Speeding Does Not Cause Accidents

Speeding Does Not Cause Accidents

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Dammit

3,790 posts

207 months

Saturday 25th October 2014
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We're starting to split hairs here, I was including that in "time to act".

This is, fundamentally, a distraction however - it's not important to chase off down this particular rabbit hole in order to (hopefully) get to the point where Phatboy has his Damascene moment with regards to faster things being faster than slower things.

Phatboy317

801 posts

117 months

Saturday 25th October 2014
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Dammit said:
No, stop changing the inputs. That's where you keep going wrong.

If everything ELSE remains constant, but the speed varies, what implication does that have for the time to react+time to act in a given situation?

Once we've established this, we can apply it to a driving situation.
Yes, yes, I know that if you throw the ball from the same distance at different speeds then the faster one is going to be harder to catch, but that's largely irrelevant when it comes to the road.

It doesn't carry across to the driving situation, because you can't just change speed and hold everything else constant.

In the driving situation, you have five parameters: speed, time, reaction time, deceleration and starting position.

But you can't change speed and still hold all the rest constant, because the position you are at any particular time, and therefore the starting position, is dependent on your speed, amongst other things - you change the speed and you also change the starting position (or the starting time)

Dammit

3,790 posts

207 months

Saturday 25th October 2014
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No. Some things are (or are close enough to be treated as) constants.

And they don't stop being true, simply because there are more variables involved - the time in which you have to react to a situation is proportional to the speed at which the situation is developing.

This holds true no matter your frame of reference - i.e. you can be stationary and something is approaching you (ball) or you can be in motion (for example in a car travelling down a road).

In absolutely every situation the higher the speed the smaller the window of time in which you have to do something - you can't argue with this, especially by saying "but if I left the house 1 second earlier", it simply doesn't change reality.

You can throw in all sorts of other variables - road condition, quality of brakes, blah blah blah, the effects of those things have to be accounted for separately, they don't remove reaction time and they don't effect it other than to make it slower (dark, hard to see etc).

trashbat

6,005 posts

152 months

Saturday 25th October 2014
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TOP TIP: Avoid costly or injurious car accidents by simply not being there when they occur. Why not try a different place and time altogether, such as having a cup of tea in the 1700s?

Phatboy317

801 posts

117 months

Saturday 25th October 2014
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No they're not.

You can't just cherry-pick a few parameters or whatever, in order to make your case - you have to take everything into consideration, which includes the laws of probability.

For instance, the simplistic argument has it that, as braking distance is proportional to the square of speed and therefore quadruples with a doubling of speed, that the probability of hitting someone within that distance also quadruples with a doubling of speed.
But, because the probability depends on exposure, (running at twice the speed through a storm halves your chances of being struck by lightning, because you're outside for half the time) the actual probability of hitting someone within your braking distance doubles with doubling speed, it doesn't quadruple.

In any event, this probability is purely statistical - if everybody drove at a constant speed speed, and people ran into the road at random times without looking, and drivers only ever reacted at the last possible moment to such events, without anticipation, slowing down or any such mitigation, then the number of accidents would increase somewhat if the speed increased, when measured population wide over a long period of time - in the order of many years to decades.
It would have no measurable effect on an individual's accident risk, and certainly no impact on the probability of any individual event.




Dammit

3,790 posts

207 months

Saturday 25th October 2014
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You're saying that we should discount probability due to it being related to statistics?

Or something else?

Might be an idea to re-write your post with an eye on clarity, as at the moment it doesn't make sense.

Phatboy317

801 posts

117 months

Saturday 25th October 2014
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Dammit said:
In absolutely every situation the higher the speed the smaller the window of time in which you have to do something - you can't argue with this
I'm not trying to.
But the above only applies within a certain narrow range.
For arguments sake, if you need, say, two seconds to react to any given situation, and you get that two seconds at speed x, halving your speed to x/2 doesn't help you in the slightest, because although you now have four seconds, you still only need two, so the remaining two are superfluous.

Phatboy317

801 posts

117 months

Sunday 26th October 2014
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Dammit said:
You're saying that we should discount probability due to it being related to statistics?

Or something else?

Might be an idea to re-write your post with an eye on clarity, as at the moment it doesn't make sense.
Are you sure you're reading it with a clear eye? The misrepresentation in your first sentence makes me doubt it.


Dammit

3,790 posts

207 months

Sunday 26th October 2014
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Phatboy317 said:
No they're not.

You can't just cherry-pick a few parameters or whatever, in order to make your case - you have to take everything into consideration, which includes the laws of probability.

For instance, [an argument comparing running through a storm with braking distance].

[b]In any event, this probability is purely statistical[edit for brevity].
It would have no measurable effect on an individual's accident risk, and certainly no impact on the probability of any individual event.[/b]
The paragraph in bold is the one I'm struggling with - you would appear to be directly contradicting yourself.

However- once again, it's a distraction.

