Adrian wrote:
Chris Tolley (ukonline really) gurgled happily,
sounding much like they were saying:
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.)
I do apologise - I thought it relatively clear that I was not referring
to the precise formula and the precise figures, but the ball-park figures
and the meaning they convey.
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.
Once again - why that particular formula resulting in figures in that
particular ballpark?
Because the numbers are easy to do. Before people had little computers
in their pockets, they could do v*v/20 in their heads. Some (inc me
still can). Back in the day there were many of these things. We called
them "rules of thumb". Simple approximations that were sufficiently
accurate when answers were not presented to eight or more decimal
places.
But the point is that it's all a very theoretical thing anyway. There
are going to be astonishingly few people who have the talent, in a
moving frame of reference, to place their vehicle at any precise
predetermined distance behind the one in front. The HC is for reading
before you go out on the road, not while you are driving. On the road,
other mechanisms, like the 2 second rule, experience of conditions, and
empathy for other road users, come into play.
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?
So I can only presume, then, that you have not taken a driving test? If
you lock the brakes on the emergency stop, you would fail the test - as
well as massively extending the stopping distance.
Oh dear, considering that elsewhere you accuse me of posting without
reading, you've clearly overlooked the previous post in which I told you
about my test. Early HCs talked about these in the context of an
emergency, which is the situation I imagine ABS comes into its own. The
commentary in the 1946 edition starts "DO YOU REALISE how long it takes
to pull up in an emergency?"
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.
sigh I'm not sure why you're being so obtuse about this.
I can only work with the material that is in front of me. If you want me
to stop pointing out the flaws in what you are writing, stop writing
them.
Look at the chart in the HC. For each speed there is a thinking distance
and a stopping distance.
Clearly, humankind has not evolved in the last half century, so we can
assume that the thinking distance has remained relatively static. So the
stopping distance is what we are discussing.
That would be an irrational and unwarranted assumption. I would argue
for assuming the opposite. Back when this thing was dreamed up, there
were far fewer vehicles on the roads. The emergencies envisaged might be
such things as a child rushing out in front of the car, and because
there are no other cars around, it's the one thing the driver focuses
on. The ball game is completely different when the emergency arises from
the erratic behaviour of another road user who is also moving, and one
is surrounded by a lot of other moving things too, as well as a forest
of signs and other distractions.
Either way, any discussion of stopping distances that ignores the amount
of time it takes to size up the situation and decide what to do about it
isn't going to produce a worthwhile answer.
In terms of general stopping distances, I personally drive in a manner
which means the HC understates how long it takes to slow down. I am not
one of those who dashes from place to place slamming brakes on. Indeed,
I often point out to SWMBO how many times other drivers are applying
their brakes while I am not doing so at all, since I've thought ahead
about what is in front of me. I had my previous car 6 years and never
replaced the brake pads, my current one is approaching that landmark,
and I haven't done so with that either.
--
http://gallery120232.fotopic.net/p9633098.html
(55 011 at Peterborough, 3 Sep 1979)