Safe Speed Forums

I extracted this from the "Who is at fault / overtaking" thread



to apply a bit of analysis to this, something that has been circulating in my brain recently:

Safespeed says that 'safety cannot be measure in miles per hour'.

Safespeed says that 'there is no link between routine speeding and crash involvment'

Taking these two comments, then, it must be true to say that the likelihood of a particular driver having a crash is the same whether they are travelling under, at or over the posted legal speed limit? (I'm not saying I necessarily agree with this, it's a deduction from the Safespeed messages).

So Driver A travels for 2 miles in a 30mph zone at 27mph, then a further 2 miles at 30mph exactly, then a further 2 miles at 33mph. All other things being equal - the same quality of road surface, amount of other traffic, conditions etc. (please don't get bogged down with driver a being more or less alert, or the possibility of having 6 miles of the same conditions, this is a hypothesis).

If they have a crash in miles 0-2, the amount of momentum of the vehicle immediately prior to the point of impact, which is directly proportional to the velocity, is 18.18% lower than the momentum of the vehicle if the same crash happened in miles 4.1-6. The kinetic energy, being related to the square of the velocity is actually 33.05% lower in the first 2 miles than the last 2 miles.

So these two hypothetical crashes have nearly a one third difference in the amount of energy to be dissipated. Note that I have used the higher figure for my calculations, if I based my percentages on the lower speed, the kinetic energy of the system in the last 2 miles would be 50% higher. everything else remaining the same, based on the safespeed position defined above. As the likelihood must be the same, which would you prefer to be hit by?

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

The simple answer is "I'd prefer not to be hit by any of them".

The argument doesn't hold up simply because it is impossible to distil speed out of an accident in this way.

It would be equally valid to point out that if car B weighs 50% more than car A then it will have 50% more momentum in any given crash.

If I buy a lighter car can I drive faster then, so long as I don't exceed some notional "momentum limit"?

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CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

And in any case, you are describing free-travelling speed, not impact speed. It is rare indeed for the two to be exactly equal - drivers notice a hazard, apply brakes, and the impact speed will be much, much lower, assuming there is an impact at all.

What you describe could be termed the "heart attack scenario" where there is no possibility of mitigating action, and no amount of legislation or enforcement can prevent that.

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Keep right on to the end of the road ...

I think this is the (by now) old fallacy. Free travelling speed is not crash-impact speed.

read it again - same car, same driver.


the point is that the amount of energy to be dissipated is 33% higher. The same car, same driver, hence same ability to brake to reduce impact speed, same deceleration, the same crash occurring at 3 different speeds. One will cause more damage than the other.

Of course the accident shouldn't happen or be avoided, but in this hypotheses the accident, the collision, is a given.

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

nope, I am describing the amount of kinetic energy in the system. Energy cannot be created or destroyed, etc. so it muyst go somewhere.

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

Explain how it is a fallacy. Immediately prior to the collision the kinetic enegy is higher in one than the other. FACT. The car is identical in both scenarios, as is the driver, so the total braking capability is the same.

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

Your scenario depends on driverless cars. Since we have no such thing it isn't realistic.

In the real world only (perhaps) one in ten incidents results in a crash. So the average impact speed in an incident is likely to be about 10% of the average speed of free travel. This average impact speed is far more vulnerable to 'failures to avoid' than it is to minor changes in free travelling speed.

Then there's this. DfT says:

a) around 60% of folk are speeding at sample sites
b) around 12% of crashes have speeding OR inappropriate speed as a contributory factor. The speeding percentage is highly likely to be under 4%.

So how is it that speeding is more prevalent than excessive speed in crashes? 'Speeding' must be safer than 'not speeding' right? And it is - but only because we slow down in areas of danger. We don't speed where there are hazards likely to cause a crash. Speeding is most common on open roads, well away from any hazard. So a) and b) above actually belong to different 'filtered' populations.

If you need more, I've got lots.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I realise that. My point was that an equally valid hypothesis is to keep the speed & driver as a "given" and vary the mass of the vehicle.

