Safe Speed Forums

buggrit ... I had to retype my post as I fouled it up somehow and forgot my last point, which was "of course, collision prevention is worth much more than protection".

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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.

Mosis you're starting to sound like a paranoid over protective obsessive mother figure.

You seem to be basing the entire speed arguement on the assumption that a collision is going to occur. In that case, yes, the energy must be dissipated.

What turns your arguement into total pigswill however, is your mistaken opinion (note opinion, not fact) that sneaking over the legal speed limit automatically equates to greatly increased danger.

You know I bought my first car at 21. I actually paid for it when I was 20, but waited a few weeks for my 21st birthday. My insurance literally halved overnight, hence my wait.

What I never understood was why I was suddenly half the liability one day compared to the day before. Similarly, why is someone driving at an indicated 70mph a decent, child loving citizen (such as yourself) yet someone driving at an indicated 71mph is a dangerous idiot with a total disregard for safety and infact life?

Like others, I am fast running out of patience with you, your personal attacks and your general tendancy to talk out your backside.

I do not know you, but if I were to meet you I strongly suspect I wouldn't like you. I get the impression that you will insult us until you either leave or are banned then you will run off to the likes of Transport 1650 crying about our lack of fairness in dealing with our critics.

I wouldn't be too quick to speak on behalf of the public either. I suspect that "the public" would pay to line up and slap you if you ever got your way with scameras.

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this just gets better

can't even get the maths right!

from wikipedia

so the maths being wrong actually understates the issue, as the kinetic energy to be scrubbed off varies with the square of the speed.

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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.

OK I was going to let this one go but since it's got this far...

...yes, it's true, the KE goes up with the square of the speed.

Let's stop to think about what that actually mans in practice.

If we consider a 40 tonne truck doing 56MPH along the motorway (as the law allows) and work out it's KE (I won't bore anyone with the maths) we find that it has about the same amount as a 1 tonne hatckback travelling at about 300MPH.

If you want to base speed limits on KE alone, do you feel we should lower the speed limit for HGVs or increase the speed limit for cars?

(Honestly, it is NOT just a matter of KE - it's MUCH more complicated than that. If it was, how would you explain that many sports cars can stop in significantly shorter distances than their saloon counterparts when travelling at the same speed and loaded to the same weight?)

This might be irrelevant anyway. It follows that if you were lucky enough to scrub off all your kinetic energy, you didn’t crash.

The significant issue here is the transfer of destructive energy (non-elastic/deformation/crushing). This is inherent if one object contacts and propels another at the same resulting speed. When striking something relatively light, like a pedestrian, this destructive energy is proportional to velocity – not V^2.

Regardless, prevention is better than damage limitation

alternatively, the KE you didn't scrub off is what you lose in the collision (albeit not with a pedestrian as the likelihood is that the collision will not result in a total stop).

wrt to the sports car question ... if a sports car was loaded with the same total weight as a larger passenger vehicle (my wife's Kia Sedona, for example, which is something like 2 tonnes before the brood get in) INCLUDING HE WEIGHT OF THE CAR then I would not expect the brakes to scrub off the speed as quckly as they do when it is loaded correctly. Weight has an awful lot to do with it, as does brake capability, including anti-fade factors such as vented or carbon discs, and the number and size of the pads.

I'm not suggesting that we limit speeds based on kinteic energy. Although it would be interesting to see what limits would be if they were based on the ability to scrub off that KE / velocity - trucks AIUI have pretty good braking capability, little 1.1 runabouts with drums all round are pretty poor by comparison?



on this we all agree!

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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.

I can assure you that the sports car loaded up to the weight of the MPV WOULD be very likely to stop rather better - even though that's an unfair test. The scenario I mentioned was a saloon of modest weight - so that it would be of similar weight to the sports car. The intention was to illustrate that there is a lot more to stopping distances than the kinetic energy of the moving body.

There are a number of reasons for this - not least better brake technology but, almost as important, more expensive tyres and a lower centre of gravity. Forget about carbon discs - that's F1 stuff and they wouldn't work very well on a road car in any case. They need to be kept hot and they tend not to have to come to a complete stop in their line of work! The third (and most important factor) is the equation we have been debating -1/2 MV^2 in other words, the mass isn't the most significant bit - it's only "1/2M" whereas the velocity is squared.

Finally, when did you last see a little 1.1 runabout with drums on the front? Early 1980s maybe?