Lets have a look at:

Phatboy317 said:
I'm not trying to(a).
But the above only applies within a certain narrow range(b).
For arguments sake, if you need, say, two seconds to react to any given situation, and you get that two seconds at speed x, halving your speed to x/2 doesn't help you in the slightest, because although you now have four seconds, you still only need two, so the remaining two are superfluous(c).
a. Great!
b. No - it applies all the time, in every situation, always.
c. This is true, kind of. Personally I like the idea of having two "superfluous" seconds in every potential crash situation. However, this is, once again, your "if you leave two seconds earlier" theory, wearing a different pair of trousers.

Now, at the risk of creating many different rabbit holes that we could charge off down - braking distance can be considered to be analogous to reaction/action time, in some regards.

Primarily that it gets longer as the vehicle travels faster.

Wait! I hear you cry - braking distances are different for different cars. This is true - my car has AP Racing brakes and 13" vented rotors. It can stop (from the same speed) significantly more rapidly than a car with less performance focussed braking hardware.

But this really doesn't matter as you have to consider the braking performance of the car with the worst brakes - probably a lorry, thinking about it (simply due to weight).

In the same way, we have to consider the reaction/action times of a statistically valid (i.e. large enough and diverse enough sample set) of the public.

So to go back to your idea (leaving earlier in different trousers) you may need half of that theoretical 4 seconds, but "average Joe" might need 5 seconds.

You (yourself, personally) are not a yardstick against which to judge things, and that's before we bring in confirmation bias, ego, etc etc.


trashbat

6,005 posts

152 months

Sunday 26th October 2014
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TOP TIP: When driving, reduce the chances of a collision by simply driving everywhere at 761.1 mph. You'll spend approximately half the usual time on the road and therefore there's a much lower chance of hitting anything.

Jon1967x

7,175 posts

123 months

Sunday 26th October 2014
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trashbat said:
TOP TIP: When driving, reduce the chances of a collision by simply driving everywhere at 761.1 mph. You'll spend approximately half the usual time on the road and therefore there's a much lower chance of hitting anything.
Including lightening

The lightening analogy is actually a good one as it shows the difference. The risk from lightening is proportional to the time exposed and not the distance travelled. The risk of an event in front of you when driving is proportional to the distance traveled and not the time it takes (ie the more bends, side junctions, schools etc you pass). The number of those events that are significant to you is then proportional to your speed as the danger window lengthens the faster you go. And finally, in the event you can't deal with one, there is a correlation between speed and impact speed and in turn outcome.

Not sure why any of us bother because those that understand this already understand this. And those that don't won't change their mind and even if they did have a penny dropping moment they're unlikely to come on here and ever admit it.

The counter arguments incidentally remind me of people who argue that because the no 7 has not been drawn in the lottery for the past 10 weeks it's more likely this week. It's a basic and fundamental misunderstanding of probability.


Dammit

3,790 posts

207 months

Sunday 26th October 2014
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Well, some reasons for continuing the discussion might include: I've just been on a long conference call with Qatar, now I'm waiting for the coffee machine to warm up, and then I expect that there will be some displacement activity prior to going out for my long weekend run.

I think we are making progress though, albeit not lightning fast progress.


Jon1967x

7,175 posts

123 months

Sunday 26th October 2014
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Dammit said:
I think we are making progress though, albeit not lightning fast progress.
Thats OK, we're constantly being told by some that being fast isn't the problem smile

9mm

3,128 posts

209 months

Sunday 26th October 2014
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Jon1967x said:
trashbat said:
TOP TIP: When driving, reduce the chances of a collision by simply driving everywhere at 761.1 mph. You'll spend approximately half the usual time on the road and therefore there's a much lower chance of hitting anything.
Including lightening

The lightening analogy is actually a good one as it shows the difference. The risk from lightening is proportional to the time exposed and not the distance travelled. The risk of an event in front of you when driving is proportional to the distance traveled and not the time it takes (ie the more bends, side junctions, schools etc you pass). The number of those events that are significant to you is then proportional to your speed as the danger window lengthens the faster you go. And finally, in the event you can't deal with one, there is a correlation between speed and impact speed and in turn outcome.

Not sure why any of us bother because those that understand this already understand this. And those that don't won't change their mind and even if they did have a penny dropping moment they're unlikely to come on here and ever admit it.

The counter arguments incidentally remind me of people who argue that because the no 7 has not been drawn in the lottery for the past 10 weeks it's more likely this week. It's a basic and fundamental misunderstanding of probability.
I not sure weight saving has much to do with it. wink

emmaT2014

1,860 posts

115 months

Sunday 26th October 2014
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Phatboy317 said:
No they're not.

You can't just cherry-pick a few parameters or whatever, in order to make your case - you have to take everything into consideration, which includes the laws of probability.

For instance, the simplistic argument has it that, as braking distance is proportional to the square of speed and therefore quadruples with a doubling of speed, that the probability of hitting someone within that distance also quadruples with a doubling of speed.
But, because the probability depends on exposure, (running at twice the speed through a storm halves your chances of being struck by lightning, because you're outside for half the time) the actual probability of hitting someone within your braking distance doubles with doubling speed, it doesn't quadruple.