I did this to illustrate that neither idea makes any sense. You have to consider the real world.

More importantly, neither idea has any real practical validity. If you were to say that I ought to drive x mph slower so that my crash momentum is lower, it would be perfectly valid for me to counter that by pointing out that my car is y pounds lighter than yours, so my overall momentum is already lower - who is the danger then?

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CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

In that situation, where a free travelling speed crash is a given no matter what you do, the only logical solution is to not drive forwards at all. At 2mph the impact will be 4x worse than 1mph, etc...

reductio ad absurdum.

the crash is a given because the hypotheses says it is, it is a given at all of the speeds as safespeed says (or said, before the last post) that there is no relation between speeding and crashes. Now, of course, he appears to be saying that people who speed are less likely to crash.

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

Nah. I was pointing out an apparent anomaly in the DfT's own stats.

But I will say this: The faster the speed of traffic the fewer crashes we get.

[edited for spelling]

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

how so? The whole system is fixed apart from the speed. in your terms, the speed should not cause any change in likelihood of an accident.


on in ten incidents result in a collision so average speed is 10% of the travelling speed? That is perhaps the worst piece of flawed logic I have ever seen.



I am not talking about statistics am I? Energy in a collision - one collision, at a potential 3 different velocities. As you say, in the real world there are hundred, probably thousands of factors involved and speed is only one of them. It is, however, one which the operator of the system is in control of, unless they are a crap driver and can't find A) their own arse with both hands and a map and B) those 2 big switches ('pedals' to those of us in the know) that have a speedy up and a slowy down effect (again, to the specialists these are sometimes called accelerator and brake).

And I'm far too polite to say that not only do you have lots, you seem to be full of them

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

Look at it this way: How are you going to force the driver to change speed?

Is it possible to make him change speed without changing anything else?

Are you going to deny him the right to adapt his speed and position to suit the environment? And if you are, what are the side effects?

As soon as we put your scenario in the real world it fails the basic test of realism.

Anyway, road safety isn't a physics subject. It's a psychology subject. You'll never get this out of a physics model.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

the energy of the impact is definitely physics - the energy of the impact defining the amount of damage we might expect to see.



(my bold)

Congratulations on being elevated to the monarchy ... or is that commiserations on your multiple personality disorder?

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

So how does that help?



Stick to the arguments. Serf.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

handy is absolutolely correct to say that the faster the crash happens the more energy will be dissipated because it's simple physics, but the reason the hypothesis fails is because it does not consider whether the speed changes to probability of a crash occuring at all. He has just taken one small piece if data out of context and attempted to extrapolate the result so there is no relationship to what happens in the real world.

Whether a crash occurs is affected by many more things than just speed, as of cousre most of round here already know!

isn't it obvious? the damage caused by a lower speed collision is less than the damage caused by a higher speed collision. The human operator essentially controls 2 things in the car - one of which is direction, the other is the speed. So by choosing to control one and reduce it by only a tiny few percentage points, the operator is able to reduce damage potential in the case of a collision.




[tugs forelock]
I will if you will and your imaginary friends do too
[/tugs forelock]

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

It's obvious that it isn't reality. (And if it was reality, I certainly wouldn't be here.)

Try this:

Government message regarding pedestrian impact:

at 20mph 2.5% - 10% die
at 30mph 20% to 50% die
at 40mph 80% - 95% die

I agree the nominal figures.

But in the real world, in 2004, the following figures apply to child pedestrians in built up areas (30 and 40mph limits):

injured: 11,999
killed: 58
percentage killed = 0.4%

What does that tell you about the average impact speed?

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Okay...


And if they don't have a crash, what then? What speed they were driving at and whether it was above, at or below the limit becomes irrelevant. To put it another way, in your scenario what are the relative chances of a collision actually occuring in miles 0-2, miles 2-4 and miles 4-6? What do your calculations for kinetic energy tell us about the risk of a collision at each of the three speeds?

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Make everything as simple as possible, but not simpler - Einstein