Stopping Distances!

[craig7584]

Why do people always use this as a yard stick for your driving knowledge?

TBH I really dont see the point of knowing stopping distances, the important thing to know is that faster you go the longer it takes to stop and that conditions will affect it.

Based on the above statement, the fact is that every car will have a different stopping distance in a given set of circumstances anyway, so what does the number we are taught represent - are they best case, worst case? How much did the car weight when they measured it and what braking system and tyres were fitted??

Surely you get to know your own car and by instinct brake with sufficient distance, that is the knowledge to have IMO. Im sure many people have had the exact same rant at some point, just wanted to re-iterate the point!

Plus the fact that even I if knew my car would stop in say 30 meters at a given speed, I have no idea how far away 30 meters is just by looking!

[untakenname]

There are too many varibles to calculate stopping distance accurately so its a moot pointm its all down to the conditions on the day.

Each varible can effect stopping distances dramitaclly just a few are the heat in the tyres/brakes, state of the road surface, quality of the tyres, what time of year (heat/moisture) how good the abs/driver is etc...

[Pork_n_Beem]

I think in absolute terms its meaningless but in relative terms its useful to know how it grows with speed but more importantly what should be taught is how it magnifies massively in wet conditions and well in snow, just forget the brakes.

If i think back over the years actually carrying out a full stop is so rare i am struggling to recall when i may have even done one, however carrying out a rapid reduction in speed and an evasive turn at the same time has been much more relevant.

So i think its just another outdated element that needs reviewing.

[Xerstead]

I rememer checking a few years ago. From what I recall one manufacturer, (but not which one,) were quoting braking* distances less than half the braking* distance listed in the highway code. Yet I was expected to learn these numbers as a fact to pass my test.
*Braking distance, not total stopping distance (reaction + braking).

[james_gt3rs]

"Only a fool breaks the two second rule"

That's what I follow in dry conditions (sometimes up to 4 secs). I don't know the stopping distances off be heart and don't see the need to.

[GravelBen]

james_gt3rs said:

"Only a fool breaks the two second rule"

O/T: The late, great Peter Brock was the face for that slogan as a road safety campaign down this side of the world for a good few years - I was always amused by the "please stay two seconds behind this car" sticker on the back of his racecar!

[WhoseGeneration]

craig7584 said:

Why do people always use this as a yard stick for your driving knowledge?

Which people use this as a yardstick for your driving knowledge?
Remeber which forum you are in.

[craig7584]

General census of agreement then! It's not just me being pedantic!

WhoseGeneration said:

Which people use this as a yardstick for your driving knowledge?
Remeber which forum you are in.

This generally comes from peers when they find out I have IAM qualification, or from police if u were making 'a little too much progress' and they have 'the talk'.

It's the same kind of thing when people find out I do martial arts, so they instantly want u to 'show them a move' or ask if I can kill them with one blow lol.

[Z.B]

craig7584 said:

I have no idea how far away 30 meters is just by looking!

Perhaps you'd find quoted stopping distances more useful if did have an idea what 30m looked like?

[WeirdNeville]

craig7584 said:

Why do people always use this as a yard stick for your driving knowledge?

TBH I really dont see the point of knowing stopping distances, the important thing to know is that faster you go the longer it takes to stop and that conditions will affect it.

And that the relationship is exponential, not linear....

craig7584 said:

Based on the above statement, the fact is that every car will have a different stopping distance in a given set of circumstances anyway, so what does the number we are taught represent - are they best case, worst case? How much did the car weight when they measured it and what braking system and tyres were fitted??

It's hugely outdated. The numbers come from the 60's/70's, and IIRC an E-Type Jaguar (a high performance car in the day)could halve the stopping distance quoted from 70Mph. It's safe to say that the stopping distances were generated by something like a Morris Traveller with drum brakes and 125 section tyres on a "mildly moist" day.

craig7584 said:

Surely you get to know your own car and by instinct brake with sufficient distance, that is the knowledge to have IMO. Im sure many people have had the exact same rant at some point, just wanted to re-iterate the point!

