Speed Cameras, are they for safety, or revenue?

[woof]

martine said:

woof said:

It's difficult to find actual current figures of how many motorist are being fined for speeding in recent years but in 2007 it was 1.9 million (in 1997 it was just 700,000) and from the current increase in new digital speed cameras tickets being issued in this "new war on motorists" I expect to see that figure be close to 4 million and raising something in the region of £500,000,000 in revenue.

I think your estimate for number and revenue is wildly out if this from 2013 is accurate (1/2 million motorists and £30m in fines)

Those figures really surprise me - no wonder they've increased the number of speed cameras!!

[Devil2575]

Phatboy317 said:

Except that they themselves seem to have some difficulty in identifying a plausible mechanism to explain the rather extreme slope of the speed/risk graph in their findings - read section 5.1.1

There they suggest that the discrepancy between their results and their own physical model may be because of the fact that human behaviour is not always random, and that people are more likely to appear in a vehicle’s path at a range of 50-60m than at 0-10m or 10-20m.
However, this is not supported by their own data. If that were the case then the distribution of distance covered before the collision, calculated from the case data, would be independent of speed, but it's not.
And there are many other sources of bias due to human behaviour, as they acknowledge themselves, one of which I can think of is that failure to slow down for a dangerous junction might simply be a symptom of the same lack of due care and attention which leads to the crash.

So what do we do? How can we be sure that we're accurately gauging the relationship between speed and accidents, without biases getting in the way?

I'm not sure what you're talking about. I've read section 5.1.1

It's a discussion as to the possible mechanisms by which increasing speed leads to increasing probability of having an accident, but it clearly states that the study was not intended to investigate these. This section is in effect just discussing untested hypothesis.

As I said before for the purpose of the study the causal factors of the accidents are not important, it is simply seeking to indentify a relationship between travelling speed and risk of having an injury accident.

Phatboy317 said:

One way is to conduct your own controlled experiments, and the only really practical way of doing this is by way of computer simulations, which are not difficult to create for anyone with programming skills.
One could, for example, simulate a vehicle, or vehicles, travelling along a mile or two of road, with objects appearing in the road at random times and points along the road, and recording a collision if there's a vehicle within its stopping distance of the object at the time it appears - and then perhaps even calculate and record the collision speed.
As long as you keep things, such as the frequency of hazards appearing, within realistic limits, and no biases are introduced by, for example, having the vehicles spaced apart by less than their stopping distances, or having the hazards appearing within a few metres of the start (real life doesn't have starting points) the results should be reasonable.
One could even run two or more simulations in parallel, with the same random conditions, to judge the effect of changing just one parameter while leaving all else unchanged.

I can almost guarantee that doing this will at least give you a fresh look at the issue.

Well why don't you produce a model then? I don't possess any programming skills.

Your experiment also fails on one score,

and recording a collision if there's a vehicle within its stopping distance of the object at the time it appears

This implies that you would change the distance from the vehicle to the object depending on it's speed (as stopping distance increases with speed) This misses the whole point of what is being pointed out. The faster you are travelling the more likely it is an object will appear inside your stopping distance so not giving you sufficient time to stop.

[Phatboy317]

Devil2575 said:

Well why don't you produce a model then? I don't possess any programming skills.

I did - many years ago.
I'll dig it out and clean it up a bit so it's easier for non-programmers to use, and then send it to you, complete with source code so you can get someone to check it to ensure that I'm not cheating.

Devil2575 said:

Your experiment also fails on one score,

and recording a collision if there's a vehicle within its stopping distance of the object at the time it appears

This implies that you would change the distance from the vehicle to the object depending on it's speed (as stopping distance increases with speed) This misses the whole point of what is being pointed out. The faster you are travelling the more likely it is an object will appear inside your stopping distance so not giving you sufficient time to stop.

I think you'll understand this, amongst other things, once you have played around with the simulator.

[Devil2575]

Phatboy317 said:

Devil2575 said:

Well why don't you produce a model then? I don't possess any programming skills.

I did - many years ago.
I'll dig it out and clean it up a bit so it's easier for non-programmers to use, and then send it to you, complete with source code so you can get someone to check it to ensure that I'm not cheating.

