Slow down!

This sign is at the entrance to our lane.  I'm quite happy with the fact that it says private as it indicates that people shouldn't be heading up the lane without a valid reason for doing so.  I'm also quite happy that it asks drivers to drive slowly.  This is good for a couple of reasons - we have elderly neighbours who aren't quite so nimble on their feet and we have lots of 'neigh'bours who go for a canter up and down the road to and from their fields in the nearby stud.

What I am less certain of is how they settled on 12 miles per hour.  Most speedo's aren't accurate enough to separate between 10 or 15 mph (unless they are the newer digital ones), so why didn't they go for a round number?  And - before anyone points it out - I know that 12 mph converts to 20 kph, but we don't use kph so that is totally irrelevant knowledge!