Don't call 'high-use' zones, 'high-risk' zones.
This is from the new edition of the Arboricultural Association's ARB Magazine that's out this week. It 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 the equivalent of the few minutes it takes to cover about 2 miles (3km) 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
We recently came across a new concept called Friction. Friction is making things difficult when they could be done much easier. It’s the obstacles between what you want to do and doing it. Generally, friction is something you want to remove, and a great example of successful friction removal is Amazon. One of the reasons Amazon has been so successful is it’s taken a lot of the friction out of buying stuff. They’ve made shopping pretty much as effortless as it can be.
It made us realise that one of the things we’ve done with VALID is to remove friction from tree risk.
At the risk assessment level, the occupancy and consequence decisions in the App have little friction. The friction that comes with bafflegab (vague words) and numberwang (difficult maths) has been designed out. With a bit of training, occupancy and consequence decisions are pretty much effortless.
On the other hand, when you want assessors to be thoughtful about their decisions you add friction. In the App, you have to go through VALID letter-by-letter before you can make a likelihood of failure decision. This is adding friction where it’s important.
Whilst putting VALID together with the Risk Professor, and Bath University, I learned many lessons about risk modelling. One lesson learned was that I’m an Arborist. I mainly know about trees. There’s much I didn’t know about risk modelling. Okay, I did know risk matrices are fundamentally flawed and can’t sensibly rank risks. And you can’t apply mathematical rules to ordinal rankings. Though I was yet to find out how much I didn’t know, I was at least smart enough to realise some people knew much more than me.
Another important lesson learned was there’s too much uncertainty in tree risk to claim single figure value accuracy. It’s not credible to measure tree risk to such accuracy as 1/4, 1/300, 1/20 000, or 1/5 000 000. Nor can you realistically measure the difference between a 1/10 000 and a 1/50 000 risk. And you really shouldn’t be compounding the error by adjusting these very accurate risks by double or single figure multipliers like 0.25 or 2, 3, or 4.
With tree risk, we’re looking to measure something with high uncertainty, and our risk ratings should reflect that. With VALID there are only four tree risk ratings. None of them is a single figure value.
Here’s our Drive-by Assessment specification that we've been working on the Tasmanian Government's Department (DSG) of State Growth.
DSG are working on their Traffic Management Plan and I'll encourage them to share that as well. This is really important because the risk of assessing trees on a high-use road is much higher than the risk of branches or trees falling on a high-use road.
It can be adapted to meet your needs. By way of explanation for how this is set up. It’s part of the Government Tree Risk-Benefit Management Strategy. So, you’ll need to be familiar with that to understand the difference between Passive Assessment & Active Assessment. And the three levels of Active Assessment – Basic, Detailed, and Advanced.
Zones of high confluence = High use + large trees
In VALID that’s 1400 or more vehicles a day. The speed limit doesn’t matter. For people, we’re looking at around 1200 per day. This translates to someone passing every minute or so between 7am – 7pm, Mon – Fri. Large trees, for purposes of zoning rather than assessment are trees with a DBH of about 50cm/20in or more.
Drive-bys can be carried out by a Basic Validator or a Validator. This is released under a creative commons licence, and you're welcome to replace these trained assessor titles with whoever would be carrying out a Drive-by Assessment and what their credentials are.
Putting tree risk into perspective.
A neat and easy to understand relative risk came up in Tim Harford’s marvellous Cautionary Tales - The Spreadsheet of Life and Death. Driving for 200 miles (320km) has a risk of death of about one in a million (1 micromort).
We know the overall risk of death from branches or trees falling each year is less than one in a million (less than 1 micromort).
That means you’re exposing yourself to a higher risk of dying by travelling in a car on a 200 miles round trip to visit friends for the weekend, than you are being hit by a branch or tree over the whole of a year.
Duty holders who don't have a strategy explaining how they manage the risk from tree failure are vulnerable to legal claims or enforcement action. Even though we know the overall risk of death or serious injury from tree failure is extremely low, a number of recent Coroner's Inquests from around the world have highlighted why having a strategy in place is so important.
Death on the highway, from the Arboricultural Association's Arb Magazine (Summer 2020), takes a closer look at the importance of Tree Risk-Benefit Management Strategies to the duty holder.
Here's a LinkedIn article about the landmark Cavanagh v Witley Parish Council Judgment. It explores the gulf between reasonable, proportionate, and reasonably practicable tree risk assessment and management, and expert evidence in some UK court Judgments.
Poll v Bartholomew 2006
Every summer, when we get long hot dry periods, concern is often raised about the risk from Summer Branch Drop (SBD). Fear not. We’ve got the risk management of SBD covered for you in our free Summer Branch Drop (SBD) Guide.
Is this SBD?
In brief, the overall risk from SBD is mind-boggling low. What that means is there’s no need to fret about putting up signs, or fencing, or pruning, unless you have a tree that’s a repeat offender.
Have a look at our Risk Management page for lots more free and handy common sense tree risk management advice and help.
When a tree might be 'dangerous'*, it'll usually have obvious features (not tree defects) that you can't help but notice.
To help you spot trees that might be dangerous, here's an illustrated Obvious Tree Risk Features Guide for you to download.
We're a not-for-profit and this is released under a creative commons licence, so we're more than happy for you to share it around.
*Dangerous = where the risk is not Acceptable or Tolerable.
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