Visibility splays: where x and y distances come from
The x distance is a setback, the y distance comes from measured traffic speeds. How to evidence a splay that the highway authority will accept.
A visibility splay is the triangle of clear sightline at your access, and the two numbers that define it come from different places. The x distance is a fixed setback into your driveway, usually 2.4 metres. The y distance is derived from how fast traffic actually travels on the road you are joining, measured rather than assumed. Get the visibility splay planning evidence wrong and the highway authority objects, which on a single-plot application is usually fatal.
Most self-builders discover this at the wrong moment, after the drawings are done and the hedge that provides the privacy also blocks the sightline.
What the splay is measuring
The splay represents the distance a driver emerging from your access needs to see an approaching vehicle, and the distance that approaching driver needs to stop. It is drawn as a triangle: back along the centreline of your access by the x distance, then out along the nearside edge of the main carriageway by the y distance in each direction.
Inside the triangle nothing may obstruct visibility above a stated height, commonly 0.6 metres above carriageway level for the main road and lower where a footway is crossed. Fences, hedges, planting, walls, parked cars, bin stores and highway furniture you install all count.
Sightlines are measured between a driver eye height and an object height rather than between two points on a plan. Manual for Streets uses an eye height range and an object height starting at 0.6 metres, which is why a 0.9 metre wall reads as an obstruction while a low kerb does not.
The x distance and why 2.4 metres is the default
The x distance is the setback from the edge of the running carriageway to the point where the emerging driver's eyes sit when they can first see along the road. Two-point-four metres is the standard figure in Manual for Streets for most accesses. It represents a driver stopped at the give-way position without their bonnet protruding into the traffic lane.
A reduced x distance of 2 metres is accepted at some low-traffic accesses, typically a private drive serving a small number of dwellings on a lightly trafficked residential street. It is a concession rather than a right, and the authority will want to see the flow and speed evidence supporting it.
Where an access serves a larger development or joins a busier distributor road, the x distance increases, and on the strategic road network the geometry comes from a different family of standards altogether. Whether your scheme needs a Transport Statement or a full Transport Assessment usually decides how much justification the splay needs.
Deriving the y distance from recorded speeds
The y distance equals the stopping sight distance for the recorded speed of traffic. That figure comes from the tables in Manual for Streets for streets and lower-speed roads, and from the design standards for the strategic and major road network at higher speeds.
Two figures appear so often that they are worth memorising. On a road where traffic runs at about 20 mph, the commonly applied y distance is 25 metres. At about 30 mph it is 43 metres.
The critical word is recorded. Authorities work from the 85th percentile wet weather speed, which is the speed at or below which 85 per cent of vehicles travel in wet conditions, and that is often higher than the posted limit on a rural lane and lower than the limit on a constrained village street. Designing to the speed limit alone is the single most common reason a splay is rejected.
Where the road curves, the splay is drawn to follow the nearside kerb line rather than as a straight line, and the plan must show that the sightline is achievable in three dimensions. A crest in the road defeats a splay that works perfectly on a flat drawing.
Speed surveys and when an authority accepts one
A speed survey uses automatic traffic counters placed at or near the access, recording for a continuous period of seven days in a neutral month. The output gives the 85th percentile speed in each direction, from which the y distance follows.
Authorities usually want the survey agreed in advance: location, duration, dates and the equipment. Undertaking it first and asking afterwards invites a challenge to the position of the counter. Avoid school holidays, avoid roadworks anywhere on the approach, and avoid the fortnight either side of Christmas.
The survey is worth doing whenever the achievable splay falls short of the figure for the speed limit. On a 60 mph national speed limit lane that is actually driven at 35 mph, a survey can turn an unachievable requirement into a deliverable one. It costs a fraction of a redesign.
Record the survey certificate, the raw data and the analysis in an appendix. The highway engineer will want to see the whole distribution as well as the headline number.
Splays that cross land you do not control
This is where single-plot applications fail. The splay must be within land the applicant controls or within the publicly maintainable highway. A sightline running across a neighbour's front garden is not deliverable.
Check the extent of highway land first. Verges are frequently highway even where a resident has been mowing them for thirty years, and the highway authority holds a definitive record. Where the splay falls on adopted verge, the authority can accept it and impose a condition requiring it to be kept clear.
Where it falls on private land in someone else's ownership, you have three options: buy or take an easement over the strip, redesign the access to a position where the splay works, or move the access entirely. A condition requiring something outside the applicant's control is unlikely to be imposed, and a permission granted on an undeliverable splay is worthless.
Hedges and trees inside the splay raise a further question. Where the vegetation is protected by a Tree Preservation Order or sits in a conservation area, removing it needs its own consent, and a tree survey showing the constraint should sit alongside the access drawing rather than turning up later.
Keeping the splay clear once it is built
Expect a condition requiring the splays to be provided before the access is first used and thereafter kept free of obstruction above the stated height in perpetuity. That condition binds successors in title, so it survives the sale of the house.
Draw it properly on the approved plan. A dimensioned splay triangle with the x and y distances labelled, the obstruction height noted, levels shown, and the highway boundary marked is what the discharge officer will check against. A vague hatched triangle with no dimensions gets queried every time.
And think about what you plant. A 600 mm hedge at handover becomes a 1.5 metre hedge in four summers, at which point the owner is technically in breach of a condition on their own home. Low planting, a post and rail fence or a gravel bed inside the triangle avoids the argument.
To check which transport documents your access is likely to need, enter your postcode on PlanScope and answer the access questions.
This is general guidance, not 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.
Frequently asked questions
What is the standard x distance for a driveway?
Two-point-four metres is the usual figure in Manual for Streets, measured back from the edge of the running carriageway. A reduced two metre setback is sometimes accepted at low-traffic private accesses serving a small number of dwellings, supported by flow and speed evidence. Busier accesses and higher-speed roads need more.
How is the y distance calculated?
It equals the stopping sight distance for the recorded speed of traffic on the road being joined, taken from the tables in Manual for Streets or, at higher speeds, from the standards used on major roads. Two commonly applied figures are 25 metres where traffic runs at about 20 mph and 43 metres at about 30 mph.
Can I use the speed limit instead of a survey?
You can, and it is often the wrong answer. Authorities work from the 85th percentile wet weather speed, which on a rural lane can be well below the national speed limit. Where the achievable splay falls short of the figure implied by the limit, a seven day automatic traffic count usually costs less than redesigning the access.
What if the splay crosses my neighbour's garden?
Then it is not deliverable unless you acquire the land or an easement over it. Check first whether the strip is publicly maintainable highway, because verges often are. A permission granted on a splay you cannot deliver is of no practical use, and conditions requiring works outside your control are rarely imposed.
How high can planting be inside a splay?
Conditions commonly require the triangle to be kept clear of obstruction above 0.6 metres measured from carriageway level, in perpetuity. That binds later owners of the property. Plant low, or use a gravel bed or post and rail detail, because a hedge that reaches 1.5 metres puts the owner in breach.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.