You say that but people will happily drive at 50+ down a narrow country road. I think the "narrow = slower" only works for a limited period of time before people get normalised to it.
The country roads one has always dumbfounded me though - why some of those have national speed limits I will never know.
The thought of doing this now scares me and I don't do this and suggest that no one else does either. But I know many people still drive like this.
Why not? Even roads with lower speed limits you're required to drive at a speed appropriate for the road, the conditions, and your vehicle; the speed limit merely sets an upper-bound, and it's not really relevant whether it's achievable. Just look at the Isle of Man where there is no national speed limits: most roads outside of towns have no speed limit, regardless of whether they're a narrow single-lane road or one of the largest roads on the island.