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.
1) The 'mathematics professor' and the risk model
The 'mathematics professor' isn't a mathematics professor. His name's Willy Aspinall and he's the Cabot Professor in Natural Hazards & Risk Science at the University of Bristol. He's a 'risk professor' who we worked with when developing VALID's risk model that does the hard work behind the scenes in the App. He's driven the model to breaking point and this is what he has to say about it:
“We have stress tested VALID and didn’t find any gross, critical sensitivities. In short, the mathematical basis of your approach is sufficiently robust and dependable for any practical purpose.”
2) Risk overvaluation? - Death by numberwang
"The risk of harm* for incidents involving motor vehicles (not motor cycles) appears to be high. There is little evidence of people being killed from cars running into fallen trees but this still apparently has a significant input to the calculated risk of harm."
This gets a bit detailed. In short, the risk isn't too high in VALID's risk model when it comes to vehicles.
There's a couple of points here. First, VALID's risk model doesn't try to measure a death. Death is too narrow and accurate a consequence for a risk that has this much uncertainty. Here's the long answer, and we shared a version of this with Peter after reading his thoughtful article.
The essence of the conundrum with traffic is that it's seldom that a death occurs unless a tree or a large branch hits the cab space of a moving vehicle. So, how do you go about modelling the consequences when they're usually a vehicle driving into a tree during its stopping distance, rather than tree part hitting the relatively small cab space?
The answer's quite complicated because it's a combination of risk modelling from published traffic accident data, the Abbreviated Injury Scale, Occam's Razor (have the least assumptions), running what's called sensitivity analysis, and ease of use.
Perhaps most importantly, 'a difference is only a difference if it makes a difference'. We don't have the data to confirm this because it doesn't yet exist, but we suspect VALID's risk model is over-valuing the consequences in some parts of traffic because of the safety measures that cars have to protect the occupants during accidents; the model's erring on the side of caution for consequences. However, does this matter? Does it make a difference to the actual risk output?
To test this, in the model, we can drop the consequences one or two orders of magnitude in each scenario, with sensitivity analysis, and see how much a difference that makes to the risk. Doing this, it's clear red risks aren't turning green. That means the duty holder is still going to do something to reduce the risk even if the consequences are overvalued.
Cranking up the numberwang to try and model trees and traffic more accurately is not only fraught with mathematical problems and increased uncertainty, but it's not going to make a difference to the decision-making of the assessor or duty holder. It would also add another layer of complexity to the risk model when it comes to decision-making where you combine traffic and people in high-use occupancy. So why try to do it?
An analogy we've used is that we're looking at a big risk picture, and in one part of the canvas that's dealing with traffic and consequences it's a bit blurry. We could get down on our hands and knees with a magnifying glass and spend ages trying to make that consequences part a bit sharper. But when we've finished and taken a step back to admire our work, the risk picture isn't noticeably different.
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 us, it's not worth the paper it's not written on.
That said, having just read this article and its compelling evidence, perhaps we should 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, the Judge spends a quite remarkable amount of the Judgment tearing strips off the defendant's expert. This commentary, by Gordan Exall, in 'Civil Litigation Brief' covers the issues in detail.
Perhaps, 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 (2006) and Cavanagh v Witley Parish Council (2017).
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.
An ISA 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.
ISA 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
We’ve got an all new Basic Tree Risk Assessment workshop!
Contact us to arrange training now.
This one day course will train your staff in how to recognise obvious tree risk features that need a closer look by an Arborist. If you’re a tree owner or manager with staff who spend a lot of time outside, this is the most cost-effective way for you to manage your tree risk.
We know duty holders prefer this training to Lantra’s ‘Basic Tree Survey and Inspection’ or ‘Highway Tree Inspection’ courses. You'll appreciate the value because the training is all about helping you fulfil your duty of care as a key part of your Tree Risk-Benefit Management Strategy.
*Basic Validators aren’t expected to survey or inspect trees. They’re able to carry out Passive Assessment (keeping an eye out for obvious tree risk features you can’t help but notice). Or, carry out an Active Assessment at a Basic level, where they’re looking for obvious tree risk features.
If you have a tree in your garden, all you need do is download this short Strategy and use a pdf editor to change the personal details. Then keep an eye out for obvious tree risk features (page 3) that you can’t help but notice in high-use areas. If you see any of these obvious tree risk features, get in touch with an Arborist who's been trained in tree risk assessment.
When you know what to keep an eye out for, trees with the highest risk are the easiest to spot. Most tree risk assessment isn’t so difficult.
Here, and in the other Strategies, Passive Assessment encourages citizen science risk assessment from those who are on the front line, enjoying the benefits that trees give us, day in day out.
This, and the other Strategies for Government and Landowner, can be downloaded from our Risk Management page.
Don't call 'high-use' zones, 'high-risk' zones.
This is from the Arboricultural Association's ARB Magazine (Autumn 2020) and can be downloaded here.
The high tree risk twilight zone
As we’re going through a hot spell in the UK at the moment, it’s time to release v5.0 of the Summer Branch Drop (SBD) Guide.
Those of you who are concerned about the risk from SBD, this should help reassure you. The overall risk from SBD is mind-bogglingly low. In this update, we have an easy to grasp explanation of what mind-bogglingly low means. The overall risk from SBD for a whole year is less than the short times it takes to cover about 5km/3mi on a drive.
200 miles (320km) drive = 1 micromort (a one in a million chance of death).
The overall risk from SBD is over one hundred times lower than this.
If you’re a duty holder, unless you’ve got a tree that’s a repeat offender, there’s no need to fanny around with confusing and ineffectual warning signs. Just download the SBD Guide from the Government section on the Risk Management page of the website.
Photo credit - Paul Barton
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