Visibility splay: x-distance, y-distance and land control
The y-distance follows measured traffic speed, not the posted limit. How a splay is constructed, who has to control the land and what counts as an obstruction.
A visibility splay is the triangle of land at a vehicular access that has to be kept clear of obstruction so that a driver emerging can see approaching traffic, and so that approaching traffic can see the emerging vehicle. It is defined by two measurements: the setback from the carriageway edge, called the x-distance, and the sight distance along the carriageway in each direction, called the y-distance.
The y-distance is derived from the speed traffic actually travels rather than from the posted limit. That single point decides more access applications than any other, because a measured speed survey frequently produces a shorter required splay than the speed limit implies.
How the splay is constructed
Draw it as a triangle for each direction of approach. The apex sits on the centreline of the access at the x-distance back from the edge of the carriageway. The other two points sit on the nearside edge of the carriageway, or on the centreline of the nearer traffic lane depending on the standard applied, at the y-distance in each direction.
The x-distance is where a driver's eye sits when they have stopped and are ready to emerge. Conventionally it is 2.4 metres, which allows a car to stop with its nose clear of the footway. It is reduced to 2.0 metres in low-speed situations where the highway authority accepts it, and increased at accesses used by long vehicles that need to see further forward before the driver's eye clears the boundary.
The y-distance is the stopping sight distance for the approach speed. It is the distance a driver travelling at that speed needs to perceive a hazard, react and stop.
Two refinements matter in practice. The splay is measured between defined eye and object heights, so a low wall can be acceptable while a hedge at the same position is not. And on a curved road the splay is not a straight line: it follows the carriageway, which means the land required can extend well beyond the site frontage on the inside of a bend.
Deriving the distances from recorded speeds
Highway authorities apply different technical standards depending on the road. Design standards for the higher-speed network give stopping sight distances that rise steeply with speed, with a figure of 215 metres for a 60 mph design speed. Street design guidance for lower-speed environments derives shorter distances from measured speeds, producing figures in the region of 40 to 60 metres for roads where traffic is actually travelling at 30 to 40 mph.
The practical route is the same in both cases.
- Commission a seven-day automatic traffic count at or close to the access position, recording speeds by direction.
- Extract the 85th percentile wet weather speed, which is the figure the standards work from.
- Ask the highway authority in writing which standard it will apply to a road with those recorded speeds.
- Design the splay to the distance that answer produces, and dimension it on the drawing.
Skipping the survey and adopting the speed limit is the default many applicants take, and on a rural road subject to the national speed limit it can turn an achievable access into an impossible one. The survey costs a fraction of a refused application.
Where the recorded speeds are far below the limit, say so plainly and attach the data. Where they are at or above the limit, the survey has still done its job by telling you before the application rather than after.
Land control and maintenance
A splay drawn across land you do not control is not deliverable, and highway authorities check.
Three questions settle it. Does the publicly maintainable highway already extend to the hedge line, as it frequently does on rural roads, in which case the splay may sit entirely within highway land. If not, do you own the land. If not, do you have an enforceable right over it, and has the owner been notified through the correct ownership certificate served with the application.
Maintenance is the other half. Authorities secure it by condition requiring the splay to be provided before first use and kept clear thereafter, by dedication of the land as public highway, or by a planning obligation where the land sits outside the applicant's control. A condition cannot bind land the applicant does not control, which is why an obligation or dedication is required in that situation rather than optional.
Where the access serves multiple plots, decide at the outset who maintains the splay in twenty years. A management company obligation, an estate rentcharge or dedication all work. Silence does not.
Vegetation, walls and parked vehicles
Obstruction is assessed against a height threshold, commonly 600 millimetres, measured above the level of the adjacent carriageway. Anything above it within the triangle is a problem unless it is slender enough to be discounted, such as a lamp column, a sign post or a tree trunk of limited girth.
The items that cause trouble.
- Hedges, which grow back and which is why the maintenance mechanism matters more than the initial clearance.
