Safe Speed Forums

I know the answer, but I don't believe the question is representative (drivers tend not to do 100mph without a good, clear motorway in front of them) so there's no point posting it.

I haven't. The prof is talking baloney. At the very best he's making a very specific statement that only applies to one of the cars in this exact scenario - and he's not letting on which car that is.

Note also that he doesn't state what speed is being halved. If his statement was generally true, then starting from 130 kph you'd knock off the last 65 kph in 5 metres. However, if you started at 65 kph (as in this crock of ***) you'd still be doing 32.5 kph with 5 metres to go. IOW, 65 = 32.5, which is plainly false, and the good prof's baloney is revealed by reductio ad absurdum.

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Will

now explain to me how braking times and braking distances are not connected? Use one of your statistical (rather than mathematic or physical) examples if you like, show me how for a roughly equivalent starting speed a braking TIME of, say, 2s, does not cover a lesser distance than a braking TIME of, say, 4s.

You can't, it can't be done, time and distance covered are inextricably linked in a moving body, that's why we have those pesky devices in our vehicles marked in a time / distance scale (you know the one, usually has MPH and KmPH on it).

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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.

But how good would road safety be if we kept having to do emergency stops? The trick - and the truth - is that whenever we get to the point of attempting an emergency stop our safety systems have failed and we're rolling the outcome dice.

Fortunately our safety systems are pretty good and emergency stops aren't that common. We need policies that help us to be more skilled and do fewer.

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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

If he is being approximate, then it could apply to both.
A mathematically correct example with slightly less -G gives: slow car = 4.60m, fast car = 5.40m, rounding to the nearest meter would give the stated result. Clear as mud, but since when have these speed obsessed people been clear and transparent?


You are right, that prof really should have been clearer as to what speed would be halved. Few people could understand what the answer must be, let alone take the time to figure it out (present company excepted of course).
He should have said "a car starting to brake at 65kph will come to a stop 5m past the point when it is at half of that speed".

Handy,

There's probably nothing wrong with the maths - I haven't tried to check and I'd probably get the wrong answer anyway. But, taking the maths as correct, it doesn't follow that road safety can be fully and adequately described in mathematical terms. This, as you well know, is the point Paul has made.

The mathematical model can accurately describe the outcome of a given situation with two vehicles, travelling at different speeds, and reacting at precisely the same moment to a developing hazard all else being equal. But, in the real world, all else ie never equal. What's important in road safety, the difference between crashing and not crashing, or between fatal crash and minor crash, is what drivers do. That comes from inside their heads and cannot be described in mathematical terms.

The video clip only explains the physics of speed and braking distances in a single hypothetical situation, but that's only part of the system (in the widest sense) of driving. The part which is far more important in determining outcomes is how well drivers identify developing hazards and react appropriately. Drivers will react to hazards because no-one (if not mentally deranged) actually wants to crash or mow down a pedestrian or cyclist. Road safety will be improved far more if drivers become better at identifying and reacting appropriately to developing hazards, but drive (on average) at the same speed (or even faster), than if they become worse and drive (on average) a bit more slowly.

Of course they're linked. However, one determines the other. While you assert that the braking distance determines the time, Paul asserts that the braking time determines the distance.

Now the distance required to stop from a particular velocity is related to the square of the velocity while the time required to stop is directly proportional to the velocity. Ergo, it's much easier to make "instinctive" assessments of safe times than distances. I know that I work on time rather than distance. I try to keep a scan of at least ten seconds ahead - i.e. where I will be in ten seconds; I follow the two-second rule in "convoys" and I adjust my speed to maintain those timings. While driving, I can't tell you how far away a particular object is but I can tell you fairly accurately how long it will take to reach it.

Another factor is the ease of measurement. It's darnded difficult to accurately measure distance while driving (I'm talking about the number of yards needed to stop, not the miles or tenths shown on an odometer). Even if you had a distance measuring device (such as a laser ranger), you'd need to operate it and read it, which would reduce your concentration on the road. In contrast, it's easy to get a reasonably accurate measure of time - and get it without taking your eyes of the road. This is what reciting, "Only a fool breaks the two second rule" etc. does; and repeated application of these timing aids soon gives you the ability to instinctively estimate the time required to brake safely etc.

Paul's right.

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Will

hmm, wonder why track vehicles don't have them then. How do all those drivers work out where to brake without having the faintest idea how fast they are going?

I don't think I make assessments in terms of times either, except to gauge an appropriate following distance. I'm prettty sure I do not approach (say) a roundabout, T-junction or any other hazard thinking (consciously or sub-consciously) "I'm now (say) ~5 seconds away at current speed so need to start braking". Nor "I'm now (say) 150m away so need to start braking".

