As some of you are aware, the Tasmanian Government (State Roads) was an earlier adopter of VALID’s approach to tree risk-benefit management and assessment. They’ve been completing the move to their new VALID Strategy from July with Basic Drive-by Assessments being carried out in high use zones as part of their 5 yearly Active Assessment programme. They’ll soon be publishing their Strategy and we’ll share it when it goes live.
One piece of work they asked us to do in this project was to write a short briefing note for the General Manager State Roads, Department of State Growth (DSG), making the case for and explaining why they were upgrading their approach from QTRA to VALID. This is some of the work we were party to with DSG’s tree risk framework review team.
We’ve not shared this before because we thought it was a confidential bit of in-house work. However, it turns out this is not the case because the DSG is a government agency and the work is in the public domain. As many of you working at the tree risk coalface might find it of interest, we’re now sharing and you can download it here:
During VALID's recent UK Summer tree risk training tour, I popped into Dunham Massey to check out the magnificent trees. Whilst walking amongst the Limes and Beech on the northern boundary, this is the sound of the traffic passing by.
Traffic Occupancy | High
I can't see the traffic because of the boundary wall, but I know the Likelihood of Occupancy on Smithy Lane (B5160) is High just by the sound.
The rhythm of this traffic is on average in the middle of High. Of course, traffic is messy and it pulses both higher and lower than the average during the one and half minutes of the recording, and during the day. But one of the many benefits of VALID is we've had a tame mathematician do the clever stuff and render complex traffic numberwang into easy to understand decision-making.
When you're a Validator, you'll have been calibrated so that you can make the O for Occupancy decisions for Very High, High, and Moderate based on what you can see (or hear) whilst you're on site.
You make this decision in the time it takes you to walk to the tree. You make it with your (calibrated) gut and it's effortless.
You're not expected to do any numberwang.
Or try to find traffic data that might not exist.
Or label it with a word that gives the illusion of communication and is open to interpretation.
If you're not sure about the Occupancy, then all you need to do is ask on the website's Community page and we'll tell you.
We know from the available data that the overall risk from Summer Branch Drop is mindbogglingly low.
The risk is so low, our risk of death from driving for the few minutes it takes to cover 3km/2mi is higher than from Summer Branch Drop over a whole year.
Yet, fear-mongering and threats from risk entrepreneurs about legal action if such a mindbogglingly low risk happens are pushing some anxious duty holders into wasting time and money on putting up confusing and ineffective signs; like this recent photo at Scone Palace, Scotland.
Not only are the costs of commissioning and managing these signs grossly disproportionate to any questionable reduction in a risk that's already Acceptable. Their ineffectiveness may backfire on the duty holder and create an opportunity for a risk entrepreneur to act as an expert in the extremely unlikely event of someone being injured or killed. It'd be all too easy for them to claim the duty holder hadn't managed the risk well enough with signs and they could've reduced it more by fencing or pruning. Or if the visitor wasn't an English speaker, why wasn't the sign translated into their language.
If you're a duty holder, or advise one, worry not about Summer Branch Drop and where, when, and how many signs to put up - or how many languages to use. We've got you covered with our handy Summer Branch Drop Guide on our Risk Management page.
It’s always valuable to see how others view your work because they’re seeing it with different eyes.
We were recently asked by a State Government client to publish a standalone summary of our Tree Risk-Benefit Management Strategy because it was the easiest way for their senior managers to understand how they were updating their approach to managing the risk.
The Strategy Summary is now available on the home page of our website.
We're updating all our publications from Tree Risk-Benefit Assessment & Management to Tree Risk-Benefit Management and Assessment.
We thought we'd gotten past looking at tree risk through the eyes of an Arborist, rather than a Duty Holder, with v5.0 of the Strategy. This is where creating a flowchart helped reveal that the most valuable risk management asset is Passive Assessment by anyone. Not Active Assessment by an Arborist.
A 5 yearly Active Assessment isn't being topped up by Passive Assessment, as we used to propose. Passive Assessment is being carried out all of the time, day in day out. It's being topped up by Active Assessment in zones of high confluence every 5 years.
Well, in the process of completing our work helping the Tasmanian Government manage their tree risk, David was asked whether we could produce a summary to explain what VALID is about. It's for the strategic decision-makers, they explained.
"'What is VALID?', does that", David said.
"In 'What is VALID?', the first picture is of a woman looking at a VALID likelihood of failure decision in the Tree Risk App. That's an Active Assessment at a Detailed level", they replied.
"That level of investigation is not the first thing in the minds of Tasmanian Government decision-makers when it comes to adopting VALID's approach to Tree Risk-Benefit Management, is it?"
Hence the upgrade and the new, 'What is VALID?'
Jeremy Barrell’s Arb Magazine version of his journal article has had some Duty Holders and Arborists ask us about where his binary, ‘High’ or ‘Low’ likelihood of occupancy approach sits with VALID.