Devil2575 said:

Your experiment also fails on one score,

and recording a collision if there's a vehicle within its stopping distance of the object at the time it appears

This implies that you would change the distance from the vehicle to the object depending on it's speed (as stopping distance increases with speed) This misses the whole point of what is being pointed out. The faster you are travelling the more likely it is an object will appear inside your stopping distance so not giving you sufficient time to stop.

I think you'll understand this, amongst other things, once you have played around with the simulator.

It's a generous offer but I neither have the time, the ability or the inclination.

My computing skills go as far as using MS office, surfing the net and playing games.

However simulation aside I'm still at a loss to understand how the result can be anything else. Higher speed = longer stopping distance. This is basic physics. If you set an object to appear in the road at a fixed distance from a car then the faster tha car is travelling the more likely it is to hit it as at a certain speed that fixed distance will be exceeded by the minimum stopping distance.

[Phatboy317]

Devil2575 said:

However simulation aside I'm still at a loss to understand how the result can be anything else. Higher speed = longer stopping distance. This is basic physics. If you set an object to appear in the road at a fixed distance from a car then the faster tha car is travelling the more likely it is to hit it as at a certain speed that fixed distance will be exceeded by the minimum stopping distance.

For you to set an object to appear in the road at the same fixed distance from a car travelling at a different speed then you'll have to change the time at which you make it appear, for the simple reason that if the car is travelling at a different speed then it won't be at the same place at the same time - that's the bit of basic physics which everyone seems to ignore.

I tried to explain this on another thread but I didn't get very far - some people don't even try to understand.
It's funny how people can immediately see how ridiculous it is to suggest that you won't be at point A at time B if you started your journey later or earlier, even though it's a perfectly valid 'parallel universe' type thought experiment - but they accept without question the idea that you can change your speed and still be at point A at time B, when this doesn't even work as a thought experiment, because you can't change speed without also changing either time or position.
But instead of saying, "OK that's not a valid way, so let's find some other way to figure it out that doesn't require bending the laws of physics", they doggedly stick to their notions.

[singlecoil]

Aha! We are back to the avoiding accidents by the powerful technique of driving faster so as not to be there at the time technique!

[Devil2575]

Phatboy317 said:

For you to set an object to appear in the road at the same fixed distance from a car travelling at a different speed then you'll have to change the time at which you make it appear, for the simple reason that if the car is travelling at a different speed then it won't be at the same place at the same time - that's the bit of basic physics which everyone seems to ignore.

I tried to explain this on another thread but I didn't get very far - some people don't even try to understand.
It's funny how people can immediately see how ridiculous it is to suggest that you won't be at point A at time B if you started your journey later or earlier, even though it's a perfectly valid 'parallel universe' type thought experiment - but they accept without question the idea that you can change your speed and still be at point A at time B, when this doesn't even work as a thought experiment, because you can't change speed without also changing either time or position.
But instead of saying, "OK that's not a valid way, so let's find some other way to figure it out that doesn't require bending the laws of physics", they doggedly stick to their notions.

Ok then, forget the fixed distance. Just set an object to randomly appear in a road on which cars are travelling. The faster a car is travelling the more likely the car will be to hit an object that appears anywhere in the road in front of it. The results of individual events (i.e. one car and one object) will vary depending on the specifics of that situation, but over a large number of events the data will show that the faster a car is travelling the more likely it is to hit the object.

Think of a stretch of road. There are two important distances:
1 The distance that the driver can see to be clear. This is independant of speed
2 The distance that the car can stop inside. This is dependent on speed.

The difference between the two distances is the space where an object can appear and the car be able to avoid hitting it.

The faster a car is travelling the shorter that space will be.

Therefore an object appearing in the road inside the distance the driver can see to be clear randomly is more likely to appear within the distance that the car requires to stop the faster the car is travelling.

Now there will be some mitigation because in some instances the car travelling faster will be past the point where the object appears (whereas the slower car won't) so will not even see it, however the relationship between speed, distance and time is linear. The relationship between braking distance and speed is not. Therefore this mitigation effect will not cancel out the increased risk of impact.
This also provides no mitigation against a case where an object is in the road from the outset rather than simply appearing and would be especially relevant to smallish object that might not be easy to see yet could do damage, such as a brick. You'd struggle to see it in the distance it would take to stop from higher speeds.

