UKTC Archive

Re: Windshield/Drive-by | Speed Limit

Subject: Re: Windshield/Drive-by | Speed Limit
From: Julian Morris
Date: Dec 03 2019 15:46:11
[Text converted from HTML]
" We wold know similar numbers for many roads and public places, but we
don't know for many other places, thus how do you determine under VALID
your three criteria."

Michael, it's a fair question, not just for VALID but for any risk
management system.

Back to basics here. risk is severity of harm x probability of target
presence x likelihood of failure. It's clear enough that the time that
should pass before a tree is re-inspected should be the time during which
(due mainly to change in condition of the tree i.e. likelihood of
failure) the tree could move from acceptable (or lower) risk to less than
acceptable. Let's call this 'the line'.

For some trees, due to size or low target probability, it could take
several orders of change of likelihood of failure i.e. a very sizeable
magnitude of change of condition before the line is crossed. Others could
be already on the line and a small change could happen quite quickly to
take them over the line.

That line, though, and the degree of compromise of each tree, is
proportional to the target probability. Quite sensibly, on roads the
probability is not just the relatively short time a car is under a tree
but also the stopping distance time, which is a function of safe driving
speed (capped at the speed limit). It's also proportional to the number
of vehicles using the road.

The bigger the tree, the closer it is to the line and the sooner it will
reach it if it deteriorates.

The busier the road, the sooner any tree will reach the line.

And that is for each tree individually.

So unless one has a perfect baseline record of the matrix assessment of
each tree that can be used to inform a regime of inspection that
differentiates between different individual trees and different sizes,
the initial or ongoing frequency of inspections should be different for
each class of tree size and for each speed limit section on each road.
All other things being equal, if all the trees on roads are about the
same size, the prioritisation of inspections should be based on speed
limit because that addresses the only variable in the risk matrix, namely
target probability.

Anyone using a matrix based system and who understands how it works
should also be applying the same principles to prioritisation of
inspection, which without a baseline equals frequency of inspection.
Which MUST be more often for faster roads, and more often for busier

Can a tree that's big enough to cause less than acceptable risk by virtue
of its size and position go from no discoverable defect to discoverable
defect, or can a defect get worse by an order of magnitude in 5 years?
Definitely. 3 years? Occasionally. Can all defects be spotted all year?
Definitely not.

I'd say a one-size-fits all frequency is possibly negligent unless it
errs on the safe side by being based on the lower sizes in the tree
population. Which makes it a little or a lot inefficient. And there's a
lot to be said for an alternating active/dormant regime.

The enemy of rationality here is the idea if frequency of inspection.
Each risk assessment for each tree should identify how soon it could be
that that tree could become a less than acceptable risk. That, and that
alone, should determine the re-insection period for that tree. On the
next inspection a different period might be deemed appropriate. And so
on. And armed with the reinspection period data for a number of trees, it
is not difficult in the slightest to group them for reinspection based on
periods. It is also possible to pick a worst-case period for the whole
population, even if it's inefficient. Clients seem to like the simplicity
of that. I'll venture to suggest that a worst-case period for trees of
various sizes, ages, species etc. along a busy and/or fast roadside is a
lot less than 5 years.

Julian A. Morris - Professional Tree Services and
0778 XXX XXXX - 0141 XXX XXXX

Sent: Sunday, December 01, 2019 at 1:20 PM
From: "Michael Richardson" <>
To: "UK Tree Care" <>
Subject: Re: Windshield/Drive-by | Speed Limit

David, after re-reading your website I am still left asking how do you

1) It's not reasonable
2) It's not proportionate
3) It's not reasonably practicable

Here I am asking for specifics, I have no idea why you propose 5 years,
4 or 6 or 7 years 2 months. In the case of KEW we know the number of
visitors per year and the number of hours workers have been on the
grounds. We wold know similar numbers for many roads and public places,
but we don't know for many other places, thus how do you determine
VALID your three criteria.


Michael Richardson B.Sc.F., BCMA
Ontario MTCU Qualified Arborist
Richardson Tree Care


On Sun, Dec 1, 2019 at 3:48 AM David Evans <>

<<David from your writings I have learned that TRAQ, QTRA, and PTI
are no

Hi Michael

I've not said they're all 'no good'.

They all have some value, but as you know they all have substantial
in their construction and in their application by the assessor. PTI
doesn't even assess risk - which, if you think about it is rather odd
you're taking all that time and effort to inspect a tree. It's why I
so much work into getting VALID going because I thought the whole
could be done so much better. As well as being so much easier on the
assessor and duty holder.

<<Can you please tell us exactly what is reasonable, practical and
proportionate for one single example?>>

I'm not being pedantic here but it's not 'practical'. It's
practicable'. The term has a very particular meaning in risk

At the risk (acceptable) of enraging the usual suspects, it's all set
in the Tree Risk-Benefit Management Strategies on the Risk Management
of VALID's website here.

Have a look at the Government one, which is the most comprehensive.
single most important document is the Policy. It's a position
Everything flows from the Policy. In the unlikely event of a claim
made, it sets out the ground on which a duty holder would mount their

Kew Gardens Questions
Have a read of the Active and Passive Assessment Note in the
Strategy, and
the three levels of Active Assessment - Basic, Detailed, and
This answers most of your questions. How frequently Kew would carry
an Active Assessment is their call - VALID proposes a 5 yearly Active
Assessment frequency in high use zones as being reasonable,
and reasonably practicable.

Really importantly, Active Assessment is forever being topped up by
Passive Assessment, which is happening all of the time. One of the
I've been doing in Tasmania is putting all their Highways Inspectors
through a one day 'Tree Risk & Obvious Trees Defects' course. If Kew
to follow what the Tasmanian Government have done, then all of their
outdoor staff would be trained to recognise obvious tree defects.
then be carrying out thousands of 'free' Passive Assessments each
week. As
such, I'd have thought they could make a very strong case to manage
risk by Passive Assessment alone, and carry out Advanced Active
on those few trees that they're monitoring, at whatever period of
time has
been specified.

<<exactly how many minutes and seconds per tree>>

<<Further can you please tell us the exact risk vs. benefit for the
massive *Quercus castaneifolia*.>>

Sorry, I can't on both counts. VALID doesn't do 'exact'. There's way
much uncertainty in tree risk assessment to be exact. I'd have to see


Acer Ventura

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The UKTC is supported by Bosky Trees arboricultural consultancy