Default 20mph for Wales

Bmblbzzz
Posts: 6964
Joined: 18 May 2012, 7:56pm
Location: From here to there.

Re: Default 20mph for Wales

Post by Bmblbzzz »

Drop in frequency and severity of collisions is only one part, perhaps the most easily countable part, of the benefits there should be from lower speeds. Less noise and less air pollution could also be expected, and of course are measurable. Increase in walking and cycling will probably lag and are harder to measure accurately. But some of the most important things are almost impossible to measure; such as decrease in social atomisation, or simply are never measured, such as increase in ease of crossing the road (and people attempting to do this).
roubaixtuesday
Posts: 6624
Joined: 18 Aug 2015, 7:05pm

Re: Default 20mph for Wales

Post by roubaixtuesday »

On the significance of the casualty figures. From 20s plenty campaign.

1000012314.jpg
User avatar
[XAP]Bob
Posts: 20244
Joined: 26 Sep 2008, 4:12pm

Re: Default 20mph for Wales

Post by [XAP]Bob »

Are those year on year changes or all compared with the same baseline?
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.
roubaixtuesday
Posts: 6624
Joined: 18 Aug 2015, 7:05pm

Re: Default 20mph for Wales

Post by roubaixtuesday »

[XAP]Bob wrote: 3 Feb 2025, 12:28pm Are those year on year changes or all compared with the same baseline?
Year on year changes as I understand it.

Was from a Facebook post

We have done further analysis on the Welsh urban/village casualty data going back 20 years. By charting the annual %age change from the previous year it shows that, apart from Covid, the 28% reduction after implementing the default 20mph in 23/24 is exceptional. Note that usually 40mph+ and 20/30mph will follow the same pattern, but in 23/24 it was markedly different. The 23/24 reduction is also 2.3 times greater than any previous reduction in casualties. #20splenty



Is be amazed if that doesn't pass any standard significance test, but I guess the question as to whether it sustains is open.
Bmblbzzz
Posts: 6964
Joined: 18 May 2012, 7:56pm
Location: From here to there.

Re: Default 20mph for Wales

Post by Bmblbzzz »

The other thing that stands out on that graph is the increase in casualties as we "returned to normal" after covid.
Pete Owens
Posts: 2547
Joined: 7 Jul 2008, 12:52am

Re: Default 20mph for Wales

Post by Pete Owens »

roubaixtuesday wrote: 3 Feb 2025, 12:42pm Is be amazed if that doesn't pass any standard significance test, but I guess the question as to whether it sustains is open.
The thing about statistical significance is that it is a very strong test. What it says is that given a particular set of figures - and considering no other evidence whatsoever - is there less than a 5% probability of the result being a random event. It is basically similar to the "beyond reasonable doubt" test at criminal trials.

Unfortunately the common usage of the word "significant" means that even strong evidence that doesn't quite meet the test of statistical significance is often claimed to be "insignificant" or even non-existent by political opponents wishing to avoid uncomfortable research findings.

If, given a very extensive intervention over an entire country with a very marked result does not meet the test of statistical significance then it is unlikely that sufficient data could be collected for any highway intervention whatsoever to meet the criterion. And if they really think that it will need 3 years of data to judge the efficacy of the scheme then they really need to stop any reversions to the limit till September 2026.
roubaixtuesday
Posts: 6624
Joined: 18 Aug 2015, 7:05pm

Re: Default 20mph for Wales

Post by roubaixtuesday »

Pete Owens wrote: 3 Feb 2025, 2:48pm
roubaixtuesday wrote: 3 Feb 2025, 12:42pm Is be amazed if that doesn't pass any standard significance test, but I guess the question as to whether it sustains is open.
The thing about statistical significance is that it is a very strong test. What it says is that given a particular set of figures - and considering no other evidence whatsoever - is there less than a 5% probability of the result being a random event. It is basically similar to the "beyond reasonable doubt" test at criminal trials.

Unfortunately the common usage of the word "significant" means that even strong evidence that doesn't quite meet the test of statistical significance is often claimed to be "insignificant" or even non-existent by political opponents wishing to avoid uncomfortable research findings.

If, given a very extensive intervention over an entire country with a very marked result does not meet the test of statistical significance then it is unlikely that sufficient data could be collected for any highway intervention whatsoever to meet the criterion. And if they really think that it will need 3 years of data to judge the efficacy of the scheme then they really need to stop any reversions to the limit till September 2026.
I'm very confident that if covid year is excluded as an outlier, that meets the standard 5% test for significance.
User avatar
[XAP]Bob
Posts: 20244
Joined: 26 Sep 2008, 4:12pm

Re: Default 20mph for Wales

Post by [XAP]Bob »

Bmblbzzz wrote: 3 Feb 2025, 1:10pm The other thing that stands out on that graph is the increase in casualties as we "returned to normal" after covid.
What surprised me was how small those "corrections" were.

If you drop 50% due to "one off" event.
To get back to where you where you need to grow by 100% the next year.
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.
mattheus
Posts: 5929
Joined: 29 Dec 2008, 12:57pm
Location: Western Europe

Re: Default 20mph for Wales

Post by mattheus »

[XAP]Bob wrote: 3 Feb 2025, 3:53pm
Bmblbzzz wrote: 3 Feb 2025, 1:10pm The other thing that stands out on that graph is the increase in casualties as we "returned to normal" after covid.
What surprised me was how small those "corrections" were.

