From a partnership survey:
This raises some questions about the safety benefits of speed limits. Clearly if someone doesn't know what the speed limit sign means, then they are unlikely to obey the limit. But it goes further. Layered on top of this are the folk who understand the signs but haven't seen them. One might even go so far as to guess that 50% or more of drievers are unaware of the speed limit in force with full confidence at any random sample site.
But when you put this in the real world you realise that there aren't anywhere near enough excessive speed crashes to spread around. For the last year on record we had 214,000 injury crashes, and latest DfT figures suggest that 12% of those probably had excessive speed as a contributory factor. That would be a bit under 26,000 crashes.
We could then compare that with the number of drivers - we have 32 million licenced drivers and we should guess that at least 2/3rds of that number are active. (i.e. they actually drive). So that would be a bit over 21 million active drivers. If half of them don't know the speed limit that would be 10.5 million.
If we subtract the 26,000 crashes from the 10.5 million who haven't got a clue what the speed limit is we still have 10.474 million who had no idea what the speed limit was yet, by some magic still didn't crash.
I say we need a bit more of that magic.