Safe Speed Forums

But is impact speed directly proportional to free travelling speed? I don't think it can be since free travelling speed is not the only thing that will effect it. Consider an imminent crash where the driver has been inattentive and taken half a second too long to react and hit the brakes. Now consider the same circumstances with the same free travelling speed but where the driver is even more inattentive and taken a second. Result - two different impact speeds where free travelling speed was the same. In this case impact speed relates not to free travelling speed but to the degree of inattention. That's without considering other factors that must go into making impact speed such as the type of vehicle and it's braking ability, how well maintained it is, the physical condtion of the driver and so on. There is simply too much involved to state that impact speed is proportional to any single factor in isolation.

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

Perhaps I could have worded that better.

In any particular incident, from the commencement of emergency braking (under good conditions) you lose roughly 20mph per second, which means that you'll come to a complete stop from 30mph in about 1.5 seconds.
The time between commencing braking and impact, and therefore the impact speed, depends on where the hazard is in relation to you at the point at which you commence braking. As this relationship is essentially random, so will the impact speed be - and could be anything between zero and your full travelling speed.
If you have a statistically large number of incidents, you will find that there is an even spread of impact speeds when plotted against braking time. However, as braking time is effectively 'slowed down' (it takes twice the time to cover your braking distance while braking as it does at your original speed) and the risk is proportional to 'pre-incident' time, you will find the numbers increasing toward the higher speed when plotted against pre-incident time.
BTW it took me (and Paul as well) quite a while to get my head around the effects of this 'slowing down' of time.

What attention (hazard awareness) does is to place an upper limit on the impact speed.

Consider this: except for incidents where you don't see the hazard untill less than your reaction time before impact, some braking will take place, and so the impact speed will always be less than your pre-braking travelling speed. A higher level of attention (hazard awareness) will increase the time between seeing the hazard and the impact (if any) and so increase the amount of speed you lose before impact. So your level of attention places an upper limit on the impact speed for any given pre-braking travelling speed.

Cheers
Peter

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

No, as I've just explained to Steve.

Cheers
Peter

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

I know mate, it was rhetorical. I'd say impact speed is only directly proportional to the degree to which the driver has cocked things up , and that can be due to a dozen different things.

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

Okay, yes, I understand it will be a statistical distribution, I understand that reaction time will affect the results as well as free travelling speed, but we still haven't quantified the relationships. What we don't know is the relationship between free travelling speed and reaction time, and a computer model cannot tell us that. What the computer model can tell us is how impact speeds vary with each of those factors when treated independently, and it is useful to work that out, because you can then start to analyse how the results would vary for different relationships between free travelling speed and reaction time.

So, accepting that reaction time also has an impact, are you prepared to give an answer to the question of how the probability of having an accident with an impact speed greater than x mph varies with free travelling speed, when reaction time is held constant? Clearly, the probability is zero when free travelling speed <= x (unless we consider head-on collisions). I speculate that it might then rise faster than linearly with speed, then at some point the rate of increase might slow, giving an s-curve type shape.

Or making some other mistake, such as not looking, or both.
How to make people change? Certainly not, as we're doing at the moment, by telling them that they're safe as long as they stick to the limit.



Yes, it might need to be extremely slow, depending on the likelihood of someone being there - I often go past parked cars at less than 10mph if the road is narrow so I have to pass closely and there's, for example, evidence of children playing in the vicinity. In cases like that, where there's a very high risk, one may not always be able to avoid a collision - and this is the one circumstance where I would advocate driving at snail's pace, so you don't do too much damage if the worst happens.

Cheers
Peter

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

We can start to get a picture by looking at the severity and frequency distribution of real world crashes.

I find it highly compelling to consider the average impact speed in a world where 9/10 incidents end in a near miss.

Then there are these views:
http://www.safespeed.org.uk/12mph.html
http://www.safespeed.org.uk/proof.html
http://www.safespeed.org.uk/killspeed.html
http://www.safespeed.org.uk/percentages.html
http://www.safespeed.org.uk/ten.html

Having looked every way I can think of at the real world crash stats, I can find no significant evidence of a relationship between driver selected free travelling speeds and impact speeds.

The final view concerns the overall frequency of injury crashes - with 32 million licenced drivers and 214,000 injury crashes in 2003, and assuming that each crash was caused by one driver (actually it's less than that because many crashes are caused by pedestrians) a simple division tells us that the average driver causes an injury crash once in 150 years.

What's he doing on all those days when he isn't causing an injury crash? It's those behaviours - on the non crash days - that makes road safety work.

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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

Just looking at this one for now, I have to take issue with this statement:

"So if we could limit speed to 12 mph perfectly nation-wide, but still had the same accidents with every crash having a delta v of 12 mph we'd expect the same number of car drivers to die."

You seem to have derived this from this previous conclusion:
"speed = 71 * (fourth root of) 0.000764 = 11.80 mph That's the calculated average impact speed of all GB car accidents affecting car drivers to give the right proportion of fatality. Let's call it 12 mph."

