My personal view is that speeds of 120mph are unnecessary on any public roads.
The simple stopping distance at this speed is almost 200 yards. That makes no allowance for reaction time which for a simple stimulus like a light coming on is 200ms. This increases to a minimum of .5 seconds for the more complex stimulus of a road situation.
These figures are for an experienced grand prix driver in his prime. Half a second at 120mph equates to about 30 yards. That's a stopping distance of 230 yards total.
Thus these figures are all the very best possible for a perfect vehicle on a road with perfect grip and a driver with the best human reaction time possible. ABS does not improve these distances for a skilled driver.
In practise given all these figures are likely to be around 30% greater than this, we have a practical stopping distance of 300 yards.
Let's say the route was from the south London suburbs to a village in Dorset - we need a route with totally clear vision for 300 yards for most of it, and a minimum of 80 yards for the slow (60-70mph) section. Any traffic lights, hold ups or even sharp road bends will necessitate longer and longer periods at higher speeds.
To average 70mph door to door we must pass the starting point at 70mph and at no point in the journey go slower than 70mph until stopping dead at the destination. A practical stopping distance at this speed is 100 yards. To average this speed safely therefore, there would have to be no part of the route where you could not see with absolute clarity for a minimum of 100 yards. No parked cars, hidden driveways or obstructions of any kind.
If speed is lowered in suburbs, before pedestrian crossing, close to parked cars, to pass cyclists, then the speeds on other parts of the trip will need to be drastically higher.
If Flecc says he is safe at these speeds I'll take his word for it. But I do wonder what is the purpose of making journeys at such speeds. What is gained? A few minutes? Half an hour? Why? But especially, is there no increased risk to other road users from these speeds? There would appear to be no safety margins in this style of driving unless you are utterly infallible and nothing unexpected ever occurs. I stand to be corrected, of course.
When Harold MacMillan was asked what he most feared in politics, he answered, events, dear boy, events. Lacking FDlecc's self-confidence in his driving I am sad to say I concur with MacMillan.