Well, yes and no. Do you have any idea how far "100 metres" is, or what it looks like when travelling at speed? Is it acceptable to use some of "your" stopping distance on the assumption that the car in front will also travel while slowing?

craig7584 said:

Plus the fact that even I if knew my car would stop in say 30 meters at a given speed, I have no idea how far away 30 meters is just by looking!

IN that case, perhaps you should find a quiet/private place and mark out a range of distances, then try a few stops from varying speeds. Not only will it give you a visual reference and a good feel for your cars braking performance, but if you choose NOT to learn stopping distances then you will at least have something to go in their place.


To get back to your original point, I think stopping distances are used because it's easy to test, and it's importnat to have an idea of worst case scenarios when planning during your driving. They may no longer be entirely relevant, but you're not exactly going to get driving instructors turning round and saying "Oh, the two second rule was Sooooo 80's, We can safely sit at 1 second now, and let the radar braking system do the work."

[craig7584]

WeirdNeville said:

N that case, perhaps you should find a quiet/private place and mark out a range of distances, then try a few stops from varying speeds. Not only will it give you a visual reference and a good feel for your cars braking performance, but if you choose NOT to learn stopping distances then you will at least have something to go in their place.

On that point, this is what I mean - the skill is in learning how far your car travels to come to a stop at varying speeds, not neccessarily memorizing the numbers behind it - Its probably easier to think of it in seconds rather than feet or meters, since people are not good at estimating distance, which is why the 2 second rule came about and proves so useful and which is my point with learning the distances in the first place, most people cant successfully estimate an inch let alone feet or meters, so the information is useless, unless you have distance markers.

It would be more useful IMO, building upon the 2 second rule, to memorize the time it takes to come to a stop from a given speed (since the 2 second rule only applies to following distance to allow for thinking time, not complete stopping distance), so u have a better idea when you are approaching a 'reason to stop', when you need to start braking.

[WeirdNeville]

No, people are far worse at estimating time given variable speeds than they are measuring distances.
Time distorts and compresses the whole time.

I shoot quite a bit, out in the field. You need to know distances, so you get adept at gauging it. I routinely put afigure on the distance, take the shot, and then pace out the distance to find out if I was right or not. You get uncannily good at it after a while. Driving is no different - you should know what 100 yards/m 30yards/m looks like and where roughly your car will pull up in it.

I agree "The two second rule" is very easy as a guideline minimum gap to leave, but if you can't remember a series of numbers from different speeds, it makes no difference whetehr those numbers are in seconds or meters or dead baby Jesii...

[RobM77]

If I may get on my soap box a little in response to the OP, then I believe this problem starts at school, where a lot of people seem to pick up the erroneous idea that knowledge and understanding can be represented by memorising factoids. Additionally and subsequently, many people see it as a way to become an instant expert in a subject (which it isn't!), and use this as a way to feel superior to other people. Pathetic, but very common.

I have a degree in Astrophysics, and when out on a starry night people seem surprised that I can't tell them the distance to Betelgeuse or the mass of Sirius. The truth is that memorising a stopping distance has as much to do with being a good driver as memorising the distance to Betelgeuse has with becoming an Astrophysicist. True understanding and ability is something completely different, but annoyingly, many people either don't get this, or don't want to get it as it will remove the superficial feeling of superiority that they get by knowing dry facts out of the Highway Code.

[craig7584]

I suppose thats down to preference, in short my point is this: ones ability to judge when to start braking to stop at a certain point, is a different one to, the ability to memorize a set of numbers - so by testing the latter it doesnt say anything, imo, about the former.

Edit:
Rob, what you said

[RobM77]

craig7584 said:

I suppose thats down to preference, in short my point is this: ones ability to judge when to start braking to stop at a certain point, is a different one to, the ability to memorize a set of numbers - so by testing the latter it doesnt say anything, imo, about the former.

Edit:
Rob, what you said

Another example is to imagine that Jenson Button brakes from 181mph to 62mph for a particular corner, time after time, day after day. Could he tell you how many metres it takes to do that stop? No. Can he repeat it? Yes. Judgement is different from encyclopaedic knowledge.

