Evidencing visibility splays at a new access

A visibility splay drawing gets refused for one of three reasons: the y distance was taken from the posted speed limit rather than from measured speeds, the splay crosses land the applicant cannot control, or there is no mechanism to keep it clear once the house is built. The calculation is the easy part. Evidencing visibility splays for planning means proving the geometry, the speed basis and the land position together.

Highway authorities are consultees rather than decision makers, but a highway objection on visibility is one of the hardest to argue away, because it is framed as a safety matter.

What the x and y distances represent

The splay is a triangle, and it has two sides. The x distance is the set-back measured along the centreline of the access from the nearer edge of the carriageway to the point where the driver's eye sits while waiting to emerge. For a private residential drive, 2.4 metres is the standard figure, and 2 metres is sometimes accepted where speeds are unusually low and the access serves a single dwelling.

The y distance is measured along the nearer edge of the carriageway in each direction from the centre of the access. It represents how far an emerging driver has to be able to see an approaching vehicle, and it is derived from stopping sight distance rather than chosen from a list.

Stopping sight distance has two components: the distance travelled while the approaching driver reacts, and the distance needed to stop. Manual for Streets sets out the derivation using a reaction time of 1.5 seconds and a deceleration rate appropriate to a residential street, and tabulates the result against speed. On a 30 mph road the figure most English authorities apply is 2.4 metres by 43 metres. The arithmetic behind that number, and where the higher Design Manual for Roads and Bridges values apply instead, is set out in our note on the stopping sight distance calculation.

Two further measurements define the envelope. Driver eye height is taken at 1.05 metres above the carriageway, and the object to be seen at 0.6 metres. Anything above 0.6 metres inside the triangle is an obstruction.

Choosing the design speed and evidencing it

The posted speed limit is not the design speed. Manual for Streets is explicit that visibility should be based on the speeds vehicles actually travel at, which is normally expressed as the 85th percentile wet weather speed.

That cuts both ways. On a rural lane signed at 60 mph where traffic rarely exceeds 30 because of the geometry, a splay based on the limit would be absurd and unachievable. On a 30 mph estate road with a long straight approach, measured speeds can exceed the limit and the required y distance grows.

So the evidence is a speed survey. Automatic traffic counters placed for a continuous period of about a week, positioned clear of the junction so that vehicles are not already slowing, give the distribution of speeds in each direction. A radar survey over a shorter period is sometimes accepted for a single dwelling, but it is easier for an officer to dismiss.

Three things to settle with the highway officer before you commission the survey:

1. The location of the counters, because a position too close to a bend or a junction will be challenged.
2. The duration and whether a weekday and weekend period are both required.
3. Whether the 85th percentile speed is to be used directly or with an adjustment for wet weather.

Agreeing that in an email takes ten minutes and prevents a second survey.

Land control inside the splay

A splay has to be deliverable. Where the triangle falls on the adopted highway, including the verge and footway, it is already under the highway authority's control and the issue is maintenance rather than ownership. Where it falls on land in the applicant's ownership, it can be secured by condition. Where it falls on a neighbour's garden, hedge or front wall, there is a problem that no amount of drawing solves.

This is the point at which applications fail quietly. The splay is drawn across next door's privet hedge, the officer notices the ownership certificate says Certificate A, and the recommendation writes itself.

Options where third-party land is involved are limited and all of them take time. A deed of easement or a licence from the neighbour, which has to be in place and evidenced. A revised access position that keeps the splay within your own frontage. Or acceptance of a reduced splay supported by a safety case, which the highway authority may or may not entertain. Remember that land needed for the access works belongs inside the red line on the location plan, and the ownership certificate follows from that.

Vegetation, walls and the maintenance condition

Achieving a splay on day one is not enough. Hedges grow, owners build piers, and cars get parked on verges, so authorities secure splays with conditions requiring them to be provided before first occupation and then kept free of obstruction above 0.6 metres in perpetuity.

Practical consequences for the design:

• A boundary wall inside the splay has to come down to 0.6 metres or be set back behind the triangle. Rebuilding it at full height later would breach the condition.
• A hedge inside the splay has to be cut back and maintained at that height, which usually means it ends up as a low formal hedge rather than the screen the owner wanted.
• Removing a countryside hedgerow may require notice under the Hedgerows Regulations 1997, and a hedge with a Tree Preservation Order on individual trees within it is a separate consent again.
• Street furniture, a telegraph pole or a lit bollard inside the splay is treated as an acceptable intermittent obstruction in most cases, but confirm it rather than assume.

Where the splay relies on land outside your control for maintenance, the condition becomes unenforceable and the officer will say so.

Drawing conventions the highway officer checks

A visibility splay drawing is examined for a short list of things, and most returns are about presentation rather than substance.

Show the splay triangle hatched or outlined on a scaled plan at 1:200 or 1:500, with both dimensions annotated and the x distance clearly measured from the nearer edge of the carriageway. Mark the centreline of the access. Include the eye height and object height in a note. Show the existing boundary treatment and what is to be removed or reduced, with levels where the ground falls away. Add a second pair of splays if the access is two-way onto a bend, and show forward visibility along the carriageway where the approach curves.

Pedestrian visibility is a separate requirement that gets forgotten. Many authorities ask for a 2 metre by 2 metre splay either side of the access across the footway, clear of walls and planting, so that a driver crossing the footway can see a child on it.

When the splay cannot be achieved

Sometimes the geometry simply does not work, and the honest options are to change the scheme or to build a case.

The case has three possible limbs. Measured speeds lower than assumed, evidenced by survey. A reduced splay that still provides adequate intervisibility, supported by a drawing showing what a driver actually sees. And the existing situation, where the access already exists and the proposal makes it no worse, which is the strongest argument available to most self-builders.

Collision records held by the highway authority are worth requesting. An access with no recorded collisions over a long period, serving an existing dwelling, is a materially different proposition from a new access onto the same road. That evidence belongs in the Transport Statement rather than in a covering letter, so that it is formally before the decision maker. For larger schemes the same material sits in a Transport Assessment with trip generation and junction capacity alongside it.

Where the access serves more than one plot, the internal geometry matters as much as the splay, and the dimensions for turning, forward visibility and refuse vehicle access are covered in Manual for Streets geometry for a small cul-de-sac.

Get the speed survey done before the drawings are finalised. Everything else follows from the y distance, and redrawing an access after the house position is fixed is the expensive way round.

This is general guidance, not legal advice, and highway standards are set locally, so confirm the required dimensions and survey method with your highway authority before relying on it. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.