Visibility splays and stopping sight distance

A visibility splay for a new access is a triangle of land that has to be kept clear so a driver leaving your site can see approaching traffic, and so approaching traffic can see them. It is defined by two measurements. The x distance sets how far back from the carriageway the driver's eye sits, and the y distance sets how far along the road they must be able to see in each direction. Most refusals on access grounds come down to the y distance being too short for the speed of traffic, or the splay crossing land the applicant does not control.

Both problems are solvable. Neither is solvable after the drawings are submitted.

What a visibility splay is and how it is measured

Picture the access from above. A point sits on the centre line of the access, set back from the nearside edge of the carriageway by the x distance. From that point, two lines run out to meet the nearside edge of the carriageway at the y distance in each direction. The triangles between those lines and the road are the splays, and they must be free of obstruction.

The measurement is three-dimensional, not two. Visibility is assessed between a driver's eye height above the access surface and an object height above the carriageway, both measured vertically. Design guidance for lightly trafficked streets commonly uses an eye height of 1.05 metres and an object height of 0.6 metres, which is why a low wall, a parked car or a rising verge can block a splay that looks clear on plan.

So levels matter. On a site where the access rises away from the road, or where the verge is crowned, a splay that works on a flat plan can fail on a section. Draw the section if the ground is anything other than level.

Where the x and y distances come from

The x distance is a set-back that allows the driver to stop clear of the carriageway while still seeing along it. For residential accesses onto lightly trafficked streets 2.4 metres is the figure usually applied, with 2.0 metres sometimes accepted in low-speed situations where the authority agrees. Shorter set-backs push the driver's eye closer to moving traffic and are resisted.

The y distance derives from stopping sight distance, which is the distance a driver needs to perceive a hazard, react and brake to a stop. Stopping sight distance rises steeply with speed, which is why the arithmetic is unforgiving on fast roads. Design guidance for streets gives figures of the order of 25 metres at 20 mph and 43 metres at 30 mph, with substantially higher values above that taken from the relevant design standard. Take the current figures from the standard the highway authority applies, because the tables are revised and local authorities publish their own requirements.

The combination is written as x by y, so "2.4 by 43 metres" describes a standard residential access onto a 30 mph street.

Speed surveys and recorded speeds against the limit

This is where money is saved. Design guidance bases sight distance on the speed traffic actually travels at, usually expressed as the eighty-fifth percentile wet weather speed, not on the posted limit.

On many rural lanes and on tight urban streets, recorded speeds sit well below the limit because the geometry prevents anything faster. A seven-day automatic traffic count at or close to the proposed access gives you the distribution, and a y distance derived from a measured 24 mph is far shorter than one derived from an assumed 30 mph.

Two cautions. Survey in a neutral period, avoiding school holidays and roadworks, and record the dates. And accept the result if it goes against you: a survey showing speeds above the limit raises the requirement rather than lowering it. Our note on traffic counts in neutral months and survey validity sets out what makes a count defensible.

Splays across land you do not own

A splay has to be deliverable. Where part of the triangle falls on a neighbour's garden, a farmer's field or an unadopted verge, you need to be able to secure it and keep it clear.

Three routes exist. The land may already be public highway, in which case the authority controls it and the splay is achievable, subject to any street furniture or planting in the way. Our note on checking the extent of the public highway explains how to establish that, and it is the first check to make. The land may be acquired or made subject to an agreement with the owner, which then supports the correct ownership certificate on the application form and a notice served on that owner. Or the works may be delivered under a highway agreement with the authority where the land is to be dedicated.

What does not work is a condition requiring a splay over land outside your control with no agreement behind it. A condition of that kind needs a reasonable prospect of being carried out, and an officer who spots the problem will recommend refusal rather than impose it.

Hedges, walls and the maintenance condition

Splays fail over time. A hedge grows, a fence is replaced with a taller one, a garden wall is rebuilt, a car is parked on the verge.

Expect a condition in two parts: the splay shown on the approved drawing must be provided before the access is first used, and the land within it must thereafter be kept free of obstruction above a stated height measured from the carriageway level. The stated height comes from the authority's own standard and is set below the sight line.

Where the splay crosses a hedge that has amenity or ecological value, options include setting the hedge back and replanting on the new line, reducing and managing its height, or in some cases retaining individual trees with clear stems where the trunk alone does not block the view. Hedgerow removal may need separate consent, and removal in the breeding bird season brings its own constraints.

Substandard accesses and the betterment argument

Many existing accesses do not meet current standards and never will. The useful arguments are these.

First, betterment. If the proposal improves visibility compared with the lawful existing position, say so with a drawing showing both. Improvement short of the standard is a material consideration.

Second, intensification. If the proposal does not materially intensify use of the access, the highway authority's ability to require a full standard splay is limited. Quantify it: existing lawful use, proposed use, net change in vehicle movements at the peak.

Third, safety record. Ask the highway authority for recorded personal injury collision data for the stretch. Where the record is clean over a reasonable period, that is evidence. Where it is not, you need to deal with it rather than hope it is not raised.

Those three points usually belong in a Transport Statement rather than a full assessment, for a scheme of a few dwellings.

Drawing the splay so the highway officer can check it

Make it checkable in two minutes. Put the splay on a scaled plan at 1:200 or 1:500 with the scale bar shown. Dimension the x and y distances with leader lines. Mark the eye point on the access centre line and the y points on the nearside carriageway edge, in both directions. Show the carriageway edge, the channel line and the verge. Annotate the eye and object heights used, and the speed on which the y distance is based, with the survey reference. Show existing obstructions, including boundary treatments to be removed, and hatch the land to be kept clear.

Add a short note stating the standard applied and the authority's published requirement. Officers approve what they can verify. Our note on turning heads, cul-de-sac length and layout geometry covers the rest of the access geometry that usually arrives in the same consultation response.

This is general guidance rather than legal advice. Confirm the required standard with the local highway authority before fixing a layout, because published requirements vary between authorities.

Enter your postcode on PlanScope to see which documents a new access proposal is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.