[WeirdNeville]

Absolutely, but it's useful to have a framework to base that judgement on, no?

I'll agree, if you have no idea what 314 feet looks like, it's pointless learning it, but for those of us that do??

[RobM77]

WeirdNeville said:

Absolutely, but it's useful to have a framework to base that judgement on, no?

I'll agree, if you have no idea what 314 feet looks like, it's pointless learning it, but for those of us that do??

good point. I guess it works one way but not the other? A new driver knowing a stopping distance off by heart will help them drive safely, but if a driver doesn't know the stopping distance off by heart, it does not make them an unsafe driver. Similarly, to go back to my Jenson Button analogy, I bet his datalogging engineers know exactly how long it takes him to slow for each corner, but I bet they can't do it! JB on the other hand probably hasn't got a clue what the exact figures are, but can do it repeatedly every time. I don't mean to say that F1 drivers are just seat of the pants jocks, because clearly they aren't, but in this specific example it's a good example of how knowledge doesn't necessarily mean good judgement, but good judgement needn't come with knowledge.

[blindswelledrat]

THis annoyes me too.
Its one of the reasons that universal speed limits annoy me so much.
I am going to be able to stop a Fireblade at 30 mph in about a quater of the time that a 70 year old man in a Micra will stop (pure guess-inculding thinking time I wouldn't be suprised if the truth was a lot more extreme than that).
Its infuriating that we are all treated as the lowest common denominator.

[The Black Flash]

WeirdNeville said:

No, people are far worse at estimating time given variable speeds than they are measuring distances.
Time distorts and compresses the whole time.

I shoot quite a bit, out in the field. You need to know distances, so you get adept at gauging it. I routinely put afigure on the distance, take the shot, and then pace out the distance to find out if I was right or not. You get uncannily good at it after a while. Driving is no different - you should know what 100 yards/m 30yards/m looks like and where roughly your car will pull up in it.

I think it's quite different, personally - one is judging the distance you are projecting something whilst you remain static, the other is judging how far you will travel whilst decellerating to a stop from a variable speed. A quite different process! For driving you need to be able to judge that you can stop in the distance you can see to be clear - I don't personally see how putting numbers on that helps - except as a classroom illustration of how that distance changes with speed and conditions.

The 2 second rule is good because it takes account of speed, albiet as an approximation, but one which works well over the normal range of speeds we drive at. You couldn't come up with a workable equivilent in terms of distance.

[Observer2]

The Black Flash said:

I think it's quite different, personally - one is judging the distance you are projecting something whilst you remain static, the other is judging how far you will travel whilst decellerating to a stop from a variable speed. A quite different process! For driving you need to be able to judge that you can stop in the distance you can see to be clear - I don't personally see how putting numbers on that helps - except as a classroom illustration of how that distance changes with speed and conditions.

The 2 second rule is good because it takes account of speed, albiet as an approximation, but one which works well over the normal range of speeds we drive at. You couldn't come up with a workable equivilent in terms of distance.

I have some firmly held views on this that I haven't seen expressed so resurrecting this old thread.

It is my contention that for 'everyday' road driving, drivers have absolutely no need to know what is their theoretical (ex Highway Code) or actual stopping distance, whether in feet, yards, metres (or stopping time in seconds).

What drivers do need to know is what their minimum stopping distance isn't. To explain: I don't need to know that I could (if needed, with maximum available braking force, stop in a specific distance (or time). I do need to know (and I think this is the way drivers actually behave), on a continuously varying basis, that at my speed for the time being the space available to me is greater than (by some factor that doesn't need to be quantified) my minimum stopping distance - i.e. that I can assuredly come to a stop (or achieve a desired reduction in speed) by or before reaching a specific, observed physical point ahead. I then continually (but subconsciously) repeat this assessment and, depending on my judgment of road conditions and other factors, commence braking and modulate the applied braking effort to achieve the desired speed reduction. The point at which braking actually commences becomes a matter of personal judgment based on a number of facors including individual discomfort and fear thresholds.