- Boundary walls and fences, including ones rebuilt by a future owner who has never read the permission.
- Earth banks, which are easy to overlook on a plan drawn without levels.
- Parked vehicles, where the splay overlaps an on-street parking bay or a habitual parking spot.
- Bin presentation points and electric vehicle charging bollards, which are a modern addition to the list.
Levels deserve their own mention. A splay across rising ground can fail on vertical alignment even when the plan geometry is correct, because the driver cannot see over the crest. A topographical survey with spot levels along the splay answers this and a location plan does not.
Showing the splay on the drawings
Put it on a dedicated drawing rather than leaving it as a note. Include the x and y dimensions written on the drawing, the eye and object heights assumed, the speed survey result and the standard applied, the existing and proposed boundary treatment, and spot levels along the splay in both directions.
Show the land ownership. Hatch the area within your control differently from the area outside it, and where the splay crosses highway land, say so and reference the highway authority's confirmation.
Cross-refer the drawing from the supporting statement so that the case officer and the highway consultee are reading the same thing. Inconsistency between the transport evidence and the site layout is a common reason for a holding objection, and it costs weeks.
Where a substandard splay can be justified
Highway authorities do agree departures from standard, and the evidence needed is predictable.
A speed survey showing recorded speeds materially below the design speed the standard assumes. Personal injury collision data for the section, usually for the last five years, showing the safety record. An independent road safety audit at the appropriate stage, with designer responses. Evidence about the intensity of use, since a single dwelling generating a handful of movements a day is a different proposition from a scheme generating a hundred. And a comparison with existing accesses along the same road, which is persuasive where they are visibly worse.
A clean collision record does not prove an access is safe, and inspectors say so. It is one input among several rather than the answer.
Where agreement cannot be reached, the alternatives are relocating the access along the frontage, using an existing access with established visibility, or reducing the scale of the proposal. Where the authority refuses to determine the application at all, the route is an appeal against non-determination, as our note on the deemed refusal appeal explains.
This is general guidance, not legal advice. Visibility standards are applied locally and vary between authorities and road types, so confirm the requirement with the highway 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.
Browse all 66 PlanScope report types to see which transport documents your access is likely to need, or enter your postcode to check the constraints on your site.
Frequently asked questions
What are the x and y distances?
The x-distance is the setback from the carriageway edge to where the emerging driver's eye sits, conventionally 2.4 metres and sometimes 2.0 metres where speeds are demonstrably low. The y-distance is the sight distance along the carriageway in each direction, derived from the stopping sight distance for the speed traffic actually travels.
Is the splay based on the speed limit?
No, on the recorded 85th percentile wet weather speed. Commission a seven-day automatic traffic count at the access, extract the figure, and ask the highway authority in writing which standard it will apply to a road with those speeds. On a rural road the difference between measured speed and posted limit can decide the application.
What counts as an obstruction within the splay?
Anything above the height threshold the authority applies, commonly 600 millimetres above the adjacent carriageway level, unless it is slender enough to discount such as a lamp column or sign post. Hedges, walls, fences, earth banks, parked vehicles, bin presentation points and charging bollards all cause problems in practice.
Can a splay cross land I do not own?
Only if you can deliver it. Check first whether the publicly maintainable highway already extends to the hedge line, as it often does on rural roads. Otherwise you need ownership or an enforceable right, and you must serve the correct ownership certificate with notice to the other owner rather than certifying sole ownership.
How is a splay kept clear in the long term?
By a condition requiring it to be provided before first use and kept clear thereafter, by dedicating the land as public highway, or by a planning obligation where the land is outside the applicant's control. A condition cannot bind land the applicant does not control, so in that case dedication or an obligation is necessary.
Can a substandard splay ever be accepted?
Yes, where the evidence supports a departure. That usually means a speed survey showing recorded speeds below the assumed design speed, five years of personal injury collision data, an independent road safety audit with designer responses, and evidence about the intensity of use. A clean collision record on its own does not prove an access is safe.
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.