A recent enquiry to a local council about tree root intrusion, kerb breakup and proximity of the trunk to the carriageway resulted in a reply that in effect gave greater priority to the tree rather than safety to the road user and structural/surface damage to the kerb and pavement. For a cyclist on a local 50kmhr road the surface breakup was sufficient to require deviation away from the kerb and reduce passing width for a car. My query is do any Councils have policies that relate to these problems and the interaction with vulnerable road users. Additionally can I get a copy of those policies.
Trees in proximity to roads is a classic asset management dilemma in cost, risk and service. I've worked in organisations that are safety driven and/or environment drive trying to resolve this issue including at a state government policy level. I hope this longish answer helps.
I don't have a specific solution but I'll offer some insights on how to frame the discussion to get a useful outcome.
When you have a tree close to a road edge you have several risks. Risk of vehicle collision, risk of branches obstructing vision or carriageways, risks of falling branches or trees. You also have identifiable costs such as pavement damage from root intrusion, vegetation care, keeping drains clear and so forth. You also have beneficial effects such as soft green environment, people friendly streetscape, micro and macro environmental benefits (shade, CO2 reduction) and social/heritage value. You further have negative effects (such as pavement roughness and reduction in safe travel speed).
The dilemma is how you manage these competing demands. This dilemma is made worse because of different human (social/political) interests and inflexibility of approach by different stakeholders. This is all compounded because you have a Boolean choice to make – a) damage or remove the tree or b) accept costs and risks that are undesirable.
The choice is clear where you have one dominant concern. For example on a high speed highway the traffic safety concern is dominant so you remove trees in impact zones. Where you have a threatened ecological community or high environment value you need to engineer a solution (crash barriers, root barriers) and accept additional costs.
It is the in between areas like neighbourhood streets and low volume rural roads that become topical and difficult. Exactly the problem situation you are looking at.
Somehow you need to be able to objectively assess and compare the risk, the service impact (benefit and reduction) and the cost across your road network to inform a valid policy.
For risk you can develop a risk assessment using your corporate risk approach that is difficult to argue with (as an approach although people will argue the details). That puts you on solid ground with respect to peoples safety
Cost you can presumably develop an estimate of the cost across the City of root damage based on proximity to the road although this would be a significant piece of work and require data. This would include costs of disruption to traffic and other services to repair damage.
Value of the tree is far more difficult to gauge because it's primary values are social and environmental. However what you can do is offer informed stakeholders a choice about how much risk and cost they are prepared to pay for having a tree. I know this is not particularly robust science but you'll almost always end for residential streets with a "we'll pay as a much as it costs and the risks are not relevant" response in residential areas. That's an answer that justifies a Council budget.
Your problem then becomes (a) when does a residential tree become a significant risk (shedding limbs, age, sight lines),(b)when do you have to intervene as a professional organisation; and (c) what is the additional cost to keep trees.
For you as an engineer I would suggest you adopt the OSH approach (modified for cost and asset damage). What is the real cost and risk of the tree? Then in order consider Eliminate (remove), Substitute (plant replacement trees away from the pavement), Engineering (crash barrier, pavement and drainage structures to divert roots), etc), Administer (warning signs, routine checks and pavement repairs) and Protection (crash and root barriers).
Whatever you do recognise that you are dealing with a political/social decision as much as an engineering/cost/risk decision. Likely you will need to propose the compromise solution and accept everyone will dislike it a little bit . You need to retain the ability to keep people safe but beyond that everything is a compromise in service, cost and risk.
Hope this is useful.
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