Visibility splays and protected trees: resolving the conflict

A tree in visibility splay arguments turn on a distinction most objectors and some officers miss: a trunk is a point obstruction and a canopy is a continuous one. A slender stem standing within the splay blocks the view for a fraction of a second as a driver approaches. A low crown spreading across the whole splay blocks it for the entire approach. The first can often be accepted. The second usually cannot, and crown lifting rather than felling is the answer.

That distinction is where the negotiation between the highway officer and the tree officer either succeeds or deadlocks.

How a trunk and a canopy are treated differently

Visibility splays are set by the highway authority, usually by reference to Manual for Streets, with an x distance back from the carriageway edge and a y distance along it derived from stopping sight distance at the design speed. A 2.4 metre setback with a y distance matched to a 30 mph road is the familiar starting point, and the authority's own standard governs.

Manual for Streets accepts that some obstruction within the splay can be tolerated. Street trees, lighting columns and sign posts are discontinuous, and the guidance recognises that occasional obstructions of that kind need not make an access unacceptable, provided they do not obscure a pedestrian or a cyclist and provided the driver still gets adequate warning.

A canopy is a different proposition. Where foliage sits below driver eye height across a width of the splay, the obstruction is continuous and the argument that it is occasional fails.

So the first thing to establish on site is the clear stem height, the crown spread and whether the lowest branches fall within the envelope between about 0.6 metres and 1.05 metres above carriageway level, which is the band the splay is normally measured through.

Measuring the obstruction honestly

Most disputes come from drawings that overstate or understate the problem.

What a credible submission contains: a topographical survey with levels along the frontage, a tree survey to BS 5837:2012 recording stem diameter, crown spread by compass point and height of the lowest significant branch, the splay drawn on the survey rather than on a sketch, and photographs taken from driver eye height at the x distance looking both ways.

Photographs do more work than anything else here. A sequence taken from inside a car at the access position, in leaf and out of leaf, settles most arguments about whether a stem obscures an approaching cyclist.

A Tree Survey or Arboricultural Impact Assessment is also where the root protection area appears, and the splay question often arrives alongside a construction question about surfacing within that area. Where the stem form is unusual, the root protection area calculation needs care, as set out in root protection area for a multi-stem tree.

Crown lifting as an alternative to removal

Crown lifting raises the lowest branches to clear the splay envelope while leaving the tree, its stem and most of its crown intact. It is almost always the first proposal to put forward.

Specification matters. Crown lifting has to be specified by an arboriculturist to the guidance in BS 3998 on tree work, because lifting too far changes the tree's form permanently, raises the centre of gravity and can leave a long bare stem that fails in wind. The standard cautions against lifting beyond a proportion of the overall crown, and a tree officer who has seen a badly lifted lime will start from scepticism.

Where crown lifting alone is not enough, a reduced specification combined with a minor change to the access geometry often is. Pruning and design should be considered together rather than sequentially.

Moving the access rather than the tree

The better answer on many sites is to move the access.

Options, roughly in order of how often they work. Relocate the access along the frontage to a point where the splay misses the tree. Share an existing access with a neighbouring property. Use an existing gap in a hedge or boundary wall. Reduce the required y distance on the basis of a measured speed survey. Or, on a private drive serving a small number of dwellings, agree a reduced standard reflecting the low vehicle numbers.

A speed survey is the most underused tool. Where the design speed assumed by the authority is based on the posted limit rather than on measured traffic, a survey of actual speeds at the 85th percentile can support a shorter y distance, and a shorter y distance may take the tree out of the splay altogether. Commission the survey early, in neutral conditions, and share the methodology with the highway officer before you run it.

When the highway officer and the tree officer disagree

This happens often, and the local planning authority has to resolve it as a planning judgement rather than deferring to either consultee.

The highway authority is a consultee on access, and its standards are advisory to the determining authority rather than binding on it. A Tree Preservation Order carries statutory weight and the council must consider whether consent for works is justified. Where the two cannot be reconciled, the decision balances highway safety against the amenity value of the tree, and inspectors have taken tree retention seriously where the measured speeds and the actual vehicle numbers do not support the full standard splay.

Note that where works to a protected tree are necessary to implement a full planning permission, the 2012 Regulations provide an exemption from the need for separate consent. That exemption depends on the permission being full rather than outline and on the works being shown in the approved details, so get the pruning specification into the approved drawings rather than leaving it for later.

Evidence that persuades both

A submission that resolves the conflict rather than presenting it contains, in one package: the splay drawn on a measured survey, the tree survey with crown spread and lowest branch height, driver eye height photographs in both seasons, a speed survey where the design speed is in issue, an arboricultural specification for any pruning with the standard cited, a plan showing the retained tree with its root protection area and the surfacing proposed within it, and a short note explaining the alternatives tested and why the chosen option causes least harm.

That last note is what officers respond to. Showing that you looked at three access positions and chose the one that keeps the tree reads very differently from asserting that removal is unavoidable.

Where new planting is offered as mitigation, be realistic about what it replaces. A 14 to 16 centimetre girth standard does not replace a mature oak in any timescale that matters, and the honest position is to say so and offer more trees elsewhere on the site, with the establishment requirements covered in tree planting conditions: species, stock size and establishment.

Conditions that secure the outcome

Standard condition wording causes the problem here, because a condition requiring the splay to be kept clear of all obstruction above 0.6 metres in perpetuity makes the retained tree unlawful the moment it is implemented.

Ask for bespoke wording. A workable condition excludes the trunk of the identified retained tree from the obstruction requirement, specifies the clear stem height to be maintained, and requires maintenance of that clearance as part of a landscape management plan. Pair it with a tree protection condition covering the construction phase, and discharge both together rather than separately, which is quicker for the reasons set out in discharging a tree protection condition without delay.

One honest uncertainty: highway authorities vary in how much discretion they will exercise on splay standards, and two neighbouring authorities can reach different answers on identical geometry. Ask the question at pre-application, in writing, with the tree shown on the drawing.

Enter your postcode on PlanScope to see which tree and transport reports your site is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.