f2k,
There is no "below 140Kt/1800m rule". That is it. It does not exist.
The 140 Kt speed is a division between two limits for VMC. As BEagle says - read the book.
The 1800m visibility requirment applies to IMC holders and is the minimum visibility that they need to take-off and land.
the two are in no way related or connected.
Regards,
DFC