W K wrote:
"PeterE" wrote in message

But what I've argued is that it isn't a steady linear or geometric
progression, it's more in the nature of a Z-curve, where risk rises
very rapidly between 0 mph and speeds where collisions may prove
lethal, then flattens out over a range of "safe speeds" where it is
possible to stop within one's field of vision, and then rises
rapidly once the maximum safe speed is exceeded.

I don't think that middle part is completely flat.
Because of:
a) unusual, unexpected, or bizarre events where an
object/person/vehicle can arrive in one's field of vision, causing a
collision.
b) the effects of anything unusual at the various speeds (ie less
risk of injury if the unexpected becomes an accident, easier to get
out of difficulties).
c) An effect of statistical mush - if you overlay similar but slightly
different conditions into one definitions, you have to overlay and
average out those Z- curves until you get something with more curved
boundaries. ie - road X on a clear sunny day might have the Z point
at 40 mph - yet various other factors might shift that between 30 and
50 for different cars or subtly different circumstances.

It's not completely flat, but over a range of speeds (which obviously does
vary according to the conditions) it rises only very gradually.

--
http://www.speedlimit.org.uk
"If laws are to be respected, they must be worthy of respect."