Visibility splays and how x and y distances are set

A visibility splay for a planning access is the triangle of land a driver needs to see across when pulling out, and it is defined by two numbers. The x distance is how far back from the carriageway the driver sits. The y distance is how far along the road they can see in each direction. Get either wrong and the highway authority objects, and a highways objection is one of the hardest reasons for refusal to overturn on appeal.

The number that decides everything is the recorded speed of traffic on the road, not the speed limit. That surprises most self-builders, and it works in both directions.

What the splay is and who decides it

The splay is drawn as a triangle. One corner sits on the centreline of your access at the x distance back from the edge of the carriageway. The other two corners sit on the nearside channel line of the road, y metres away in each direction. Everything inside that triangle has to be kept clear of anything that would block the view.

The local highway authority sets the standard, not the planning department. In a two-tier area the county council is the highway authority and the district is the planning authority, so the standard comes from a consultee rather than from the case officer. Ask the highway authority for its adopted access standards before you design, because those standards vary between authorities and some still apply older, more demanding figures.

Visibility is measured from a driver eye height of 1.05 metres to an object height of 0.6 metres. That object height matters: a 600 millimetre wall is an obstruction, a 500 millimetre one generally is not, and a parked car is always one.

The x distance and what governs it

The x distance, sometimes called the setback, is the shorter of the two and gets the least argument.

Manual for Streets gives 2.4 metres as the normal figure for residential and lightly trafficked situations, on the reasoning that this is roughly where a driver's eyes sit when the car's nose is at the give-way line. A reduced 2.0 metres can be accepted for a single private drive in a low-speed, low-flow context, where a driver can edge forward safely.

Larger setbacks of 4.5 metres come from the older design tradition and are still applied by some authorities for accesses onto busier or faster roads, and for junctions rather than driveways. The practical consequence of a larger x is that the whole splay gets wider, and the land you need at the roadside grows with it.

Where the access crosses a footway you will also need a vehicle crossing under section 184 of the Highways Act 1980, which is a separate consent from the highway authority with its own fee and its own specification.

The y distance and its relationship to speed

The y distance is the stopping sight distance for the road: how far a driver travelling along it needs to see your emerging car and still be able to stop.

Manual for Streets derives it from speed, and the figures are considerably shorter than the values used before 2007.

Two points follow. First, an access onto a true 20 mph street needs a very modest splay, and applicants often over-provide because they have been given a figure intended for a 30 mph road. Second, an access onto a fast rural road can need a splay of well over 100 metres in each direction, which is usually the binding constraint on whether the site can be developed at all. That situation is dealt with in access onto a national speed limit rural road.

Check which standard the authority applies. Some highway authorities still use pre-2007 figures for the same speeds, which roughly double the y distance at 30 mph, and there is no point discovering that after the drawings are done.

Why recorded speeds beat the posted limit

The standard is based on how fast traffic actually travels, and the convention is to use the 85th percentile wet weather speed: the speed below which 85 out of every 100 vehicles travel in wet conditions.

That is measured, not assumed. A survey uses automatic counting equipment or radar over about a week, in free-flow conditions, positioned far enough from junctions and bends that the readings are not artificially depressed. The output gives a speed distribution and the 85th percentile figure, and wet weather speeds are used because they are the conditions in which stopping distances are longest.

This cuts both ways, and self-builders should understand which way it cuts for them.

On a 30 mph road through a village where traffic averages 24 mph because the road is narrow and parked up, a survey can reduce the required y distance and rescue an access that fails against the posted limit. That is a legitimate and frequently successful argument, and the survey typically costs a few hundred pounds.

On a 30 mph road where traffic actually does 37 mph, the survey will increase the requirement, and the highway authority may ask for one precisely because it suspects that. Commission the survey knowing the answer may not help you.

Measuring across land you do not control

A splay is only worth anything if it can be kept clear, and that means the land inside it has to be either yours or adopted public highway.

Verge in front of a site is often, though not always, adopted highway. Check the highway authority's records rather than assuming from appearances, because a strip of grass that looks like verge may be in private ownership and a neighbour under no obligation to keep it cut.

Where part of the splay crosses third party land, you have four options: acquire the land or a right over it, agree with the neighbour and secure it by deed, use a Grampian style condition preventing occupation until visibility is provided, or redesign the access to a position where the splay works. A condition requiring something outside the applicant's control is only lawful where there is a reasonable prospect of it being achieved, so a Grampian condition is not a way of parking an insoluble problem.

The forward visibility of an approaching driver to your access also needs checking, which is a different measurement from the emerging splay and is easy to forget on a bend.

Hedges, walls and keeping the splay clear

Almost every access permission carries a condition requiring the splays to be provided before first use and kept clear of obstruction above 600 millimetres in perpetuity.

Three practical consequences follow. A hedge inside the splay has to be cut back and kept back, which may mean losing it entirely on a narrow verge. A boundary wall has to be rebuilt lower or set back. And you cannot plant anything in the triangle that will grow above 600 millimetres, which rules out most hedging and all but the smallest shrubs.

Before removing a hedge, check two things. A countryside hedgerow may be protected under the Hedgerows Regulations 1997, which require notification to the council before removal and allow it to serve a retention notice on an important hedgerow. And nesting birds are protected between roughly March and August, so hedge removal should happen outside that window or after a check by an ecologist.

Trees inside the splay may be covered by a Tree Preservation Order or sit within a conservation area, either of which requires a separate application or notification.

When reduced splays are accepted

Highway authorities do accept less than the standard, and knowing the grounds is worth more than arguing about the geometry.

The most useful argument is an existing lawful access with a fallback position: where an access already serves the site and you are proposing a modest intensification, the comparison is with the existing situation rather than with a blank field. Collision history supports or undermines this, and the way to use that data properly is set out in using personal injury collision data properly.

Other routes include a speed survey producing lower figures than the limit, physical measures that reduce speeds past the access, a limitation on the number of dwellings served, and relocation of the access to a better position on the frontage. Temporary arrangements during construction are treated separately, as covered in temporary construction accesses and their conditions.

Put the evidence in a short Transport Statement with a dimensioned splay drawing, the speed survey, and photographs from driver eye height in both directions. That package answers the highway officer's questions before they are asked.

PlanScope drafts Transport Statements and supporting documents structured to UK planning policy, flagging where survey data and dimensions still need to be supplied. Reports are AI-assisted drafts that should be reviewed before submission, and we do not guarantee planning approval. Councils expect visibility evidence from a suitably qualified author, and professional review is strongly advised. This is general guidance, not legal advice: confirm the applicable standard with your local highway authority before designing the access.