I suspect “high speed motorway driving” is faster than ”low speed town driving”.
It's not a myth, it's the ACPO/NPCC guidelines."72 mph indicated"? Is this in the highway code?
There is a myth, propagated by social media that you can add 10% to the national speed limit and not get booked. It remains a myth.
Keeping up with UK traffic on a free flowing motorway these days means travelling between 75 and 85 mph!