Safe Speed Forums

Fair point, but that 25% figure has the potential to be highly misleading. For a start it's quite low - we'd do better in a general sense to look at what worked in the 75% not at what didn't in the 25%.

But note these points:

• The sample was "ambulance called" junction crashes. One presumes that there would have been a pretty significant proportion of crashes excluded because damage was light and an ambulance wasn't required. We'd expect most of the non-braking crashes to be more severe, so the "25%" becomes the same size slice of a much bigger pie. In the UK damage only crashes outnumber injury crashes by around 10:1 so this could well be a factor of 10.
• There would also be a large number of near misses where pre-crash conditions were indistinguishable from the crashes, yet driver response saved the day. The pie gets bigger again. This could well be another 10 fold increase, and if it was the proportion of non-braking incidents would be down to 0.25%.
• Some of the non-braking incidents will be down to drunk, reckless or lawless drivers. We can reach those groups with speed management.

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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

Strange that when speed limit setting was done using the 85th percentile and enforcement was only used to prosecute those driving markedly in excess of the safe speed for the road and the conditions:

a) Most people drove in a very narrow speed range just below the 85th pecentile speed, with the most skilled driving somewhere up to the 95th percentile.

b) Fatalities fell.

Since road "safety" policy started concentrating on lowering limits and enforcing them without discretion:

a) The fall in fatalities first stalled.

b) It now appears to have reversed.

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http://www.itsyourduty.org.uk

Your argument appears to be that because the event (the impact) is random, all of the effects of it must also be random. That's simply spurious. Once the necessary components for an accident event are in place (i.e. where a risk of collision exists), although the final impact speed (if the impact occurs) may not be precisely quantifiable, all of the inputs which may affect the impact speed will fall within a determinable range of values - ergo the impact speed is not random.

I fall between the two camps here. We do know that in a vast majority of incidents, no impact takes place. We can theorise with certainty that a percentage of impacts take place at free travelling speed.

Between those two extremes that largest determinant of impact speed must be the instant in time when the road users involved became aware of the risk of collision. Awaking from inattention (for example) is clearly something that could take place at any time.

However, awaking from inattention can only happen the once in the run up to a collision. Assuming (reasonably) that states of inattention have finite duration, the probability of awaking from inattention must increase progressively throughout the danger period. Thus the probability of inattention must increase as we wind the clock backwards from the moment of impact.

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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 question (is there a relationship between speed and crash severity?) has been boiled down by earlier posters to the following - is impact speed related to cruising speed? This is reasonable because, although we know that many factors effect the severity, speed is known. Even though other factors are hard to predict, on average we know that slower impacts involve smaller forces and cause less damage.

In many cases, impact speed is related (or is even highly related) to cruising speed, and in those cases, the outcome involves the highest forces. In those cases where impact speed is not related to cruising speed, the outcome is, on the average, milder than the worst case.

This means that we are less interested in cases where significant braking has broken the link between impact speed and cruising speed. Unfortunately, it is not possible to say, a priori, how much braking will be achieved in a particular incident by a particular driver, so in law we must treat all alike. That is why speed limits (and their enforcement) seem such an imposition to drivers who hold a high opinion of their own skills. Of course, holding a high opinion of your skills is not the same as actually possessing those skills!

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I stole this .sig

Agreed. And if the 'waking from inattention' occurs (say) 1 second before impact (based on free travelling speed), the impact speed will increase in proportion to the free travelling speed.

I don't know so much. I'd also postulate a relationship between free travelling speed and the probability of inattention. In particular anyone free travelling at well below an optimal safe speed is far more likely to be inattentive.

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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

But the reason why it is important to ask (and answer) the question ("is impact speed related to cruising speed?") is to establish what inputs will most affect the impact speed. The free travelling speed is an important one but, for a reasonably probable range of speed values, I think we see that braking time is far more important. So, although not discounting the speed input, we must pay attention to the other variables which will influence impact speed more than the probable range of variations in free travelling speed.

I don't know how you can be doubtful about the self-evident truth of my unqualified statement.

I can see some merit in your 'postulation" but that introduces another variable which wasn't present in your original question. And the difficulty that introducing it presents is that (in my judgment) there is no single "optimal safe speed". It will vary from one driver to another and, for the same driver, at different moments in time.

Anyway, I'm not sure whether it is necessary to try and determine it or allow for it. Surely what we want to do is measure the effects (on impact speed) of a reasonably probable range in the value of 'inattention gap' (and perhaps other variables) and compare it/them to the effect on impact speed of a reasonably probable range of free travelling speeds.

I only care about braking time (and/or distance, for they are but two side of the same coin) when it is short. If braking time is long, I couldn't less care about it because it either considerably reduces the impact, or allows no impact at all. I want LONGGG braking times, which you get if you leave a good gap and LONGGG braking distances, which you get if you drive slowly. In essence, I want people to drive like I tell my children to come down stairs, i.e. carefully and slowly, leave a good gap and no messing around.

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I stole this .sig

We've heard this methodogy from you countless times before BW, and to me it sounds like an expedient approach rather than the best approach. It presupposes that the worst-case-scenario treatment is always the best, when in reality it is a cheap, one-size-fits-all solution (eg. speed reduction to an arbitrary legal limit, then gradually winding down resources aimed at all other priorities).



