IAM make a woeful mistake
Discussion
The IAM have just issued a press release that concludes from these statistics that “20mph roads are not delivering
fewer casualties”. A similar conclusion was made by the Sun in August 2012 from the 2011 DfT casualty report. The
BBC “More or Less” program then felt that this was such a blatant misrepresentation that they featured the story in
a programme. Their conclusion was that the claims were “phonus ballonus” and without a consideration of how the
number and length of 20/30mph roads had changed then no conclusions could be made as to whether setting a
20mph limit made the roads more dangerous.
One valid statistic that can be gleaned from the 2013 casualty figures is the risk of death for casualties in 20mph and
30mph roads. On 20mph roads in 2013 there were 6 deaths out of 3,164 casualties (0.2%) whilst on 30mph roads
there were 538 deaths out of 111,186 casualties (0.5%). Hence on a 30mph road any casualty is 2.5 times more likely
to result in a fatality than on a 20mph road.
http://www.20splentyforus.org.uk/PRel/bogusIAMrep....
Rod King MBE, Founder and Campaign Director of 20’s Plenty for Us commented:-
“We are amazed and disappointed that the Institute of Advanced Motorists should make such a woeful mistake in
their conclusions from the DfT statistics. In fact, wherever 20mph limits have been piloted, on looking at the
casualties in detail, councils have concluded that there was a positive effect on road safety and subsequently widened
the implementation across most areas. Now 20% of the UK population live in towns, cities or villages where the
Traffic Authority is convinced that 20 is plenty for most streets. The IAM conclusion is bogus and reflects a poor
understanding of either the changing numbers and success of 20mph limits or basic statistics”
fewer casualties”. A similar conclusion was made by the Sun in August 2012 from the 2011 DfT casualty report. The
BBC “More or Less” program then felt that this was such a blatant misrepresentation that they featured the story in
a programme. Their conclusion was that the claims were “phonus ballonus” and without a consideration of how the
number and length of 20/30mph roads had changed then no conclusions could be made as to whether setting a
20mph limit made the roads more dangerous.
One valid statistic that can be gleaned from the 2013 casualty figures is the risk of death for casualties in 20mph and
30mph roads. On 20mph roads in 2013 there were 6 deaths out of 3,164 casualties (0.2%) whilst on 30mph roads
there were 538 deaths out of 111,186 casualties (0.5%). Hence on a 30mph road any casualty is 2.5 times more likely
to result in a fatality than on a 20mph road.
http://www.20splentyforus.org.uk/PRel/bogusIAMrep....
Rod King MBE, Founder and Campaign Director of 20’s Plenty for Us commented:-
“We are amazed and disappointed that the Institute of Advanced Motorists should make such a woeful mistake in
their conclusions from the DfT statistics. In fact, wherever 20mph limits have been piloted, on looking at the
casualties in detail, councils have concluded that there was a positive effect on road safety and subsequently widened
the implementation across most areas. Now 20% of the UK population live in towns, cities or villages where the
Traffic Authority is convinced that 20 is plenty for most streets. The IAM conclusion is bogus and reflects a poor
understanding of either the changing numbers and success of 20mph limits or basic statistics”
OTBC said:
One valid statistic that can be gleaned from the 2013 casualty figures is the risk of death for casualties in 20mph and 30mph roads. On 20mph roads in 2013 there were 6 deaths out of 3,164 casualties (0.2%) whilst on 30mph roads there were 538 deaths out of 111,186 casualties (0.5%). Hence on a 30mph road any casualty is 2.5 times more likely to result in a fatality than on a 20mph road.
I don't believe that conclusion is correct. You need to factor in the miles of road at each speed and the miles travelled on those roads.I would hazard a guess that there are far more than 2.5 times miles of 30 mph road than 20 mph road, from which you could therefore draw the conclusion that the 20 mph roads are more dangerous.
PurpleMoonlight said:
OTBC said:
One valid statistic that can be gleaned from the 2013 casualty figures is the risk of death for casualties in 20mph and 30mph roads. On 20mph roads in 2013 there were 6 deaths out of 3,164 casualties (0.2%) whilst on 30mph roads there were 538 deaths out of 111,186 casualties (0.5%). Hence on a 30mph road any casualty is 2.5 times more likely to result in a fatality than on a 20mph road.
I don't believe that conclusion is correct.PurpleMoonlight said:
You need to factor in the miles of road at each speed and the miles travelled on those roads.
Not to make the statement OTBC made you don't.PurpleMoonlight said:
I would hazard a guess that there are far more than 2.5 times miles of 30 mph road than 20 mph road, from which you could therefore draw the conclusion that the 20 mph roads are more dangerous.
