Ah yes... I was suffering from a units bypass! That would be 40m/s^2
which (near enough) is indeed 4g
v = u + at
0 = 40 + a * 1
-40 = a
(Which for a crash from 80 does not sound like much really!)
From 80 to rest in say 1m may be more realistic... Time taken for
arrest would be time to cover 1m at avg speed (20 m/s) or 50ms
0 = 40 + a * 50e-3
40 / 50e-3 = 800 m/s^2 or 80g
That sounds a bit more like the sort of "oops I have just powdered my
sturnum!" kind of force required ;-)
Vastly different guesstimates, aren't they! You're right to point out
that taking a whole second to decellerate from 40m/s to 0 would require
you to cover a non-trivial distance - 20m (vee-squared = 1600 2.40.20). Somewhere between our two oversimplified, uniform-acceleration
estimates lies what would actually happen in a crash at high speed: the
crumple zone will indeed spread out the body's decelaration over a
greater distance and time than the 1m you mention, but there'll be a far
from uniform decelleration both over time and for different parts of the
body - heads being heavy and less well restrained than torsos and legs.
25G is actually sustainable for over a minute, conscious.
If prone, and the force is pushing you down on your back.
Bloody nasty, but survivable.
(not exactly relevant to car crashes, as the delta-v rarely approaches
Yes, the point being that a head flying about puts peak G > 200 on
certain parts of your anatomy.
My whole thesis is that really, no one inside a car needs to die in just
about any crash they could possibly have up to about 150mph.
The fact that they do is car design and driver habit.
This doesn't solve the pedestrian/cyclist/motorcycle problem however.
4g is nothing. Not even bruising.
Work out what g it takes to stip a car from 30mph in the space of its
crumple zone. That is a very survivable accident.
I've seen a racing mini go from 100mph to 0 in the space of 10 feet of
tyrewall. The driver got out and walked away.
Michael Schumacher only broke his legs hitting a tyrewall at 130mph
estimated. Decelerative distance about 20 feet max. Modern F1 cars would
not even cause that injury anymore.
Conversely I have seen a woman killed by being hit at 20mph. She landed
on her head and broke her neck. A fall of less than 8 feet onto the road
is actually what KILLED her after the car had tossed her up over its bonnet.
Perhaps we should build houses that do not hacve upper floors in case
people fall out of them.
And add disabled ramps to all the cliffs in britain, to stop people
pushing their wheelchairs over them.
I'd rather reserve that for those who actually get involved in accidents.
Now the last person in my experience who wrote off a car, was my dear
old mum, who drove into the back of a stationary trailer stopped in the
middle of the road, on a bright, clear, dry, day. with at least 100
yards visibility doing no more than her customary 25mph.
She continiued to drive for 4 years after that, until an accident at
home put a stop to it. She still insists she is perfectly safe to drive,
despite the fact that she can't remember my name half the time, and has
no clear idea of what town she currently lives in.
The lads in the BMW's are only half the problem. In my list of accidents
I have witnessed/been involved in, only one of many were in that
category..Sunday drivers with full family on board having a row...2.
Old lady stepping on front of car doing under speed limit ..1
Old boy dithering and doddering at a pedestrian crossing, and completely
confusing the cars behind before finally deciding to slam on the brakes
and stop in the MIDDLE of the crossing...1
Poor road design leading to unforeseen blind spots...2
White van man with no rear three quarter vision pulling out in
front...1..almost killed me that one did. Poor van design? They have
better mirrors these days. I don't hold it against him.
Temporary road surface, no markings, and in shocking condition ....3
No. Most accidents are caused by drivers behaving in what they think are
perfectly reasonable (nad indeeed perfectly legal) ways, but being
totally unaware of how unreasonable they really are. And that's an issue
for the very inexeperienced, and the very old.
yup. 5-6g is where you start to black out/red out if persisted with..
but in a pressure suit up to 10g is doable for minutes on end...10-50g
will black you out pretty much instantly, but won't kill you but make
bruise..50-200g is more or less bruising, including internal bruising,
loss of consciusness, but may not be permanent damage. Over 200 things
start getting seriuosly damaged, and beyond about 500g death is probably
So up to 5g not a lot happens at all...5-20g will mess up blood flow,
but not do any permanent damage..20g-50g is bruises...50-200g is severe
and possibly dangerous bruising, but no broken bones...200-500g is lots
of bad bruising and broken bones, and over 500 is dead.
Using v^2*s where v is velocity, s is distance, and a is acceleration,
its instructive to see what a 30mph crunch into a totally immovable
object in a car with a progressive 4 foot of crumple zone actually does
to the passenger cell..I make it a piffling 7.5g. I.e. totally
survivable if the passngeer cell remains intact and you don't get
whiplash or bash your head on anything and are wearing a belt.
100 mph to nothing in 4 feet is 83g. Shaken up, but if well belted in, a
walk away job.
200 mph to nothing in 4 feet - that's a high speed crash into a concrete
wall at indianapolis - is about 350g. No one has walked away from one of
those, but quite a few have survived with smashed legs etc.
Now out of sheer morbid curiousiy, let's look at the acceleration of a
brain that say stops in an inch when falling from a height of 6 feet...
