UKTC Archive

RE: When tree risk assessment becomes like adding up hopeless hedgehog cakes

Subject: RE: When tree risk assessment becomes like adding up hopeless hedgehog cakes
From: David Evans
Date: Aug 16 2021 13:23:10
Hi Dave

Thanks for your comments.  I appreciate that you and Jeremy Barrell must've 
put a lot of effort into this training and workbook.

Here's my tuppence worth, and apologies for the length, but there's a fair 
bit to cover.

The table/matrix thing is indeed an example of tree risk assessment, as it 
says.  However, it's the only example you and Jeremy use in workbook for a 
Highway Tree Inspector to refer to when they've completed the training.  So, 
why use a demonstrably disastrous example?  If you and Jeremy were to publish 
just one example of how a well-made bed might look, illustrating it with a 
soiled mattress dumped on top of a skip might not be the best idea.

You're right, I don't know what's explained about risk assessment in the 
training.  However, why isn't that in the workbook for them to take home?

How is this 'example' of a dreadful tree risk assessment method supposed to 
help Highway Tree Inspectors make good decisions?

<<Whatever any one says risk posed by trees cannot be quantified.>>

Does that anyone include what Professor Willy Aspinall says?  He's the Cabot 
Professor in Natural Hazards & Risk Science at the University of Bristol, and 
is the brains behind the VALID risk model.  He wouldn't be able to model risk 
without numbers.  Without quantifying it.

If you disagree with what a Risk Professor says about how to measure and 
model tree risk, and say we can't use numbers, what makes you think you can 
do it better with words?  Words, with their vague definitions, elastic 
boundaries, and malleable meanings.  How do you work out the combination of 
three independent and complicated variables so they produce a credible risk 
that can be reasonably imposed on a third party by using words?  BTW, you 
know you and Jeremy are 'quantifying' tree risk in your table/matrix example? 
 After you've added up the hopeless hedgehog cakes, you quantify the risk 
with mind-blowing accuracy by setting the urgency of competent inspection 
timescales at;  High = 10 working days, Moderate = 3 months, and Low = 6 
months.  You're effectively quantifying the risk by saying it's acceptable 
for these exact periods of time.

<<Risk in relation to trees relies very heavily on the individual's 
experience and knowledge (Heuristic method) to arrive at any kind of answer 
that includes the two unknown elements i.e. if and when will the part fail 
and if it does will it hit a target.>>

Two? Surely the third variable in a tree risk assessment, consequences, needs 
to be part of it?

The experience, heuristics, and biases that Arborists bring with them is one 
of the more challenging problems we face when modelling tree risk.  Because 
the overall risk from trees is so extremely low, an Arborists' experience of 
it is such a small sample size that it's hardly statistically relevant.  On 
top of that, we're poorly calibrated to make sense of these random, 
subjective experiences because we haven't been trained to do it.  To compound 
these limitations, we've got the 'peak-end rule' bias that seems to drive a 
lot of fear-mongering and over confidence.

Going beyond the tree risk assessment part, sorry to say, there's not much I 
agree with in your and Jeremy's workbook.  I was contacted about it by vexed 
and horrified highway authority Duty Holders and Arborists.  It comes across 
to me as setting the duty of care bar so high, with its exhausting list of at 
least 18+ questionable 'obvious defects' and all ivy, that a highway 
authority will either quickly blow their budget on hiring Arborists to carry 
out 'competent inspections'.  Or, and much more likely, the Highways Tree 
Inspector and Duty Holder won't come close to getting over the bar that's 
been set.  For them, I worry that if a risk is realised, it'll be all too 
easy for an 'expert' to find one of these 18+ obvious defects and claim it 
should've been found and a 'competent inspection' carried out.  Your and 
Jeremy's workbook could easily be rolled up and used to beat a defendant 
black and blue in court.

Beyond do-it-now emergency and priority work, I think it's expecting too much 
for the 'target audience' (Highways Inspectors) to assess tree risk at this 
level.   Surely, all that's reasonable, proportionate, and reasonably 
practicable for a Highways duty holder is to have their Inspectors alert 
Arborists when they come across an obvious risk feature that need a closer 


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