Indeed the civil engineer who allowed such an egregious speed limit is at fault here.
In the UK most b-roads are 'national' speed limit (i.e. 60mph or 70mph). It is expected that the driver selects the appropriate speed for the conditions, visibility, road etc etc.
Of course, we're humans, which means errors are made, people are impatient, we don't balance risk very well etc etc.
(Based on my experience driving in the US, at least.)
Posted speed limits are a function of the 90th percentile speed (usually based on typical conditions).
If you set low speed limits because "think of the children" you get other (safety) problems.
[uncited opinion]