Safe Speed Forums

You are thinking of MDBS (Mean Distance Between Surprises) which is an interesting calculation. However, if, on average, there is a near-linear relationship between the number of times you get surprised and the number of crashes you have, then MDBF and MDBS are just different units of the same thing – risk measurement.



Indeed – MDBS are close shaves, and although they gives a far higher resolution result for an individual, they are (unlike crashes) not measurable at all and totally subjective. I expect thousands of people have close shaves every day and don’t even notice!



A crash has rather more impact than a near miss! But as I say, MDBS and MDBF (or MDBC if you like) both measure the same thing. The idea is to have a long one, whatever units you use!

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I was saying two things really:

1) Crashes are too rare to be useful. It's perfectly commonplace for a crap driver to go 20 years without crashing. I stand by this assessment.

2) Working with 'lower level' errors is a golden opportunity to improve road safety. We shouldn't miss that opportunity because some of the data is subjective.

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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

And what is wrong with competitive driving exactly?

I'd be far happier sitting next to (or behind) a driver who has competed in some form of motorsport as opposed to sitting next to (or behind) someone who has only driven for a few months.

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I agree with you. But for us today, discussing as we are how to 'predict' risk, the specifically measurable events are the crashes, and MDBF/C gives an idea of how long you might expect to go before you have one. As such, it encompasses countless 'surprises' which are also directly related to the circumstances of a driver.

While using MDBS is a good way to communicate the risk of errors, it is hard to define the limits of what a 'lower level' error is – there is little ambiguity about a crash, so you are talking about a different (but closely related) thing.

Another question – could keeping to the speed limits be one of the factors than effects MDBF/C (or MDBS)? And in what way?

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I tell you what's interesting - you're looking for an objective measurement because you favour 'external' (state?) control. I'm happy with a subjective measure because I believe individual responsibility is key.



You could get no answer unless you also examined the reason for speed limit adherence. For example if everyone started driving to the speed limit because the government had convinced them that the speed limit was the true safety limit then crashes would go through the roof. Same story with massive enforcement - I'm certain that the distraction and paranoia would far outweigh the potential benefits.

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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

From one point of view, competitive operating of any dangerous tool or machinery is plain stupid. We don’t encourage people to have milling machine races, nor competitions to see who can fastest cut logs, trim the hedge, climb a ladder, chop meat etc. unless we want casualty to be full of people with broken arms and bits chopped off!

For some reason, even more dangerous things like car driving is quite OK, even if casualty IS full of people with broken arms and bits chopped off!

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Last time I was in casualty , it was full of sporting injuries ( I myself had broken my arm whilst mountain biking ) , do you suggest we perhaps put more rigorous legislation in place to stem what is a clear waste of much needed NHS monies ????????

Yes - I’m explaining a measurement based on facts, not feelings, but not for the reason you say. I want to give people their MDBF/C as a form of feedback so that they can change their ways. What form of feedback do you provide with your subjective measure and who takes the readings?

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Actually we do :-

Factories are in competition for productivity both internally and between competitors; the higher the productivity the better and the more money they make.

Lumberjack competitions are a traditional sport and very competative with the prise going to the person who cuts through the log or fells the tree the fastest. They are even televised; lots and lots of references on google.

The same applies to everything that humans do. We are a naturally competitive species, and only the most PC advisors to teaching advocate that there should not be loosers in anything. It is human nature, and cannot be changed - if it wasn't this way we still be living in caves.

It's in the manifesto - we'd appeal to their sense of self preservation by informing them that mistakes or surprises foretell crashes.

So it's self-reading, self policing, self improving but with training and help line style assistance available. I'm convinced that the clear majority would heed the warnings if they were equipped to recognise them.

We'd need very different systems at the low driver quality end of the scale where atitudes are suspect or even dangerous. I would favour a system of crash reporting for these types. But I'm far from convinced that the Police would have the resources. Maybe the present Police responsibilities are about right, but obviously we have far too few officers dealing with bad driving these days.