I don't think, in normal driving, a driver ever needs to know what his minimum stopping distance is because we never (or never should) in normal driving plan or expect to use maximum braking effort. I think the assessment that a driver (or is it just me?) makes is to judge what his minimum stopping distance isn't - i.e. identify a point to start braking (or be prepared to brake) which he (I) knows is earlier than the latest possible braking point and then modulate the actual braking effort during braking to achieve the desired deceleration. Of course, for other reasons, a driver (I) may start braking closer to or further away from the latest braking point, but I doubt that I have ever planned to start braking at that latest point.

Sure you do. All the time. It's just the definition of 'latest' that we have to haggle over.

We choose the latest point, consistent with comfort and safety, all the time.

And we know that we have a good margin because we're never planning to brake in a way that isn't consistent with comfort or safety.

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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

Of course. I had defined it (or intended to) as the latest braking point assuming maximum braking effort.

you've missed the point ... I'll give you the benefit of the doubt and say that it was by accident rather than on purpose ...

TIME and DISTANCE are INEXTRICABLY LINKED in a moving body. The important part of the reference to the speedometer is the "p" bit ... stands for "per".

Now Observer gets the point:


[my bold]

and this is quite crucial. On this site there is a common tendency to state that the driver who is going faster is always going to be paying better attention to the road, therefore (as the "other things", like time taken to spot hazard, are NOT equal). I have seen no proof to support this statement (paragraphs starting "when I drive faster ..." don't count as proof), or to put it a different way, it's just as likely that the driver with a higher velocity is distracted as the driver with the lower velocity.

So, given that a percentage of drivers are not paying adequate attention to the road, and there is NO proof that those driving fastest on public roads have any different distribution of adequate attention, those driving even a little bit slower have by definition given themselves MORE time (remember, TIME and DISTANCE are still INEXTRICABLY LINKED, even if a speed measuring device is not present, Johnsher) to deal with issues.

Driving AT the limit, where limit is the capability of the car, driver, road, other road users ability, gives no safety margin. Driving UNDER the limit of capability, driver etc. gives a safety margin in terms of more time and required braking effort. This is true regardless of the posted speed limit.

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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.

What sort of 'proof' would you like?

How about a logical proof: if speeds are appropriate they are NEVER a problem (because if they were a problem, they wouldn't be appropriate).

I've already given you ample evidence from accident severity and frequency patterns that clearly demonstrate that we're not working in the region where marginal alterations to free travelling speeds might be expected to have an influence on crash outcomes.

Or have a look at international motorway risk comparisons...



Do you see any possibility of a speed / crash correlation in the chart?

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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

There is proof that those around the 85 percentile point are on th ewhole safer than those driving well above or well below.

Nobody on this site has ever suggested the fastest drivers are the safest.

We do think though that being forced to drive at an artificially slow speed has no benefit and in some cases can have be detrimental to safety.

To the best of my knowledge, I have never stated that. What I did say and attempt mathematical proof of upthread was that inattention is far more dangerous than a few kph of extra speed and illustrated that by showing the effect on collision speed of taking your eyes off the road for one second (the speedo check) at exactly the wrong moment.

Speed is a secondary factor of road safety that is determined by the primary factors (COAST). However current policy dumbs down road safety to only consider that secondary factor. Worse, it sends a strong message that the speed limit is invariably safe and too many motorists today seem convinced that they will be good and safe drivers if they rigidly obey the speed limit - even while otherwise driving like a prat or where the speed limit is too fast for the conditions. This is why dumbing down road safety to speed enforcement is so wrong.

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Will

I think that's a circular proof more than a logical proof. The question it begs is "how do you define 'appropriate speed'?", which of course "can't be defined in miles per hour".

My definition would be the lower of:

• a speed at which you make adequate progress while giving enough space and time to observe your surroundings and anticipate, recognise, and deal with any hazard before it becomes a problem.
• a speed at which you have full control of the vehicle.

JMHO,

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Will

By definition, as you have said, higher speed carries higher risk if ONLY the physics model is considered. But if we consider ONLY the physics model we lose sight of the much more important psychological element (which I think you recognise).

To counter the sections I emboldened:

- I don't think it is common for posters on here to assert (in substance) that "driving faster is driving safer. The argument "I drive more safely because I drive more quickly" is absurd in isolation. However, if expressed as "I drive more safely when I am fully engaged with the driving task", and if, as a matter of fact, I am more fully engaged when driving at the speed I would naturally choose rather than a slower speed which is imposed by some external factors, then the statement is correct.

- "It's just as likely just as likely that the driver with a higher velocity is distracted as the driver with the lower velocity". I think that's probably not true. Driving faster requires more concentration so the tendency will be for faster drivers to be paying more attention. It doesn't follow that they're driving more safely than a slower driver or that there isn't a higher risk (considering only the physics model), but neither does it follow that they represent a greater risk than the slower driver.

- I believe it has been shown that the least crash involved drivers are those who drive at or around the 80-90%ile speed (for any given road/conditions). That doesn't mean (and I acknowledge that some people erroneously draw this inference) that it is safer to drive at (say) 85%ile speed.