Edited by Devil2575 on Tuesday 27th January 14:10

[Phatboy317]

Devil2575 said:

Ok then, forget the fixed distance. Just set an object to randomly appear in a road on which cars are travelling. The faster a car is travelling the more likely the car will be to hit an object that appears anywhere in the road in front of it. The results of individual events (i.e. one car and one object) will vary depending on the specifics of that situation, but over a large number of events the data will show that the faster a car is travelling the more likely it is to hit the object.

Think of a stretch of road. There are two important distances:
1 The distance that the driver can see to be clear. This is independant of speed
2 The distance that the car can stop inside. This is dependent on speed.

The difference between the two distances is the space where an object can appear and the car be able to avoid hitting it.

The faster a car is travelling the shorter that space will be.

Therefore an object appearing in the road inside the distance the driver can see to be clear randomly is more likely to appear within the distance that the car requires to stop the faster the car is travelling.

Now there will be some mitigation because in some instances the car travelling faster will be past the point where the object appears (whereas the slower car won't) so will not even see it, however the relationship between speed, distance and time is linear. The relationship between braking distance and speed is not. Therefore this mitigation effect will not cancel out the increased risk of impact.
This also provides no mitigation against a case where an object is in the road from the outset rather than simply appearing and would be especially relevant to smallish object that might not be easy to see yet could do damage, such as a brick. You'd struggle to see it in the distance it would take to stop from higher speeds.

Edited by Devil2575 on Tuesday 27th January 14:10

I never suggested that the risk doesn't increase with speed, but I do take issue with people coming up with the highly exaggerated relationship which they do.

I'll give you a better answer this evening - work to do now

Edited by Phatboy317 on Tuesday 27th January 14:32

[singlecoil]

Phatboy317 said:

I never suggested that the risk doesn't increase with speed, but I do take issue with people coming up with the highly exaggerated relationship which they do.

Please give us an example of the sort of thing you are referring to, failing that we will have to assume this is yet another straw man.

[Devil2575]

Phatboy317 said:

I never suggested that the risk doesn't increase with speed, but I do take issue with people coming up with the highly exaggerated relationship which they do.

But that "highly exaggerated risk" is based on data.

Your assessment of the risk is based on theory, which may not take into account other factors, such as other roads users not expecting you to be travelling faster than a certain speed.

[Phatboy317]

Devil2575 said:

Phatboy317 said:

I never suggested that the risk doesn't increase with speed, but I do take issue with people coming up with the highly exaggerated relationship which they do.

But that "highly exaggerated risk" is based on data.

That's why I'm suspicious of their methodology.

They find that the risk doubles between 60km/h and 65km/h, but the theoretical model can only explain a 5% increase in risk, and even less if you use their assumed driver reaction time of 1.5 seconds.
There's a very far cry between 5% increased risk and 100% increased risk.

Devil2575 said:

Your assessment of the risk is based on theory, which may not take into account other factors, such as other roads users not expecting you to be travelling faster than a certain speed.

As an example: I'm standing at a junction waiting to cross. There's one car coming along which I correctly judge to be doing 30mph. I also judge that it's going to take me four seconds to clear the junction, and that it's around 60 metres away so it will take a bit more than 4 seconds to reach the junction without having to slow down, so I pull out of the junction and cross safely.
But if I'd misjudged its speed, then how fast would it have to be going before it would result in a collision? It would have to be doing close on 60mph to not be able to stop in time, so I would have had to grossly misjudge the speed before it became a massive problem.
Had it been doing 35 to 40mph then it would only have had to slack off a bit to avoid a collision.
However, had I wrongly judged that it would only take me two seconds to clear the junction, I might have pulled out when the car was less than 30 metres away, so it would have to do an emergency stop to avoid hitting me even if it was only doing 30.

[MGJohn]

singlecoil said:

Aha! We are back to the avoiding accidents by the powerful technique of driving faster so as not to be there at the time technique!

Correct. The Warp Factor scenario ...

It's the head gasket Cap'n ... They all do that !

[MrTrilby]

Phatboy317 said:

As an example: I'm standing at a junction waiting to cross. There's one car coming along which I correctly judge to be doing 30mph. I also judge that it's going to take me four seconds to clear the junction, and that it's around 60 metres away so it will take a bit more than 4 seconds to reach the junction without having to slow down, so I pull out of the junction and cross safely.
But if I'd misjudged its speed, then how fast would it have to be going before it would result in a collision? It would have to be doing close on 60mph to not be able to stop in time, so I would have had to grossly misjudge the speed before it became a massive problem.
Had it been doing 35 to 40mph then it would only have had to slack off a bit to avoid a collision.
However, had I wrongly judged that it would only take me two seconds to clear the junction, I might have pulled out when the car was less than 30 metres away, so it would have to do an emergency stop to avoid hitting me even if it was only doing 30.

