Alistair J Murray wrote:
David Taylor wrote:

[...]

Is there no road in Britain good for 71mph?

Not sure, know of several good for 72mph though.

There is no connection between posted limits and appropriate speeds.

There is generally some correlation, except that 3-lane dual
carriageways with grade-separated junctions are usually safer than
country lanes to drive fast, yet often carry a lower speed limit (50
compared to 60).

From a mathematical point of view, let us say that we know that 150mph
is too fast and that 30mph (on a motorway in good conditions) is too
slow and that the safe range is somewhere in between. There is probably
a curve somewhere that marks how "safe" a speed is. At some point there
is a threshold below which we don't want to go, so we could say that
one particular speed where that threshold is crossed should be the
"speed limit" although the actual safest speed (the "target") is likely
to be somewhere lower (it is highly unlikely that 70mph is the safest
speed and then 71mph is unsafe).

Of course, what the actual safest speed and threshold are will be
variable, based on the conditions of the road at the time.

One day we may have the technology to have signs that can post variable
speeds based on the road conditions.

As Paul Smith wuuld say, a good driver shouldn't need them. A good
driver will be able to judge from the conditions of the road what the
safe speed is. Unfortunately, the roads are not full of good drivers
and people need guidance.

A sign displaying two speeds might be the most ideal. One would be a
posted "target" speed that is considered the optimal safe speed for the
conditions. The other would be an absolute limit, beyond which you know
you will get fined if caught.

It might be that in a certain road condition, the target speed is set
at say 65mph and the absolute limit at 80mph. Anyone doing 81mph should
not claim to be "unlucky" because they are 16mph over what has been
given as the target "safe" speed.

Of course at the moment this is all speculative as we don't have such
technology.