Adrian wrote:
Chris Tolley (ukonline really) gurgled happily,
sounding much like they were saying:
This may come as a shock to you, but the tables never had anything
to do with real performance. They are a simple mathematical model
linking the speed in mph with the stopping distance in feet. Anyone
with GCSE maths should take no more than 2 minutes to deduce the
formula that is used..
Sure. But the formula was based on a roughly representative family
car of the period - the 105E Anglia, allegedly.
The formula is far too simple to be based on anything real.
No, but the formula would have been worked so that the resulting
stopping distances are approximately correct for "something real".
Perhaps you should offer some evidence for this contention.
Put it this way - I didn't bother to memorise the distances for my
driving test. I memorised the formula, and as I say, it links mph to
feet. The only way in which it would have any resemblance to reality is
if there were some universal driving constant whose value happens to lie
in the region of 1/5280, being the conversion factor from miles to feet.
So did this formula get plucked from thin air for a totally random result?
I see mathematics is not your strong suit. Formulas do not give totally
random results. (And no, I'm not going to enter into a debate about
chaos theory.)
Why 75m from 70mph?
Why not 200m or 20m?
The thinking distance is merely the speed in mph expressed in feet. The
stopping distance is merely the speed in mph squared and divided by 20,
then expressed in feet.
vmph v ft thinking + v*v/20 stopping
20mph --- 20 ft thinking + 20*20/20 stopping ---- 40ft
30mph --- 30 ft thinking + 30*30/20 stopping ---- 75ft
40mph --- 40 ft thinking + 40*40/20 stopping ---- 120ft
...
70mph 70 70*70/20 ---- 315ft
Your "75m from 70mph" is merely the stopping part of that - 245ft. It
should be 315ft. Manifestly, you are driving without thinking.
As for your Ford Anglia allegation, the Highway Code predates Ford
Anglias by several decades. The same figures were included in the 1946
HC, and may have been in versions before that; I can't be bothered to
look them up.
I respectfully suggest that you could have looked this up just as easily
as I did, and if you had done so you would not have gone on to make it
look quite so much as if you don't know what you are talking about.
It no doubt gives and always gave a safety margin. But until every
relevant vehicle has ABS
Which doesn't actually make the slightest difference to stopping
distances, since it does absolutely nothing at all unless the driver
cocks up in a way that would have failed them their driving test.
Never having had a car so fitted, I wouldn't know. The only evidence I
have to go by is that the continuous rubber smears on the road tend to
be longer than that dashed ones, from which I infer that ABS reduces
stopping distances.
Did you miss the "unless"?
No. I did not.
I explained that I would not know from personal experience what
difference ABS makes, and then went on to present information that I can
glean about ABS. Could you not tell that from what I wrote?
Personally, I drive according to the two second rule. That's much more
straightforward. I observe that many of my fellow road users think they
are considerably better drivers and can get away with a 0.5 second rule.
I don't think anybody's said anything to contradict that.
But if these figures purport to be a typical "stopping distance", do you
not think it might actually be useful if they were?
They purport to be a "thinking and stopping" distance. There are some
who believe that the thinking element is insufficient, and that overall
the distances quoted in the HC are reasonable as they are. For example -
here's one guy who claims to have *proved* it.
http://www.peterjackson.org.uk/Stopp...0distances.htm
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http://gallery120232.fotopic.net/p9683753.html
(142 083 at Lancaster, Mar 1991)