In any event, this probability is purely statistical - if everybody drove at a constant speed speed, and people ran into the road at random times without looking, and drivers only ever reacted at the last possible moment to such events, without anticipation, slowing down or any such mitigation, then the number of accidents would increase somewhat if the speed increased, when measured population wide over a long period of time - in the order of many years to decades.
It would have no measurable effect on an individual's accident risk, and certainly no impact on the probability of any individual event.
So probability does and does not have a significant effect! Which is it?

Phatboy317 said:
I'm not trying to(a).
But the above only applies within a certain narrow range(b).
For arguments sake, if you need, say, two seconds to react to any given situation, and you get that two seconds at speed x, halving your speed to x/2 doesn't help you in the slightest, because although you now have four seconds, you still only need two, so the remaining two are superfluous(c).
Not everyone will and does react in the Perception Reaction Time (PRT) so that is best kept to a maximum reasonable time. To maintain that time the distance between cars needs to be increased as the speed of the vehicles increases.
The PRT has quite a wide variance and seldom stays the same even for individuals. It is the variance in PRT that is a significant factor in rear-end shunts in motorway and dual-carriageway collisions. In practice the PRT of 0.67s that is chosen as the illustration of thinking time and a part of the overall stopping distance in the Highway Code is better than that seen in practical tests. The additional two seconds is never superfluous because you just don’t know when you will need it.
I was tempted to simply say you are an idiot. Perhaps an explanation of why you are is better.

Phatboy317

801 posts

117 months

Sunday 26th October 2014
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Jon1967x said:
The lightening analogy is actually a good one as it shows the difference. The risk from lightening is proportional to the time exposed and not the distance travelled. The risk of an event in front of you when driving is proportional to the distance traveled and not the time it takes (ie the more bends, side junctions, schools etc you pass)
You seem to be suggesting that if you slowly walk a mile through a storm that you have less chance of being struck by lightning than if you run half a mile, because you're covering less distance?

The risk of an event in front of you at the time you happen to be there is proportional to the time you spend driving.
It's true, as you say, that an event happening anywhere along the road increases with the opportunities for such an event, like junctions and the like, but at the time they happen you may be 20 miles away, or behind your desk at work, so they probably don't affect you.
If the frequency of events was great compared to the distance travelled, eg people were running across the road all the time every few metres, then your point would be valid.
But the actual frequency of such events is tiny compared to distance travelled.
For instance, how many times, in your lifetime, has someone actually run out in front of you, causing you to have to brake?

Yes, the probability of a collision does reduce somewhat with reducing speed, but not as much as you imagine.




Edited by Phatboy317 on Sunday 26th October 10:20

Phatboy317

801 posts

117 months

Sunday 26th October 2014
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emmaT2014 said:
So probability does and does not have a significant effect! Which is it?
No practical effect.

A driver, on average, drives their whole lifetime without running down a pedestrian. Slowing them down to 20mph isn't going to make the slightest difference to that figure.
Just like you'll probably never be struck by lightning, or win the lottery.

emmaT2014 said:
The PRT has quite a wide variance and seldom stays the same even for individuals. It is the variance in PRT that is a significant factor in rear-end shunts in motorway and dual-carriageway collisions. In practice the PRT of 0.67s that is chosen as the illustration of thinking time and a part of the overall stopping distance in the Highway Code is better than that seen in practical tests. The additional two seconds is never superfluous because you just don’t know when you will need it.
I was tempted to simply say you are an idiot. Perhaps an explanation of why you are is better.
The discussion wasn't about PRT, and as the insistence was that all things should be kept equal, that's what I did.

BTW, I'll ignore the adhom - I won't sink to your level.

Dammit

3,790 posts

207 months

Sunday 26th October 2014
quotequote all
You're confusing a few things there.

During the course of a storm we have a certain number of things that we can work with - the period of the storm (p), the area of the storm (a) and the total number of lightning strikes (n).

Now if for a second we decide that n is evenly distributed throughout both a and p we can see that the smaller our exposure is to a and p then the greater reduction in n.

i.e. the faster we travel through the storm the lower the chance of being struck by lightning.

This is a valid case of a higher speed being safer, in that it reduces the probability of being struck by lightning - all other factors being equal.

It's got nothing to do with braking distance.

It does of course count on the small being very small in area, and the range of speeds available to the car being high enough, but we'll leave that aside. The most important factor is time.

And the risk of being struck by lightning hasn't got anything to do with the odds of a pedestrian walking into the road without looking, or a ped walking out onto a zebra crossing whilst the driver stares out of the side window, wondering whether that's a deer or a pony.



Edited by Dammit on Sunday 26th October 10:30

Phatboy317

801 posts

117 months

Sunday 26th October 2014
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Dammit said:
...It's got nothing to do with braking distance.
Quite

Dammit

3,790 posts

207 months

Sunday 26th October 2014
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So! What have we learned so far?