge thread, can anybody explain to me the
logic that "unsigned roads are subject to a 30mph limit" ?

I sort of remember this from when I took my driving test some 20+
years ago but thought it was stupid (and ambiguous). However, I keep
reading reports of people have been prosecuted for allegedly going too
fast where no speed limit was posted.

How can this be ? As far as I am concerned, whenever I see a speed
limit sign facing me, that speed limit is in force until I see another
one (lower or higher than it), right ?
(Obviously, some signs get vandalised and obscured)

but what on earth constitutes an "unsigned road" ? How far does one
have to travel on a road without seeing a sign, before declaring it
"unsigned" ? I seem to remember that this "default 30mph limit" only
applies (a) in urban areas, (b) where the street lamps are 185m
apart.

If I see a 50mph sign, travel for several km at say 47mph, and
an urban area (discuss definition) shows up, does that mean I have
to slow to 30mph. How do I know when I can speed up again ?

This seems a very poor definition.

Is it something like the NSL marker where, (for an ordinary car)
wherein the speed limit automatically changes 60/70 as the road becomes
single/dual carriageway ?

Richard [in PE12]