If you drop 50% due to "one off" event.
To get back to where you where you need to grow by 100% the next year.
[The casualties were only down 28%! But i think you know that ... :) ]

Yes, it is a little surprising. I think a lot of people don't realise how slowly UK traffic levels have recovered. Presumably this is mainly down to increased WFH, but i'm just speculating.
it's like our brains don't remember 2019 accurately, we just note that this year is more than last year, etc etc .. And we only really notice exceptionally busy days/weeks - we notice the quiet days less.

A quick google gives:
Main points. Road traffic volume in Wales in 2023 increased by 2.1% compared to 2022, to 29.8 billion vehicle kilometers (bvk) but remained around 2.9% below pre-pandemic levels (30.7bvk in 2019).
29 Aug 2024
n.b. that's only for 2023.
Bmblbzzz
Posts: 6964
Joined: 18 May 2012, 7:56pm
Location: From here to there.

Re: Default 20mph for Wales

Post by Bmblbzzz »

In '21 there were still lockdowns, and more I think in Wales than in England (not sure about Scotland or NI). But also in 20/21 casualties continued falling on 30mph roads but rose on 40s.
User avatar
[XAP]Bob
Posts: 20244
Joined: 26 Sep 2008, 4:12pm

Re: Default 20mph for Wales

Post by [XAP]Bob »

mattheus wrote: 3 Feb 2025, 4:16pm
[XAP]Bob wrote: 3 Feb 2025, 3:53pm
Bmblbzzz wrote: 3 Feb 2025, 1:10pm The other thing that stands out on that graph is the increase in casualties as we "returned to normal" after covid.
What surprised me was how small those "corrections" were.

If you drop 50% due to "one off" event.
To get back to where you where you need to grow by 100% the next year.
[The casualties were only down 28%! But i think you know that ... :) ]

Yes, it is a little surprising. I think a lot of people don't realise how slowly UK traffic levels have recovered. Presumably this is mainly down to increased WFH, but i'm just speculating.
it's like our brains don't remember 2019 accurately, we just note that this year is more than last year, etc etc .. And we only really notice exceptionally busy days/weeks - we notice the quiet days less.

A quick google gives:
Main points. Road traffic volume in Wales in 2023 increased by 2.1% compared to 2022, to 29.8 billion vehicle kilometers (bvk) but remained around 2.9% below pre-pandemic levels (30.7bvk in 2019).
29 Aug 2024
n.b. that's only for 2023.
I was using easy numbers for illustration - not suggesting they were on the chart.

My memory of 2020 is not positive, though it probably should be, I didn't die.

Traffic levels did drop, for obvious reasons, and whilst 20/12 also had similar restrictions to travel.... But the major drop really didn't get "corrected" at all.
So that's a 22% drop for covid, then another 8% the year after, followed by just 6% and 5% (that's only back to 80% of 2018/9).
Maybe that's just picking up the longer term trend... the growth in intermediate years is actually just an artefact of the collapse in covid.

Then we get another 28% drop takes us to 58% of the 2018/9 level.
That's massive.... properly massive.
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.
deeferdonk
Posts: 292
Joined: 11 May 2019, 2:50pm

Re: Default 20mph for Wales

Post by deeferdonk »

It's a bit mind bending to think that some of us may be alive today purely because of the Covid epidemic. The commuter driver that was "destined" to knock us off never made the journey because he was told to work from home.
Carlton green
Posts: 4574
Joined: 22 Jun 2019, 12:27pm

Re: Default 20mph for Wales

Post by Carlton green »

deeferdonk wrote: 4 Feb 2025, 7:39am It's a bit mind bending to think that some of us may be alive today purely because of the Covid epidemic. The commuter driver that was "destined" to knock us off never made the journey because he was told to work from home.
Quite. Added to that some of us might live longer because we cycled (gaining improved health and healthy habits) during the lockdown periods. Maybe not living in such a dangerous place as Wales is a plus too? :wink:
Don’t fret, it’s OK to: ride a simple old bike; ride slowly, walk, rest and admire the view; ride off-road; ride in your raincoat; ride by yourself; ride in the dark; and ride one hundred yards or one hundred miles. Your bike and your choices to suit you.
pwa
Posts: 18226
Joined: 2 Oct 2011, 8:55pm

Re: Default 20mph for Wales

Post by pwa »

Carlton green wrote: 4 Feb 2025, 8:48am
deeferdonk wrote: 4 Feb 2025, 7:39am It's a bit mind bending to think that some of us may be alive today purely because of the Covid epidemic. The commuter driver that was "destined" to knock us off never made the journey because he was told to work from home.
Quite. Added to that some of us might live longer because we cycled (gaining improved health and healthy habits) during the lockdown periods. Maybe not living in such a dangerous place as Wales is a plus too? :wink:
Dangerous in Wales? I must have a look out of the window to see what on earth is going on... :lol:
roubaixtuesday
Posts: 6624
Joined: 18 Aug 2015, 7:05pm

Re: Default 20mph for Wales

Post by roubaixtuesday »

pwa wrote: 4 Feb 2025, 3:22pm Dangerous in Wales? I must have a look out of the window to see what on earth is going on... :lol:
Beware - there be dragons!!
Post Reply