To recap. The heart of the article is three legal cases cherry-picked by Jeremy where Jeremy acted as an expert and are Jeremy’s interpretations of these cases where Jeremy’s evidence was a key factor.
When we first went through the article, we were disappointed to find it appeared to be less about reasonable, proportionate, and reasonably practicable tree risk-benefit management (which is what the Courts and Coroners want) and more about setting out what Jeremy expects to contest as an ‘expert’.
Back to the binary ‘High’ v ‘Low’ likelihood of occupancy. VALID’s likelihood of occupancy categories are based on log base 10, like the Richter scale uses to measure earthquakes. If we show the likelihood of occupancy to scale as 10 x 10 canvases, and set High at the centre, we can compare VALID’s and Jeremy’s likelihood of occupancy categories. Jeremy’s High occupancy spans 4 orders of magnitude. Or, another way of looking at it. A Richter scale 4 earthquake is categorised the same as a Richter scale 7 earthquake.
We know from pedestrian data the centre of built-up areas have a likelihood of occupancy that means on average more than one person is exposed to the risk. Occupancy is Very High.
We also know the busiest roads have a likelihood of occupancy that means on average more than one vehicle is exposed to the risk. Occupancy is Very High.
Where we have busy roads next to busy footpaths in towns and cities, we know that the combined occupancy of people and traffic means on average more than one person AND one vehicle is exposed to the risk. Occupancy is Very High.
From the training we’ve delivered to those who have upgraded their approach to tree risk, we know that tree risk assessors have been poorly trained to recognise Very High and High Occupancy.
What all this means is that unless you’re using VALID, and have had likelihood of occupancy training (it’s really easy), you’ll be undervaluing the risk where it matters most.
You’ll most likely be undervaluing the risk by at least a whopping factor of x10 just on the ‘targets’ you’ve chosen.
In the previous post we looked at VALID's Goldilocks Likelihood of Occupancy canvas to explore categories that are not too wide, and not too narrow, that are just right.
This Likelihood of Occupancy canvas is useful to examine tree risk decision-making at the High Court in the UK’s landmark Poll v Bartholomew Judgment.
In Poll, a motorcyclist was seriously injured by a falling Ash stem. The Judge found for the Claimant because the tree was ‘High Risk.’
In their reports, the Claimant’s expert said the tree was ‘High Risk’ and the Defendant’s expert didn’t mention any risk. Yet in their Joint Statement, the experts agreed the tree was a ‘Medium Risk.’
Naturally, the expert’s opinions left the court scratching its head and it had to ask them to produce a Second Joint Statement to define what they meant by high and medium risk.
In the Second Joint Statement, the experts told the court the tree was ‘High Risk’. However, they concluded the risk was high after they'd assessed the Likelihood of Occupancy for a minor road at 50%, when in fact it was 1%. The experts overvalued the occupancy by a whopping factor of 50. This gaffe was so enormous the tree was in fact a ‘Medium Risk’ and not a ‘High Risk.’
The Judge would’ve found for the Defendant if the tree was a ‘Medium Risk.’
In the Scale of the Problem we saw the overall risk from branches and trees falling is so extremely low we need a microscope to see it.
The only sensible way to measure risks that go this low is to use a logarithmic scale. It turns out the Goldilocks logarithmic scale for tree risk that’s not too narrow, and not too wide, that's just right is log base 10. Just like the Richter Scale is for measuring earthquakes.
So far so numberwang. What does that mean for you?
Well, when it comes to Likelihood of Occupancy decision-making, the advantages of log base 10 are obvious when drawn to scale. There’s 5 colour-coded Likelihood of Occupancy categories in VALID. If we centre High, then you can only see a bit of the heel of Very High. Nearly all of it’s off the screen. You can make sense of Moderate but you can barely make out Low. And you can’t see Very Low at all.
So, the first decision a Validator makes with Likelihood of Occupancy is what 3 categories can’t it be? Which 3 make no sense? Once calibrated this is an effortless decision. Then it comes down to one of two. Usually, which one is the most obvious because they're huge canvases. If in doubt you go for the higher one.
We regularly use visuals to convey both the context of tree risk and how you can measure it. Recently, we shared a post with Professor David Ball's quote that the prospects of reducing the risk below the current level were comparable to finding a microscopic needle in a gargantuan haystack.
To help convey that finding a microscopic needle in a gargantuan haystack simile further, we've had a play around with illustrating the tree risk context using coloured spectrums.
In the graphic, the top spectrum shows the reality of tree risk to scale. We know that compared to other everyday risks we readily accept, the overall risk to us from branches or trees falling is extremely low. Our annual risk of being killed or seriously injured is less than one in a million. At this scale, we can't see the amber and red risks. We'd need a microscopic.
If we take the risk spectrum to a scale where we can just make out the risks that we're trying to find and manage, we have to overvalue the base-rate risk by a factor of more than 1000.
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