VALID has four easy to understand traffic light coloured risk ratings, and this is where they sit in the Tolerability of Risk Framework (ToR).
The Tolerability of Risk Framework is an internationally recognised approach to making risk management decisions where the risk is imposed on the public.
The ToR triangle gets fatter and redder where more attention and resources should be allocated to managing the risk. It gets thinner and greener where less attention and resources should be allocated.
Where ToR is amber the risk is Tolerable if it’s ‘as low as reasonably practicable’ (ALARP) - where the costs of the risk reduction are much greater than the value of the risk reduction.
VALID has applied ToR to tree risk but has removed the numberwang because:
1) Tree risk has too much uncertainty to credibly measure at single figure accuracy with risks like 1/4, 1/300, 1/20 000, or 1/5 000 000 000.
2) Risk outputs as probabilities create friction in communication because many people struggle with numbers. Research shows that about 25-33% can't rank 1:10, 1:1000, and 1:100 risks from highest to lowest.
3) The risk assessor and duty holder are spared the complexity of numerical cost-benefit analysis in the amber ALARP zone.
Tree failure threshold 70% hollow or 30% solid | t/R = 0.3
Recently, We caught a podcast where a tree was declared 'safe' if it's less than 30% hollow. We think they meant 70% hollow. Either way, this isn't right for several reasons.
We've posted about this before, but as long as this kind of mistake is being broadcast we think it's worth repeating so the message gradually gets home.
The heart of the confusion is the t/R = 0.3 fallacy. t/R = 0.3 is when a residual wall thickness (t) is 30% of the stem radius (R). It's often cited as a failure threshold. It's not. The 'Why t/R Ratios Aren't Very Helpful' pdf explains why in detail.
In short, one reason is because of a geometric property called section modulus. Wind load and material properties remaining equal, if you double the diameter you increase the load bearing capacity of a tree by 8 times.
To add to the confusion, t/R 0.3 is often referred to as 70% hollow. In fact, a 0.3 t/R ratio is only 50% hollow. 70% is the radius, which is one dimension. t/R 0.3 is the area, which is two dimensions.
This graph from Paul Muir shows the relationship of central hollowing on:
A = Cross Sectional Area
Z = Section Modulus
t/R = 0.3
A = 49% loss of cross sectional area
Z = 24% reduction in load bearing capacity
To make matters worse. A tree with a t/R ratio of 0.3 can have a very high likelihood of failure, or it can have a very low likelihood of failure.
If all that wasn't enough, it's seldom that where decay is of concern we're dealing with a cross sectional area of a tree that's a circle.
"The implications of recent English legal judgments, inquest verdicts, and ash dieback disease for the defensibility of tree risk management regimes"
We've had several requests for a better quality image that's part of a discussion about this article on the UKTC (attachments on this group have to be below 180kb). You can download the article here. Click the image to download it.
This is the post that's been polished up a bit and coloured to make it clearer.
I'm genuinely surprised the article has been peer-reviewed let alone published in a journal. It's not research. Some obvious key points of fact don't make sense, even within the questionable logic of its own risk ecosystem. I've sketched them out in the attached image and described them below. I'm surprised they weren't picked up in the peer review.
Yet, in both the risk frameworks, when another 'High' is added to a 'High & Low = Medium Risk', the risk is then LOWER.
And that's before we consider the really important stuff, like what does High, Medium, and Low really mean, and how are they measured?
Or, why is there no mention of the overall risk?
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. This base rate, overall risk, is essential context for Judges and Coroners to be aware of if we're going to take a reasonable, proportionate, and reasonably practicable approach to managing the risk.
Passive Assessment | The invisible gorilla in the room
There's a famous psychological experiment called the invisible gorilla. In it, you're asked to watch a short video of six people passing a basketball. Three of them are wearing white shirts and three of them black shirts. You're asked to count how many passes are made by the white shirts. Most people get the number of passes right. Because they're focused on this, what half the people don't see is a gorilla walk amongst the players, stop, face the camera, thump their chest, and walk off.
To half the people, this very obvious gorilla is invisible.