It's like treating healthy people for an illness just in case they are likely to contract it, and that treatment itself has a minor detrimental effect on an otherwise strong and healthy person which they are then forced to suffer because "you aren't always going to be as healthy as you think you are". Your statement assumes that the majority of drivers aren't as good as they think they are, but many might be as good or better and if they are then what you are suggesting could make things worse rather than better......

If the driver has proven through years of accident free driving that they are in fact better than the accident prone, then what incentive is there to maintain that standard if it is assumed that they aren't as good as their record proves??? Or worse, why inflict on them a 'treatment' designed to help others but will actually hinder their inherent 'healthiness'??

If we're talking purely physics your statement would be simple and self-evident truth.

I don't believe that it's safe to confine ourselves to physics because any variation in speed that may be achieved in the real world has to use the driver's mind as an "agent". The driver's mind is affected in many ways by speed reduction strategies, and MUST NOT be neglected.

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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

You're ignoring the obvious if you focus on speed to the exclusion of other factors which determine 'safe speed' and create the maximum possible 'braking distance'.

I agree. But, before we try to measure a psychological effect, surely it makes sense to thoroughly examine the effects which physics can measure (i.e. attention gap, road positioning etc)?

So it was just "terms of reference". Sorry about that.

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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'm with you, but to use a pedestrian or other car that won't be there an instant later destroys the point of the scenario. I think we're still on slightly different wavelengths. You seem to be thinking about the effect on potential crashes, some of which may become actual crashes. In that scenario I'm only considering the latter, and not all of them. Just a very specific one that would allow us to compare the effect of different initial speeds on a collision and see how much that changes when we start factoring in all the other things that do occur in real crashes.


You're much too kind Peter. Pretty remote? Very flattering, mate. I think it's lottery odds myself . Thing is, it doesn't have to be a likely situation for this purpose. As long as it's possible we can use it to show that other factors play a much larger role in determining how severe a crash is than initial speed. Unfortunately the circumstances in which we could actually go to a crash scene and see that for ourselves are very rare.

Yes, it depends on several factors coming together at a precise time, but that also applies to all crashes in the real world. I feel that nearly all the factors are quite realistic (condition and weight of vehicle, driver ability and physical condition, time taken to notice hazard and so on). They will all play a part in real crashes. What is very unlikely is one car overtaking another and a hazard presenting itself inside the stopping distance of both at the precise point of the overtake. Bloody unlikely, I'm the first to admit. But not absolutely impossible, so it works as a way to consider the effects of different initial speeds from the same distance. The least likely thing about it is me cutting the tree down - A) I wouldn't, and B) I don't really own a chainsaw. But we could always say the wind blew it over at the critical moment. That might be even more improbable, but like I say, this only meant to be hypothetical so anything that's not actually impossible is ok.


No, I don't know the ones you mean. But I can't believe they're too similar. I feel that far from underpinning "speed kills" that sort of scenario undermines it. They show a relationship between speed and severity as long as all other factors are ignored. A soon as you chuck in passengers and a bootful of luggage, a pothole or something on the road, or a distracted driver who sees the object way too late, almost anything really, the relationship between initial speed and crash severity starts coming apart.

On a side note, does anyone know if I can make a flow chart in a post? I don't have a web site that I can stick an image on so I can link to it.

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

You may well get a better offer mate, but I have a web page which I use to stick charts and photos for these forums. If you want to email your chart to me. I'll stick the chart gif on my site and email the url to you. my email address is reply@siis.fsnet.co.uk

There are probably other easier ways to do it, but the IT wizzes will be able to help with that.

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Fixed ideas are like cramp, for instance in the foot, yet the best remedy is to step on them.

Ian

If you don't have a website, you can upload photos and other images at:

http://www.fotopic.net/

Virtually all ISPs give subscribers some webspace, and you can create a directory to upload stuff without having a website as such.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

I think we're at cross-purposes here. I'll try to explain it in a bit more detail:

t is the time from when you step on the brake pedal to the time that either you collide with the hazard or you stop. As we're talking about collisions we'll ignore the latter case. t depends on a) the time at which you see the hazard (random) your reaction time (random within a range) how far you are from the hazard when you see it (random) and deceleration (within a small range).
t can be zero, ie you didn't have time to get to the brake pedal before the collision up to the time it takes to come to a complete stop. If we multiply t with an appropriate factor, we can say that for the latter case t = 1. This factor will obviously vary with initial speed and deceleration, but for a given incident it will be constant.
Now we have t as being any value between 0 and 1. If t = 0 the impact speed is equal to the initial speed. If t = 1 we come to a stop just as we reach the hazard.

Now, taking the equation (we've already factored t for deceleration):

i = s - (s * t)

As t is a random number between 0 and 1, i is a random number between 0 and s.

I hope this makes it a bit clearer.

Regards
Peter

IanH and PeterE, thanks for the offer and advice respectively. Yeah I have ISP webspace, but never felt the need for our own site we've never looked into it. I didn't know you could use the space without a site though. Not sure how to go about it, so I'll check fotopic out. If that's no good IanH I'll get back to you.
Ta again.

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