Not sure what you mean there, but for the conclusion you're trying to draw do you need to take into account that there are over 35 times as many casualties on 30mph roads as 20mph roads?SK425 said:
Not sure what you mean there, but for the conclusion you're trying to draw do you need to take into account that there are over 35 times as many casualties on 30mph roads as 20mph roads?
I guess it depends on the miles of roads applicable at each speed limit.If there is 100 times more roads at 30 than 20, wouldn't that make the 30 safer?
The 6 fatalities in 20mph zones is a low number - I'd like to see someone with more statistical knowledge than me say if that is statistically conclusive or not.
People using words like "amazed and disappointed" or "woeful" also smacks of someone with an agenda to push, which may not be based on facts. If it so obvious that IAM are wrong, I'd like to see a full statistical analysis of all the data to prove it.
People using words like "amazed and disappointed" or "woeful" also smacks of someone with an agenda to push, which may not be based on facts. If it so obvious that IAM are wrong, I'd like to see a full statistical analysis of all the data to prove it.
Edited by creampuff on Friday 4th July 19:27
OTBC said:
One valid statistic that can be gleaned from the 2013 casualty figures is the risk of death for casualties in 20mph and 30mph roads. On 20mph roads in 2013 there were 6 deaths out of 3,164 casualties (0.2%) whilst on 30mph roads there were 538 deaths out of 111,186 casualties (0.5%). Hence on a 30mph road any casualty is 2.5 times more likely to result in a fatality than on a 20mph road.
Ten years of STATS19 data (2001 to 2010) for pedestrian casualties in built-up areas reveals that there were 10 deaths out of 1739 casualties (0.58%) on 20mph roads, compared with 785 deaths out of 105626 casualties (0.74%) on 30mph roads.PurpleMoonlight said:
I guess it depends on the miles of roads applicable at each speed limit.
If there is 100 times more roads at 30 than 20, wouldn't that make the 30 safer?
Depends what you mean by safer. If you want to use statistics to back up your conclusion you need to be precise about what you're claiming. I'd note that the OP didn't say "20 limits are safer", he just compared the proportion of casualties that are fatailities in 20 limits and in 30 limits. If there is 100 times more roads at 30 than 20, wouldn't that make the 30 safer?
SK425 said:
Depends what you mean by safer. If you want to use statistics to back up your conclusion you need to be precise about what you're claiming. I'd note that the OP didn't say "20 limits are safer", he just compared the proportion of casualties that are fatailities in 20 limits and in 30 limits.
Many 20mph zones, I would only feel comfortable doing 20mph due to pedestrian activity or other factors. OTOH many 30mph limit areas, I would feel comfortable doing over 30mph, except the speed limit is 30mph.
Thus the lower casualty rate may be due to the nature of the road, where drivers are more cautious anyway regardless of the speed limit.
t.fangio said:
Good grief, are people really this stupid? Fangio, in year one there are ten injuries on a hundred miles of 20mph roads. In year two there are fifteen injuries on three hundred miles of 20mph roads. Does that suggest to you that the risk increases? I mean, really?OTBC said:
The IAM have just issued a press release that concludes from these statistics that “20mph roads are not delivering
fewer casualties”. A similar conclusion was made by the Sun in August 2012 from the 2011 DfT casualty report. The
BBC “More or Less” program then felt that this was such a blatant misrepresentation that they featured the story in
a programme. Their conclusion was that the claims were “phonus ballonus” and without a consideration of how the
number and length of 20/30mph roads had changed then no conclusions could be made as to whether setting a
20mph limit made the roads more dangerous.
One valid statistic that can be gleaned from the 2013 casualty figures is the risk of death for casualties in 20mph and
30mph roads. On 20mph roads in 2013 there were 6 deaths out of 3,164 casualties (0.2%) whilst on 30mph roads
there were 538 deaths out of 111,186 casualties (0.5%). Hence on a 30mph road any casualty is 2.5 times more likely
to result in a fatality than on a 20mph road.
http://www.20splentyforus.org.uk/PRel/bogusIAMrep....
Rod King MBE, Founder and Campaign Director of 20’s Plenty for Us commented:-
“We are amazed and disappointed that the Institute of Advanced Motorists should make such a woeful mistake in
their conclusions from the DfT statistics. In fact, wherever 20mph limits have been piloted, on looking at the
casualties in detail, councils have concluded that there was a positive effect on road safety and subsequently widened
the implementation across most areas. Now 20% of the UK population live in towns, cities or villages where the
Traffic Authority is convinced that 20 is plenty for most streets. The IAM conclusion is bogus and reflects a poor
understanding of either the changing numbers and success of 20mph limits or basic statistics”
But there's numerous equally bad and erroneous assumptions in all the pro-20 points you make as well! But then you are blind to that aren't you.fewer casualties”. A similar conclusion was made by the Sun in August 2012 from the 2011 DfT casualty report. The
BBC “More or Less” program then felt that this was such a blatant misrepresentation that they featured the story in
a programme. Their conclusion was that the claims were “phonus ballonus” and without a consideration of how the
number and length of 20/30mph roads had changed then no conclusions could be made as to whether setting a
20mph limit made the roads more dangerous.
