Is the local authority liable?The question arises, therefore, to what extent is a local authority with the responsibility for the highway and trees on it, liable for loss and damage caused in the unfortunate event of a vehicle striking an overhanging tree?
In the vast majority of these types of incident, the high vehicle strikes either the stem of the tree or a major limb close to the stem. The reason for this is that the trees naturally grow toward the light, which in an urban area generally means growing away from the nearby buildings and toward the road. Should the danger of overhanging trees be completely abated in cities it would mean the wholesale felling of trees including those on some of the most distinctive avenues in the country.
The first issue to be addressed is which authority has the responsibility for inspecting and maintaining the highway. It will be that authority in which the ownership of the tree will vest, irrespective of whether the tree was planted or sown before or after the highway was adopted and became maintainable at public expense.
Claim in nuisance and possible defencesThe correct cause of action in such a claim is in nuisance although the distinction between nuisance and negligence has largely been removed.
A tree will not be considered a nuisance simply by virtue of it overhanging the highway. It must obstruct the free passage of vehicles or constitute a danger to road users. However, even if a tree has obstructed the free passage of vehicles on the highway but the tree has been inherited by the occupier of the land on which it sits, the occupier will not be liable unless they knew or ought to have known that it posed a nuisance (British Road Services v Slater ).
If it is found that an overhanging tree posed a foreseeable danger to users of the highway, the court will then take into account the reasonableness of the local authority in not having addressed it. This may involve consideration of the amenity value of the tree, possibly with reference to the Capital Asset Value for Amenity Trees formula (CAVAT), and the costs of remedying such a nuisance in light of budgets and other priorities.
However, it is not open to an authority to criticise a bus driver for driving too closely to the kerb. The court in Hale v Hants & Dorset Motor Services Ltd and Another  categorically rejected such arguments stating that a driver is entitled to use the whole of the highway unless there is some special reason to prevent him from doing so.
Importance of inspection regimeThe success of defences in these types of claim is largely based on reasonable foreseeability and it is important that an authority can show a reasonable inspection regime of its roadside trees. The argument afforded to the local authority is that it has a reasonable inspection regime in place; no similar incidents have occurred; and the road layout is such that tall vehicles can pass and repass without striking the overhanging stem or branch.
It will be noted that collisions commonly occur close to the kerb and if a branch is found to have struck a tall vehicle close to the middle or crown of the road, then this may indicate that an insufficient inspection has been undertaken. Should an adequate inspection have taken place, then such low hanging branches would, of course, have been identified.
What action should be taken?In terms of what action an authority should take in order to alleviate the danger of overhanging trees, its amenity value should be considered. As discussed above, the court will consider the likelihood of the incident occurring against the cost of undertaking remedial action and the impact to the locality generally. The CAVAT formula will be essential in these instances as if a tree has a particularly high amenity value and the only method of abating a potential nuisance is for its removal, the court is likely to be persuaded that it would be unreasonable to expect a local authority to remove such trees as a consequence of a relatively small risk.
Although not directly relevant to overhanging trees, in Berent v Family Mosaic Housing and another  the Court of Appeal explicitly stated that it was not reasonable for a claimant to expect the desertification of the city as a consequence of the potential for trees causing subsidence. The same is almost certainly true in respect of trees overhanging the carriageway.
Such claims will obviously turn on their own facts but are clearly defendable. It is important that local authorities have a defendable tree risk policy and can demonstrate a sound inspection regime of their trees.