I think why on earth should the speed limit be 60 when there aren't even any lane markings and two people can barely pass each other on the road!
You mean like this?
Or this?
It can seem quite illogical when you can be driving along a fairly good two-lane road with a 40 or 50 mph speed limit, only to turn off onto a twisty little back lane and the first sign you see is one for 60 mph!
But it makes some sort of sense when you realize that the speed limit on those little roads is 60 not because somebody decided that it's a suitable speed (which it clearly isn't), but simply because no order for a lower speed limit has ever been made for the road in question. Up until mid 1960's, those same roads had
no absolute speed limit, just the general requirement to drive at a speed reasonable and prudent for the conditions (as existed in states like Nevada prior to the introduction of the national 55 limit in the U.S.).
A brief historical overviewA major overhaul of motor vehicle legislation took place in 1930, and it was at that time that the definition of a "built up" area was set as being any road on which there are street lights placed no more than 200 yards apart. The speed limit in such built-up areas was set at 30 mph, with
no overall speed limit elsewhere. That's why the sign which today means "National Speed Limit" appears the way it does, because originally it meant "End of speed limit." The definition of a built-up area and the default 30 mph limit in such areas both still exist in legislation today.
In the years which followed, other speeds such as 40 mph zones were introduced, but these were submitted and decided on a case-by-case basis, so could only be applied if a specific application for a limit on particular stretch of road was approved. That worked both ways, i.e. for applying a limit to a road which would otherwise have been unrestricted and for applying a 40 mph or higher limit to a road which is by definition in a built-up area and would thus be 30 mph by default.
The introduction of the national speed limit in the 1960's (initially 70 mph, later changing to the split 60/70 limits of today) imposed the upper limit on all roads which had previously been unrestricted, but basically left everything else as it was.
So that's why today a road can be 30 mph if built-up or 60/70 mph if not by default,
except where a specific order to change the limit has been made for a particular stretch of road. The non-default speed limits are still decided on individual merit, and in general applications only get approval if there's something more than just "60 mph would be insane on this twisty road." There needs to be something more concrete demonstrated, e.g. "This road is outside the built-up area with street lights, but there are still lots of houses and shops with many cars turning and pedestrians about, so 60 mph is unreasonable and it should be 40." Or "This might be a street-lit road through the middle of the city, but it's a major thoroughfare with no pedestrians about, capable of being driven safely at 40 to 50 mph, so the default 30 is unreasonably slow for a major traffic artery."