One valid statistic that can be gleaned from the 2013 casualty figures is the risk of death for casualties in 20mph and
30mph roads. On 20mph roads in 2013 there were 6 deaths out of 3,164 casualties (0.2%) whilst on 30mph roads
there were 538 deaths out of 111,186 casualties (0.5%). Hence on a 30mph road any casualty is 2.5 times more likely
to result in a fatality than on a 20mph road.
http://www.20splentyforus.org.uk/PRel/bogusIAMrep....
Rod King MBE, Founder and Campaign Director of 20’s Plenty for Us commented:-
“We are amazed and disappointed that the Institute of Advanced Motorists should make such a woeful mistake in
their conclusions from the DfT statistics. In fact, wherever 20mph limits have been piloted, on looking at the
casualties in detail, councils have concluded that there was a positive effect on road safety and subsequently widened
the implementation across most areas. Now 20% of the UK population live in towns, cities or villages where the
Traffic Authority is convinced that 20 is plenty for most streets. The IAM conclusion is bogus and reflects a poor
understanding of either the changing numbers and success of 20mph limits or basic statistics”
OTBC said:
One valid statistic that can be gleaned from the 2013 casualty figures is the risk of death for casualties in 20mph and
30mph roads. On 20mph roads in 2013 there were 6 deaths out of 3,164 casualties (0.2%) whilst on 30mph roads
there were 538 deaths out of 111,186 casualties (0.5%). Hence on a 30mph road any casualty is 2.5 times more likely
to result in a fatality than on a 20mph road.
Unfortunately, that's not how statistics work. You can't simply take the difference between the two and use that as evidence of a significant difference. If you really want to do it properly you need to conduct a significance test and work out the p value which will tell you how strong the difference is. That's after you've worked out the null hypothesis of course.30mph roads. On 20mph roads in 2013 there were 6 deaths out of 3,164 casualties (0.2%) whilst on 30mph roads
there were 538 deaths out of 111,186 casualties (0.5%). Hence on a 30mph road any casualty is 2.5 times more likely
to result in a fatality than on a 20mph road.
If you want to use an incidence rate (which is what it appears you're trying to do) in order to measure risk you need more data than you have already and you also need to calculate it properly usually by a+b/c+d which will give you a ratio of odds. For risk, you'll need to do something similar but again, you don't have the data.
Sorry but your analysis is completely wrong.
Edited by Osinjak on Saturday 5th July 00:36
OTBC said:
One valid statistic that can be gleaned from the 2013 casualty figures is the risk of death for casualties in 20mph and
30mph roads. On 20mph roads in 2013 there were 6 deaths out of 3,164 casualties (0.2%) whilst on 30mph roads
there were 538 deaths out of 111,186 casualties (0.5%). Hence on a 30mph road any casualty is 2.5 times more likely
to result in a fatality than on a 20mph road.
Very poor interpretive skills. 30mph roads. On 20mph roads in 2013 there were 6 deaths out of 3,164 casualties (0.2%) whilst on 30mph roads
there were 538 deaths out of 111,186 casualties (0.5%). Hence on a 30mph road any casualty is 2.5 times more likely
to result in a fatality than on a 20mph road.
Say scenario 1 presents a 0.00001 risk of death. And scenario 2 presents a 0.00000001 risk of death
OMG! SCENARIO 1 IS 1000 TIMES MORE LIKELY TO KILL YOU THAN SCENARIO 2! BAN SCENARIO 1 IMMEDIATELY'
Except that statistically scenario 1 is incredibly safe. Unbelievably safe. Truly, implausibly safe. In this instance, you could be run over in a 30 zone 200 times before you're killed. How many of those lives have you used up? None?
Concentrate your efforts on fat people eating too much sugar and fat. You'll make more of a differnece.
Edited by anonymous-user on Saturday 5th July 00:23
SK425 said:
Depends what you mean by safer. If you want to use statistics to back up your conclusion you need to be precise about what you're claiming. I'd note that the OP didn't say "20 limits are safer", he just compared the proportion of casualties that are fatailities in 20 limits and in 30 limits.
Surely to compare that you are 2.5 times more likely to die in an accident on a 30 mph road than a 20 mph road infers that the 20 mph road is safer?Forums | Speed, Plod & the Law | Top of Page | What's New | My Stuff