It's interesting to note that the Highway Code seems to suggest that everything requires the same distance to stop from a given speed. Sports cars, hatchbacks, saloons, lorries, you name it! Could this have been produced by the same competent authorities that dreamt up the current speed limits?

If that was all there was to it, you'd have no argument from me - or from anyone else, for that matter.
But the fact that a car at a higher speed will take more distance to stop from the same point than a slower one is almost totally irrelevant, simply because they're not going to be at the same point at exactly the same time as the necessity arises to brake.
A hypothetical situation often quoted in established research papers is the one where you have two cars, one at a higher speed than the other, alongside each other when a pedestrian steps into the road, and because the faster car takes longer to stop it will hit the pedestrian whereas the slower car will stop in time. It all makes sense, until you consider that the probability of two cars at different speeds (the one passing the other) being exactly alongside each other at the precise moment when a pedestrian steps into the road at such a distance ahead that the slower car will just be able to stop but the faster one won't, is vanishingly small.
If you're travelling at any particular speed and something happens in front of you, you may be able to stop or you may not - depending on the distance you are away - which in turn depends on many factors beyond your control, like what time you started your journey etc. etc.
Now it's easy to say that if you were travelling slower then you would have had more time to stop. But, had you been travelling slower you would have been nowhere near that point at the time that something happened. And if you had been travelling a bit faster you would have been long past the same point at the time that that something happened.
Furthermore, if you need to brake hard to avoid something, it in all probability means that you haven't been paying sufficient attention to the road ahead. to put things into perspective - it takes you around 20 metres to stop from 30mph (including 'thinking' time) and around 32 metres from 40mph. This sounds like a considerable distance, doesn't it? But how far can you really see ahead? When you're driving along a road, how many streetlights can you see ahead of you? One? two? three? more? Streetlights are usually spaced about 60 metres apart - so if you can see three of them then you can see 180 metres ahead. So if you don't see something until the last 30 metres or so then you probably ain't looking. And if there are parked cars and things so you might not see something until the last moment, well 100+ metres is plenty of space to slow down to a crawl with no more than gentle braking.

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

Pete317. Can I just say I think you put that very well.

A collision is two objects coming together at a given moment in time; the influencing factors are much wider than their relevant speeds.

If you aren't careful you start wondering whether it is fate. 'Had you not been on that road on that day you wouldn't have had that accident'; sort of thing.

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Shooting is good for you and too good for some people.

we agree on something! We agree on the fact that collisions are thankfully rare, and that avoidance is preferable to protection, however in any system the failsafes have a break point and at that point the requirement is to ensure that the "break" causes the least possible harm.


right on cue ...
I should have extended that to "or the environment was different, or the faster car would have been long gone etc.

I agree with many of your points, but they are confusing the issue. It's almost like saying "I wouldn't have got that speeding ticket if I had made my journey before they put the speed camera up". The environment around the collision can stretch as far back as the childhood environment of the drivers and passengers if you really want reductio ad absurdum. The immediate environment of a collision is far more important.


fair point, I have to confess that I haven't looked closely at a 1.1 runabout since the eighties! Not my kind of thing really.

The comparison with a family saloon car is an interesting one, whereas the sports car is likely to have no assist or a smaller brake assist capability than the family saloon (due to weight) and possibly even no ABS, there are some losses and gains on both sides. The sports car probably would stop in a shorter distance in the dry, perhaps not so in the wet. It wouldn't be a huge difference though.


yes, there is the thinking time and action time for the brake press, but bringing a moving body to rest is ALL about converting the kinetic energy of the body to something else - usually heat in the discs.

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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.

Handy, I have a slightly different 'take' on this issue.

I can find no evidence that free travelling speed has ANY influence on impact speed within the range of free travelling speeds selected by responsible drivers.

Impact speed is determined by 'degree of driver error' and is on average less than 10% of free travelling speed. One simple illustration of this is to consider the relationship between near misses and crashes. Near misses outnumber crashes by at least 10:1 (for {some definition of near miss} so one should immediately conclude that the average impact speed of an incident is less than 10% of free travelling speed. But it doesn't stop there (and some folk find the example difficult), this effect is present across the board and also appears in Heath and Safety thinking as 'Heinrich's Accident Triangle'. Here's how I described it before I had ever heard of Heinrich: http://www.safespeed.org.uk/ten.html

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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

huh?