But I don't believe this is valid. That average will be composed of many accidents, some with a delta v greater than 12mph, some less. And the impact speed leading to that would need to be at least as high as the delta v, generally higher, and free travelling speed higher still. So the average free travelling speed leading to these results would be expected to be much higher than 12mph. If you set a 12mph speed limit nationwide, all the impacts above the average would be eliminated, drastically reducing mean accident delta v. If all cars never exceeded 12mph, this would be the max free travelling speed, mean impact speed would be lower than this, and mean delta v lower still.

Even if what you are saying is that it would be theoretically possible to have the same number of accidents, and for all of them to result in a delta v of 12mph, this would still produce far fewer casualties due to the concave nature of the curve.

Also, whilst it is undoubtedly useful to look at driver injuries in this way, the treatment doesn't include pedestrian injuries at all, so we shouldn't draw general road safety conclusions from the results.

Steve

I'm still working on this one.
I consider it more useful to calculate the probability of a fatal injury, for example, given various combinations of factors.
But I've not yet come across a completely satisfactory relationship between impact speed and, say, pedestrian fatality.
The Ashton and Mackay curve is not much use, as it's a cumulative graph - and therefore requires knowledge of the distribution of collision speeds.
The work of Jocksch is more promising, as it uses a simple fourth-power relationship, however it does not say at what speed the probability can be taken as 100%, for example.

Any ideas?

Cheers
Peter

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

Don't get bogged down in the detail (for now).

Look at the overview - average real world crash severity is far far below anything we might expect from observing vehicle speeds

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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 suspect because the likelyhood of finding such information is directly proportional to the number of fatal accident reports you have read.
How many will that be?

Hello Steve (style is a giveaway):

That would only outline the relationship between free-travelling speeds and impact speeds at fatal accidents, wouldn't it, thereby distorting statistics dramatically?

Having said that, checking the correlation in just fatals would be an interesting exercise. I would venture to suggest that the trend of this is increasing, ie, impact speeds in fatals are getting closer to free-travelling speeds. As the camera era takes a hold of more motorists, the collective average point count gets closer to licence loss. This in turn leads to much more speedo-gazing and sniper-spotting instead of concentrating on real hazards, I submit that as this happens - and it has started already but is now climbing fast, correspondingly more crashes involving fatalities occur with at least one party failing to scrub off speed before impact.

Basing solutions on only the most severe occurences is a critical mistake that was recognised by (what we now call) the health and safety community many decades ago. We have to look at the whole range of human behaviour to find the important errors that occasionally lead to the most serious outcomes.

While many fatal crashes may involve speeding, we know that speeding wasn't the critical failure leading to the outcome because speeding is so very commonplace while fatal crashes are so rare.

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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 many have you read? I assume from this that you have access to such reports.
If so, how about passing them onto us so we can see in black and white just how wrong we are.

Is that too much to ask?

Cheers
Peter

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

I assume multi-lane roads means dual carriageways, in which case it is desirable to have speed differentials between the different lanes.

In the case of single carriageway roads (one lane each way) I think it is unrealistic to expect to achieve a situation where everyone will be happy to travel at the same speed, in which case it is inevitable that some overtaking will take place. All I would ask is that people learn to leave decent gaps so that others can overtake and slot in comfortably. That is the sort of thing the Highway Code envisages, but a lot of drivers seem to be quite oblivious to that, not realising how badly behaved they actually are. If that message could be put across effectively in some form of campaign I believe it would be quite a useful aid to improving safety.

BTW Paul, I'm still having to log on each visit and it's a pain in the bum. Are you trying to get rid of me?

Best wishes all,
Dave.

Auto log on is working fine. It does however depend on a cookie on your machine. If you're disabling cookies, that will break it. If you're visiting with cookies enabled, then perhaps you have a corrupt cookie that will need to be deleted.

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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

OK Paul, thanks for that.

I don't really understand computers so it might be a cock-up at this end. Sorry to be blaming you unfairly.

Best wishes all,
Dave.

This isn't necessarily true. The commonplace event (speeding) could be a necessary, but not sufficient, condition for a fatality. Just because a commonplace event doesn't always lead to a rare event doesn't mean the rare events would still happen if the commonplace occurrence were stopped. Not that I'm suggesting that is the case for speeding, just that the logic above of "we know x because y" is not, IMO, valid.

Consider, for example, scuba diving. Embarking on a scuba dive is a necessary but not sufficent condition for a scuba diving death. Some people die whilst scuba diving each year, but deaths are relatively rare. Clearly scuba diving does not, in itself, cause death, but if you stopped all scuba diving, you would have no more scuba diving deaths.

You can derive a probability distribution from a cumulative distribution and vice versa:
http://www.weibull.com/SystemRelWeb/a_b ... utions.htm
Or do you mean that you don't know the equation behind the graph, so can't calculate the derivative? You could just do it by hand by sampling the gradient at intervals along the graph.

I don't think that's a valid comparison.

If you substituted 'driving' for 'scuba diving' in the last paragraph it would be valid.

If, however, you substituted 'speeding', it would depend on the presupposition that you have speeding deaths.

Cheers
Peter

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