You make a number of undeclared assumptions.

You assume that the oncoming driver is also able to accurately judge that you will clear the junction in time. In your example of "if I'd wrongly judged it would take 2 seconds clear the junction", you seem to be saying that you pull in front of the oncoming car when it is 30m away, and so it has plenty of time to stop: that assumes the oncoming car correctly anticipates the collision, and begins an emergency stop immediately, rather than thinking "hang on, I'm not sure he's going to clear that in time... oh wait, he's definitely not, I'd better do an emergency stop". Just as the person crossing the junction don't always perfectly judge whether they have time, the oncoming driver will also not always perfectly judge when an emergency stop is required and when "it'll just about be ok", and may leave it too late. Your model does not account for this.

More fundamentally, you assume both that the person crossing has even observed the oncoming car, and that the oncoming car has observed the person crossing. Both assumptions are risky, and if false, the "driver has time to perform an emergency stop" action becomes irrelevant, because the driver potentially never initiates the emergency stop, or at least, not until very much later. You cannot assume that the likelihood of the two participants observing each other is unrelated to speed, or that it is a simple relationship.

Edited by MrTrilby on Tuesday 27th January 22:41

[Devil2575]

Phatboy317 said:

Devil2575 said:

Phatboy317 said:

I never suggested that the risk doesn't increase with speed, but I do take issue with people coming up with the highly exaggerated relationship which they do.

But that "highly exaggerated risk" is based on data.

That's why I'm suspicious of their methodology.

They find that the risk doubles between 60km/h and 65km/h, but the theoretical model can only explain a 5% increase in risk, and even less if you use their assumed driver reaction time of 1.5 seconds.
There's a very far cry between 5% increased risk and 100% increased risk.

Where did you get 5% from. Also what model?

Phatboy317 said:

Devil2575 said:

Your assessment of the risk is based on theory, which may not take into account other factors, such as other roads users not expecting you to be travelling faster than a certain speed.

As an example: I'm standing at a junction waiting to cross. There's one car coming along which I correctly judge to be doing 30mph. I also judge that it's going to take me four seconds to clear the junction, and that it's around 60 metres away so it will take a bit more than 4 seconds to reach the junction without having to slow down, so I pull out of the junction and cross safely.
But if I'd misjudged its speed, then how fast would it have to be going before it would result in a collision? It would have to be doing close on 60mph to not be able to stop in time, so I would have had to grossly misjudge the speed before it became a massive problem.
Had it been doing 35 to 40mph then it would only have had to slack off a bit to avoid a collision.
However, had I wrongly judged that it would only take me two seconds to clear the junction, I might have pulled out when the car was less than 30 metres away, so it would have to do an emergency stop to avoid hitting me even if it was only doing 30.

You've created a hypothetical situation to produce a result to make your point.

The problem is relative risk doesn't deal with individual situations. What it does is tell you what will happen in a large number of events.

The probability of getting heads when I toss a coin is 0.5. But that doesn't tell me the result of a coin toss and in fact it's perfectly possible to get a run of 10 tails. What it does tell me is that with a large number of coin tosses you will get 50% heads.

[Phatboy317]

Devil2575 said:

Phatboy317 said:

Devil2575 said:

Phatboy317 said:

I never suggested that the risk doesn't increase with speed, but I do take issue with people coming up with the highly exaggerated relationship which they do.

But that "highly exaggerated risk" is based on data.

That's why I'm suspicious of their methodology.

They find that the risk doubles between 60km/h and 65km/h, but the theoretical model can only explain a 5% increase in risk, and even less if you use their assumed driver reaction time of 1.5 seconds.
There's a very far cry between 5% increased risk and 100% increased risk.

Where did you get 5% from. Also what model?

You said my assessment of the risk is based on theory, so I assumed you accepted the theoretical basis.
But if you need it explaining then I'll try to accommodate you - but not before tomorrow evening - I'm off to bed now and I have a very busy work schedule tomorrow,

Devil2575 said:

Phatboy317 said:

Devil2575 said:

Your assessment of the risk is based on theory, which may not take into account other factors, such as other roads users not expecting you to be travelling faster than a certain speed.