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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

Our daily life no matter what we choose to do is fraught with danger of some description. By virtue of that fact, all competition holds some sort of danger as well, even the simple act of running (I'm sure most people here have seen the pic of the olympic hurdler with a leg that breaks on landing after a hurdle).
What you are suggesting is that we remove an element that has been present in man since day one, well I'm afraid that is just part of our make up and will never happen no matter what.

Both of my children have been competing in motorsport since they were 3 years of age.
Last weekend, my eldest (9 years old) boys average lap speed was 62mph, and his highest jump at the track in question was recorded at 10.72 metres at 67mph. The younger (7 years old) one attains average speeds in the region of 50mph when riding.
Now either they have been incredibly lucky (along with the 1000's of other schoolchildren who partake in the same sport), or maybe the simple fact is that motorsport is no more dangerous in reality than crossing the road.

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What makes you think I'm drunk officer, have I got a fat bird with me?

You'll always have a few saddos like that. I blame thier parents.

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Given the size and power of most competitors I wouldn't accuse them of this to their face...

Yeah - or make sure YOU are the one with the axe!

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I know - that's why we need to force the Neanderthal element to obey with speed cameras. In the mean time, at least let’s not talk it up too much – they might find something else to do.

PS: looking back at that, I can see that it is off-the-cuff and could be inflamatory to you. It wasn't meant to be, so sorry. I'm sure your children are great little guys!

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It works better to round the baddies up, give 'em a good talking to, boot 'em up the arse, and tell to get lost. I can’t imagine why we aren’t pulling over 1000’s more of the road slobs and fining them to death, if need be!

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I certainly agree that we should be doing that (although we might have a bit of a scrap about the definition of a 'baddie'.), and I further agree that it's a central plank of any worthwhile road safety policy.

But I still think there are huge gains to be had from subtle improvements to the responsible majority.

Most of us need carrots. A few of us need sticks. The great beauty of the carrot is that it encourages people to 'in'. At present we're pushing them 'out'.

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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 speed camera's don't force the "neanderthal" element to obey at all.

The true "neanderthal" road user hasn't given a shit since day one, and he (or she) doesn't give a flying for the camera because they are already in a "disposable" vehicle.

The "neanderthal" element that I assume you actually refer to, are actually decent, responsible, law abiding individuals who will not even consider driving beyond what the given conditions allow.

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What makes you think I'm drunk officer, have I got a fat bird with me?

Specifically, we mentioned the driver who thinks he’s good but isn’t. That driver will never listen to SafeSpeed’s advice – he’s already Mr. Perfect. Like (almost) all blokes, he has an enormous ego. Ask your wife, if you are married, for an honest assessment! I just hope it doesn’t end your marriage!!

Look, blokes are like that, gixxer. We don’t ‘listen’ to people telling us how crap we are. We just get out our size 12 club and beat them over the head, end of story. At least, a good proportion do.

And those are the ones in my sights. For those guys, you need a size 22 club - a speed camera!

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The important word there is "if". If going more slowly increased MDBF then I'd agree, but is there any reason to suppose it does? I don't think it does for the simple reason that even the governement concede that only a minority of crashes are caused by excess speed - and we know damn well that not all of those will be excess speed that is also over the limit.

Secondly, you're only talking about crashes. Failure in this case is some kind of driver error, and as Safe Speed has said it doesn't necessarily have to result in a crash. If I fail to see someone coming as I turn right that is a safety failure, even if one or other of us stops in time to avoid a collision. Does the fact that no crash occured lessen the seriousness of my error? The simple answer is that if a crash was only avoided through luck then no, the error is just as serious as if a crash had occured as a result. Mean time between crashes is easier to measure, but it's only the number of times a failure has a serious outcome, not the actual number of failures. To be meaningful we'd need to know how the percentage of safety failures that result in a crash for each individual, and suddenly it becomes much more complicated. Easier if drivers are honest with themselves and say that a failure is every emergency stop/manoeuvre. Call it mean time between incidents if you prefer, but the bottom line is that a driver who's having to do an emergency stop once a week is obviously a concern even if he only crashes once every couple of years. What we can tell from mean time between crashes is that he goes about two years between crashes, but what MTBF (or MTBI if you prefer) is that he makes a potential crash causing error 100 times more frequently.


No, I rate safety more highly than merely going slowly or dumb adherence to the limit.

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