I recently found one of my invisible gorillas. Whilst putting a flowchart together for VALID's Tree Risk-Benefit Management Strategies, I realised my invisible gorilla was Passive Assessment.
Passive Assessment, and not Active Assessment, is a duty holder's most valuable tree risk-benefit management asset because;
This tree risk assessment review article by Peter Gray, from the Summer 2020 issue of Arboriculture Australia's 'The Bark', might be of interest to you.
We've grown up being told when we assess tree risk we should look out for tree 'defects'. The problem with this approach is what are commonly labelled as defects often aren't defects at all. Hollows, cavities, decay colonies, and deadwood, are natural features of older trees that are usually valuable habitat benefits. It’s seldom these natural features are risks that are not Acceptable or Tolerable. So, why are we labelling them defects before we carry out a risk assessment?
Those of you who know the origin story of VALID might remember the D-word dilemma. Vitality, Anatomy, Load, Identity are all neutral. On the other hand, because Defect means something that's a shortcoming, an imperfection, or a flaw it's not neutral. Defect is pejorative.
Defect is also a begging the question decision-making problem because usually, you can only work out whether a feature is a defect after you've evaluated it and the risk, not before.
Last year, the word ‘defect’ was removed from all of VALID’s Tree Risk-Benefit Management Strategies. Obvious Tree Defects was replaced by Obvious Tree Risk Features. Now, Defect is finally going to be removed as the D-word in VALID.
Occasionally, you'll come across an Arborist who claims to have anecdotal evidence about tree risk which they insist is the truth of the matter.
We've not given anecdotal evidence much weight. Not since we met a bloke down the pub who told me, it's not worth the paper it's not written on.
That said, having just read this article and its compelling evidence, We're going to have to update our priors.
Can we rely on an expert witness telling us what the courts are expecting when it comes to tree risk?
In short, the answer is no because they’re an expert to the court, and not an expert for the court.
A competent arboricultural expert witness knows their limitations. Namely, their role is limited to being an expert to the court. They’re stepping way outside their field of expertise if they claim a Judge’s wisdom about how the law will evaluate tree risk-related evidence in the next case.
Claims by Arborists that they’re experts for the court should ring alarm bells. In a similar way to a Judge who, with no arboricultural training or qualifications, claims they could carry out an advanced tree risk assessment with a Static Load Test on a tree that has extensive root decay because they’ve seen it done.
In the UK, we’ve had several tree risk-related Judgments where the Judge has spotted an expert straying outside of their lane, and dipping into their legal dressing up box. Most recently in Colar v Highways England.
What’s of much greater concern is when a Judge is not aware that the evidence an expert gives them is critically short on expertise. Highly questionable expert evidence appears to have been pivotal in two landmark Judgments in the UK, Poll v Bartholomew and Cavanagh v Witley Parish Council.
This article explores the gulf between reasonable, proportionate, and reasonably practicable tree risk assessment and management, and expert evidence in these cases.
Recently, we had a couple of enquiries asking for a copy of this article. It reviews qualitative and quantitative approaches to tree risk assessment and looks at how we could do better.
It's over two years old now and was written at the time VALID was entering the home stretch. Though VALID has evolved further, much of the article is still relevant today.
This makes for an interesting tree risk assessment case study.
A TRAQ, QTRA, and VALID tree risk assessment were carried out on the same Pine trees in Western Springs, Auckland | NZ.
It involves around 200 Pinus radiata. From a risk of branch or tree failure perspective, the trees of particular interest are those that could fall onto a footpath or property.
The reports are linked.
TRAQ | August 2019
QTRA | December 2019
Random tree part or tree onto footpath
1/400,000 (Size Range 4)
1/500,000 (Size Range 3)
1/1,000,000 (Size Range 2)
<1/1,000,000 (Size Range1)
1 Not Acceptable
50 Not Tolerable
If you'd like to stay in touch and be kept up to date with an occasional short and snappy newsletter every few months or so, please subscribe.
The kind of thing we'll share with you are some of the choice
cuts from this News page. We'll also let you know when there's
new training dates.
What is VALID?
© VALID is a not-for-profit organisation