As an example: I'm standing at a junction waiting to cross. There's one car coming along which I correctly judge to be doing 30mph. I also judge that it's going to take me four seconds to clear the junction, and that it's around 60 metres away so it will take a bit more than 4 seconds to reach the junction without having to slow down, so I pull out of the junction and cross safely.
But if I'd misjudged its speed, then how fast would it have to be going before it would result in a collision? It would have to be doing close on 60mph to not be able to stop in time, so I would have had to grossly misjudge the speed before it became a massive problem.
Had it been doing 35 to 40mph then it would only have had to slack off a bit to avoid a collision.
However, had I wrongly judged that it would only take me two seconds to clear the junction, I might have pulled out when the car was less than 30 metres away, so it would have to do an emergency stop to avoid hitting me even if it was only doing 30.

You've created a hypothetical situation to produce a result to make your point.

The problem is relative risk doesn't deal with individual situations. What it does is tell you what will happen in a large number of events.

The probability of getting heads when I toss a coin is 0.5. But that doesn't tell me the result of a coin toss and in fact it's perfectly possible to get a run of 10 tails. What it does tell me is that with a large number of coin tosses you will get 50% heads.

No. That example was to lend some perspective to your point about other roads users not expecting you to be travelling faster than a certain speed - it has nothing to do with the relative risk factors.

[Phatboy317]

MrTrilby said:

You make a number of undeclared assumptions.

You assume that the oncoming driver is also able to accurately judge that you will clear the junction in time. In your example of "if I'd wrongly judged it would take 2 seconds clear the junction", you seem to be saying that you pull in front of the oncoming car when it is 30m away, and so it has plenty of time to stop: that assumes the oncoming car correctly anticipates the collision, and begins an emergency stop immediately, rather than thinking "hang on, I'm not sure he's going to clear that in time... oh wait, he's definitely not, I'd better do an emergency stop". Just as the person crossing the junction don't always perfectly judge whether they have time, the oncoming driver will also not always perfectly judge when an emergency stop is required and when "it'll just about be ok", and may leave it too late. Your model does not account for this.

More fundamentally, you assume both that the person crossing has even observed the oncoming car, and that the oncoming car has observed the person crossing. Both assumptions are risky, and if false, the "driver has time to perform an emergency stop" action becomes irrelevant, because the driver potentially never initiates the emergency stop, or at least, not until very much later. You cannot assume that the likelihood of the two participants observing each other is unrelated to speed, or that it is a simple relationship.

Edited by MrTrilby on Tuesday 27th January 22:41

Try to read it within the context of the point I was replying to.
It seems you're reading things into it which aren't there.

[MrTrilby]

Phatboy317 said:

Try to read it within the context of the point I was replying to.
It seems you're reading things into it which aren't there.

I did. And what you've said is simply wrong.

[Phatboy317]

MrTrilby said:

Phatboy317 said:

Try to read it within the context of the point I was replying to.
It seems you're reading things into it which aren't there.

I did. And what you've said is simply wrong.

I can't decipher what it is that you imagine I meant, so I'm not going to try.

[MrTrilby]

Phatboy317 said:

I can't decipher what it is that you imagine I meant, so I'm not going to try.

You're being obtuse, presumably because you don't like admitting that you're wrong. I pointed out that you'd made some significant assumptions, that are flawed, and undermine your argument.

[Phatboy317]

MrTrilby said:

Phatboy317 said:

I can't decipher what it is that you imagine I meant, so I'm not going to try.

You're being obtuse, presumably because you don't like admitting that you're wrong. I pointed out that you'd made some significant assumptions, that are flawed, and undermine your argument.

You said:

In your example of "if I'd wrongly judged it would take 2 seconds clear the junction", you seem to be saying that you pull in front of the oncoming car when it is 30m away, and so it has plenty of time to stop

What I actually said:

However, had I wrongly judged that it would only take me two seconds to clear the junction, I might have pulled out when the car was less than 30 metres away, so it would have to do an emergency stop to avoid hitting me even if it was only doing 30.

...which is quite the opposite of what you think I said.

Try reading things properly before jumping to conclusions.