(my bold)

I can't see how you make the link between near miss : crash ratio to the travelling speed : impact speed ratio? Can you explain with figures please? Before you say read the link, I have, and it doesn't explain it.

Also, on the link you say the following:

2 Far too little time is available to avoid the impact. 300,000 slightly injured annually in the UK

1 We brake, but far too late and crash hard. 35,000 seriously injured annually in the UK

0 Immediate crash at significant speed 3,400 fatal accidents annually in the UK

There is no information or facts to say that the 3400 deaths were caused only by zero braking, I suspect (in fact I know) that braking for several seconds prior to an impact has killed people.

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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.

Imagine 10 vehicles travelling into a risky situation at 30mph. 9 avoid crashing. 1 crashes. Total impact speed 30mph average impact speed of the ten 3mph.

If this general pattern existed in isolation it would be easy to dismiss, but it's absolutely everywhere (which is why I referred to Heinrich).


No - you have taken it too literally. It's the pattern that needs to be considered. The log scale of severity cannot be created with a physics model - you have to have a psychological model instead.

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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

ah ha, the obfuscation of averages.

Using the same logic if someone wanted a baby in a hurry they would get 9 women pregnant and have one in a month.

The real average in the scenario you quote would be 30mph, as each vehicle is at 30mph, multiply by 10 vehicles = 300, 10% of the vehicles crashed hence 30mph collision.

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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.

I really don't agree. The nine didn't impact. That's no impact at all and no impact speed at all. But it doesn't matter if you are unhappy with the illustration - because the pattern is everywhere.

And I'm sure that you wouldn't be happy if we had the ten crashes at 30mph. It really is important fact that most crashes are avoided. And the behaviour that avoids also mitigates in other circumstances. Anyway you phrase it or inperpret it the good part is the same - mostly we avoid crashing altogether. But there is a risk that this avoidance is fragile and vulnerable. See: http://www.safespeed.org.uk/tiger.html

And of course in the real world we don't have nice round numbers in nice clean compartments. We have many different behaviours and many different circumstances smoothd by 'statistics' into likely outcomes.

And the outcomes reflect the exact same pattern. Not just in road safety but in all manner of accidents.

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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 don't follow this logic. If the object is to calculate average impact speed of vehicles that crash, you have to exclude the vehicles that don't crash from the population. No conclusion can be drawn from the average impact speed of vehicles that haven't crashed because they haven't crashed.

The way I understand it is that it demonstrates how the "physics model" beloved by camera supporters doesn't work.

If we consider the example scenario in terms of physics then we have a system where we feed in ten cars all travelling at 30mph and the result is one 30mph impact and 9 impacts of zero mph. If the "zero mph impact" doesn't sit well then imagine it as a situation where 9 cars brake and just come to a standstill as they contact the obstruction, where the tenth just ploughs on at 30mph.

So in terms of physics we have a 30mph input and an average 3mph impact. Plainly this is ludicrous, but the whole point is that it can only be explained by factoring in the driver response, which is back to psychology again!

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

Hmm, yeahhhh. We're getting mighty close to revisiting the protracted discussion that Paul and I had some time back here; http://www.safespeed.org.uk/forum/viewt ... sc&start=0

I reckon I was making the approximately the same point as you Observer; including all the vehicles that didn't crash could extend to each and every vehicle each and every second of the day.

No matter what your views on cameras, I don't really see how there is any getting away from the fact that, all other things being equal, a faster travelling speed increases the likelihood of a collision, and increases the severity of a collison should it occur.

Our visibility should always be greater than our stopping distance (isn't this the definition of 'safespeed'?). But should something unexpected happen, say a car pulls suddenly out of a side road, you have less time to react. And the car is arguably more likely to pull out if you are travelling faster, as it may underestimate the safe distance needed.

I agree with handy that the 'time and place' arguments completely fudge the issue. To reductio ad absurdum in extremis, if your father hadn't impregnated your mother at the exact time he did, probably down to the millisecond, you would be a different person, and probably wouldn't be sitting there reading this forum. You may avoid an individual potential incident, by being in another place and time, but overall (and again, all other things being equal) you are driving at elevated risk if you speed.

I would agree that excessive focus on the single issue of speed at the expense of other issues can be detrimental, but I think engaging in all these mental gymnastics you sometimes appear to be suggesting speed is no issue at all, which only gives cannon fodder to your opponents.