Trees and highway visibility splays: resolving the conflict

Tree removal for a visibility splay is the most common reason an otherwise resolved scheme stalls at the access. The highway authority wants clear sight lines in both directions from the new drive. The tree officer wants the mature oak at the frontage retained, and if it carries a Tree Preservation Order the council has already decided it is worth keeping. Both positions come from the same authority in most places, and neither officer has the power to overrule the other.

The resolution almost always lies in the geometry rather than in the argument. Change the speed assumption, move the access, or change the crown, and the conflict disappears. Argue it as a balance of harm and you will lose months.

Where the two requirements collide

A visibility splay is defined by two measurements. The x distance is the set-back from the carriageway edge, commonly 2.4 metres for a residential access. The y distance runs along the carriageway in each direction and is derived from the stopping sight distance for the recorded or observed vehicle speed.

Within that triangle, nothing may obstruct visibility between roughly 0.6 and 2.0 metres above the carriageway. A tree trunk is an obstruction. A clear stem above 2.0 metres, with a canopy that starts higher, generally is not, which is the single most useful fact in the whole argument.

The problem is that the y distance grows quickly with speed. Moving from a 20mph to a 30mph assumption can double the length of the splay and pull it across a frontage tree that the shorter splay cleared comfortably. Speed surveys are therefore worth their cost on any site where the design speed is contested.

Who decides when highways and trees disagree

The local planning authority determines the application. The highway authority is a consultee, and in a two-tier area it is the county council rather than the district that holds that role. Its objection is a material consideration and carries real weight, because a refusal reason based on highway safety is the one most likely to be defended successfully at appeal.

The tree position is different in kind. If the tree is protected by a Tree Preservation Order, consent to fell is a separate application under the tree regulations, determined by the local planning authority. Where the felling is part of a development proposal, the practical route is to deal with it inside the planning application, but the order does not fall away just because permission is granted for a layout that requires removal. The applicant still needs the tree consent, and an approved layout does not automatically give it.

In a conservation area, works to trees over 75mm in diameter need six weeks written notice to the council, which may respond by making an order. That six weeks sits on the critical path and is regularly forgotten.

Where the frontage tree is a street tree in the adopted highway, the position changes again, because the highway authority owns it. The practical consequences are set out in street trees, highway works and utility trenching.

Crown lifting before removal

Crown lifting raises the lowest branches so that the clear stem sits above the visibility envelope. On a mature tree with a naturally high canopy it may require nothing at all beyond confirming the existing height. On a lower crowned specimen it can mean removing two or three lower limbs.

Three constraints apply. The proportion of live crown removed matters, and taking too much in one operation stresses the tree and provokes epicormic regrowth that blocks the splay again within three seasons. The species matters, because some respond badly to the removal of large lower limbs. And the wound size matters more than the number of cuts, which is why a specification written as "crown lift to 2.5 metres" is worse than one that names the limbs.

Write the work properly. A specification that names the branches, the final clear stem height and the British Standard for tree work gives the tree officer something to approve, and the approach is covered in writing pruning specifications properly.

Moving the access rather than the tree

This is the option applicants reach for last and officers suggest first.

Shifting the access by four or five metres along the frontage often clears the root protection area entirely, and the root protection area is the constraint that survives even where the canopy is not the problem. Under BS 5837:2012 that area is calculated from stem diameter, capped at 707m² for a single stem, and any driveway construction inside it needs a no-dig cellular confinement build-up rather than an excavated sub-base.

Other geometric options are worth testing before conceding removal.

• Reducing the x distance to 2.0 metres, which some authorities accept on low speed residential streets where the footway is wide.
• Using the actual measured 85th percentile speed rather than the speed limit to set the y distance.
• Widening the access so that the driver's eye position moves forward and outward.
• Accepting a splay over third party land and securing it by agreement, which is fragile and usually refused.

The first two are the ones that work. Both need evidence rather than assertion.

The evidence each side needs

The highway case rests on a drawn splay at scale, a speed survey where the design speed is contested, and a statement of the standard applied. Cite the standard by name, because a splay drawn to a rural trunk road standard on an estate road is the most common source of unnecessary tree loss.

The arboricultural case rests on a tree survey to BS 5837:2012 with each tree given a retention category, the root protection areas plotted on the layout, and an arboricultural impact assessment that says plainly what the access construction does to the tree. A tree survey and arboricultural impact assessment that does not overlay the splay on the root protection area is not answering the question the officer has.

Where retention is proposed through construction, the method statement and tree protection plan carry the weight, and the practical detail is in retaining a tree through a construction programme.

One honest caveat. Where a tree really is in poor condition, saying so early is better than being caught out by the council's own arboriculturist. A category U tree in the splay resolves the conflict in a single sentence.

Replacement planting where removal is unavoidable

Councils expect replacement, and on a protected tree the replanting duty is close to automatic. Expect a condition requiring a replacement of similar ultimate size in a location where it can reach maturity, planted in the first planting season after removal, with a five year replacement obligation if it fails.

The location is where applicants get it wrong. A replacement planted three metres from the new drive in a 600mm wide verge is a future removal application. Plot the mature canopy and the root volume on the layout, and if the site cannot take a replacement of equivalent class, say so and offer a commuted sum or off-site planting rather than proposing a specimen that cannot survive.

On larger schemes the replacement sits inside a wider landscape strategy and reads better there than as an isolated condition discharge.

Recording the resolution

Put the agreement in the documents before determination, because a resolution reached in a phone call is not a resolution.

The layout drawing should show the splay, the root protection areas and the retained trees together on one sheet at a readable scale. The arboricultural impact assessment should state which trees are removed for highway reasons and why alternatives were tested. The design and access statement should record the speed survey result and the standard applied. If an officer asked for a specific change, note the date and what was agreed.

That record does two things. It gives the case officer a defensible position to write up, and if the application goes to appeal it shows an inspector that the tree loss was the residue of a tested design rather than the first idea on the drawing board.

Enter the postcode on PlanScope to see which reports a site with protected trees and a new access is likely to need.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.