The ratio of the deceleration to the acceleration, assuming terminal
velocity is approached, is the ratio of the two distances - 72:1 = 72g
So in fact a fall from 6 foot onto a fairly hard surface - i.e. tripping
over backwards on a carpet over concrete, IS JUST A ALMOST AS DANGEROUS
AS BEING WELL STRAPPED IN A CAR WITH A DECENT PASSENGER CELL STOPPING
FROM 100mph IN JUST 4 FEET!!!!
If all motorists drove cars equipped as racing and rally cars are, with
safety cages, full harness strapped up tight, and crash helmets, and
equipped with crumple zones, thats the sort of *walkaway* accident you
could have. Head on at 100mph into a concrete bridge pillar.
Now where pedestrians are concerned, as I have said earlier, and out of
personal experience, a fall onto tarmac from 8 feet on yer head will
kill someone. Its the hardness of the tarmac that does it. Or a
particularly unfortunate angle that breaks a neck...
The energy to raise eg. a 10 stone (140lb) person 8 feet is a 1120 pound
feet or 36,000 foot poundals. To guarantee less kinetic energy than that
in a one ton car (a small one) means a speed of 4 feet per second, or
about 3 mph....
What this means that in places where pedestrians don't exist, head on
accidents of 100 mph - either one car into concrete - or two 50mph cars
head on, both built to best safety standards with passengers well
contained, are simply walkaway shocked and shaking, but not injured,
But where pedestrians are concerned, there is no safe speed whatseover
above a 3mph crawl. And arguably even that would not someone tripping
and falling under the wheels. Or falling flat on their backs onto concrete.
The figures show just how important it is to have a crash helmet on, by
the way - even an inch of padding that crushes, will _hugely_ reduce
peak acceleration on the skull, and neck supports that prevent spinal
dislocation, are also massively useful - all F1 drivers wear them.
The answers are staring you in the face. Where
pedestrians/motorcyclists/cyclists are not, speed limits of 60mph on
single, and 120mph on dual, are probably about right, if you expect
these sorts of collisions.
This leaves entirely out of the equation whether such accidents are
likely at all, more likely travelling fast, or in fact less likely
travelling fast. IME a fast collison is a long and slow one to happen -
cars go all over the place, rolling and tumbling...but do not come to a
halt quickly. Whereas 30mph into a lamp post is pretty hard stuff.
And pedestrians, and cyclists should be totally and completely separated
from traffic. There is *no safe speed in residential below which
survival is guaranteed*.
Motorcycles and horses which have no safety cells are inherently
dangerous, and anyone who rides one should do so at their own risk. You
can kill yourself just falling off a horse onto concrete or tarmac with
no crash helmet.
The 2 situations quoted are very different.
At 2x the speed a car has 4 times the kinetic energy.
OTOH a head-on crash would not be very different to a collision with a
stationary and unyielding object at the same speed.
On reflection you are right, the head on crash at 50 is a lesser event
in terms of loss of energy, being equivalent to two cars simultaneously
driving into opposite sides of a bridge at 50.
Well that is even less argument in favour of low speed limits.
Since we can see that at similar weights, a head on collision between
two cars at 100mph each is the same as hitting a solid wall at 100mph...
All you are stying is there is no such thing as a herad on crash..its
alwsy some kind of glancing blow.
When saw the mini hit a wall at 100mph, the thing actually rose up on
its rearwheels about 6 feet. So the deceleration of the driver was a lot
less than that of the front bumper, even without teh crimpling effects
of the engine area.
As I said, the driver walked away. The same could not be said of the
The problem with all of these calculations, while mathematically correct,
they don't take account of the tremendous kinetic energy of the event and
for foreign objects, shapes of cars, etc. Unfortunately, actual crashes
aren't uniform and thing protrude into cabin space, hit at funny angles,
snap things like necks that weren't braced for impact, etc. etc.
In a racing car, you're strapped hard into the correct position and usually
braced for impact. However, at 50mph, a stray wheel could hit you in the
head and decapitate you, long after your team-mate has walked away from a
220mph spin through a gravel trap and into a nicely designed tyre wall.
Chaos theory keeps the world dangerous and unusual!
Thats is not actually an issue. So teh car gets broken and hotter, as
does the wall.
That is a matter of design. Aytron senna died because trackriod punched
through his helmet, not because he ran into a wall at 150mph.
That is design. Not intrinsically irrevocable. Yes, the parcel in the
back seat can smash your skull. Put it in the boot.
Which is why they are tied on these days.
All I am saying is that with good car design, crashes can be made
infinitely more surviveable for the passengers. We have seen huge
strides amde with soft collapsible steering columns, air bags, safety
belts, crumple zones, side impact bars and head restraints, none of
which were on the cars I learnt to drive.
Nor were disk brakes - vented discs and 4 pot calipers were not really
even invented - radial tyres, low profile tryes, ABS, traction control
A head on at 30mph often resulted in driver death and severe passneger
injuries. These days its almost a non event.
Indeed. buit statistics means that if cars are better designed with
crumple zones oustidee a tough passenger shell that is well padded, and
te passengers are bolted down, far far highre impact speeds than most
people think are een surviveable, will result in very little physical
IF we lifted speed limits on motorways, would we in fact get better
flow, reduced journey times, less trafiic as a result, less bunching and
The german experience suggest that its possible. There you can always
make up time you lose in congestion on the empty streches at 115mph,...
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