Deriving visibility splays from recorded vehicle speeds

A visibility splay recorded speeds exercise starts with a measurement, not with a road sign. Highway authorities assessing a new or intensified access expect the sight distance to be derived from the speeds vehicles actually travel at, normally the 85th percentile wet weather speed from a survey at the point of the access, and then converted into a stopping sight distance using Manual for Streets. A 30 mph limit on a straight suburban road where cars run at 36 mph produces a longer splay than the sign suggests. The reverse is also true, and that is where self-builders win arguments.

Many rural lanes signed at 60 mph carry traffic at 25 to 30 mph because of width, bends and surface. Measure it, and the splay you need may already exist.

Design speed is not the posted limit

The posted limit is a legal maximum. Design speed describes how the road is actually used, and the geometry of an access has to suit the second.

Authorities use the 85th percentile speed, the speed at or below which 85 per cent of vehicles travel, because designing to the fastest driver would make most accesses impossible and designing to the average would be unsafe. Wet weather speeds are used where they are available, since braking distances lengthen in the wet.

This cuts both ways. If a road is signed at 30 mph but speeds run higher, a 43 metre splay will not satisfy the authority. If it is signed at 60 mph but speeds sit near 28 mph, a survey is the only way to establish that, and without one the authority will apply the limit.

Measuring the speeds properly

The survey needs to be defensible, which means agreeing the method before it is commissioned.

What a highway authority normally expects:

1. Automatic traffic counters set at or close to the proposed access, on both approaches.
2. A continuous period of at least seven days, covering weekdays and a weekend.
3. A neutral period, avoiding school holidays, August, the Christmas fortnight and any period with temporary signals or diversions nearby.
4. Output reported as 85th percentile speeds by direction, with mean speeds and flows for context.
5. Raw data retained and made available if the authority wants to check it.

Radar guns and short manual counts are sometimes accepted for very minor accesses, but they are easier to attack. A week of logger data costs little and settles the point.

Tell the authority what you are doing before you start. A survey carried out in the wrong week, or on the wrong side of a bend, is money spent twice.

Turning speed into sight distance

Stopping sight distance combines the distance travelled while the driver reacts and the distance needed to brake. Manual for Streets tabulates the result, and the values commonly applied at low speeds are 25 metres at 20 mph and 43 metres at 30 mph, with intermediate and higher speeds tabulated in the guidance itself.

Above about 40 mph many authorities move to the Design Manual for Roads and Bridges instead, which produces longer distances. Some authorities also retain older local standards that predate Manual for Streets and require more, and they are entitled to apply their own adopted standards.

Check the figures against the current edition of the guidance and your authority's own standard before drawing anything. These values move, and local standards override national ones where the authority has adopted them.

Setting the splay out on a drawing

The splay is a triangle. From a point on the centre of the access, set back the x distance behind the edge of carriageway, draw lines to points at the y distance along the nearside edge of carriageway in each direction. Everything inside the triangle has to be free of obstruction above the stated height.

Draw it on a scaled topographical survey, not on a mapping extract. Kerb lines, verge widths, boundary positions and existing vegetation all have to be shown as they are on the ground, because the splay is tested against reality. Annotate the drawing with the measured 85th percentile speed, the source of the sight distance figure and the date of the speed survey.

Where the carriageway curves, the splay has to follow the nearside edge rather than cut across it, which is the point at which forward visibility on the bend becomes a separate test with its own geometry.

The land you have to control

A splay you cannot deliver is worthless, and this is where most self-build accesses fail.

Every part of the triangle must lie either within the public highway or within land you own or control. A splay crossing a neighbour's front garden depends on their agreement, and an authority will not grant permission on the assumption that it will be given. Check the highway boundary with the authority rather than assuming the verge is highway, because the two frequently differ.

If the splay falls on third party land, the options are to buy or take an easement over the strip, to relocate the access to a point where the splay fits, or to reduce the speed environment by agreement with the authority, which is slow and rarely available. And relocating is usually the realistic answer, often at lower cost than any of the alternatives.

Hedges, walls and keeping it clear

The splay has to stay clear for the life of the development, so expect a condition requiring it to be provided before first occupation and maintained thereafter free of obstruction above a stated height.

That has design consequences people miss. A 1.8 metre fence across the frontage will not be acceptable within the splay, so the boundary treatment has to step down. Hedges grow, so plant them behind the line rather than on it. Parked cars inside the splay defeat it, which means the layout has to keep parking clear of the triangle.

Where a hedge or wall on the splay line is protected, by a Tree Preservation Order, by conservation area status or because it is an important hedgerow, removing it needs its own consent and may not be granted. Check that before you design the access, and record protected vegetation in the tree survey for the application.

What to submit, and what it answers

For a single dwelling or a small infill scheme the access evidence is usually a speed survey report, an access drawing showing the splays with dimensions and heights, a note on land ownership or control, and a short statement explaining the derivation. That package sits comfortably inside a Transport Statement for smaller schemes, or inside the highways chapter of a Transport Assessment where the scheme is larger.

It also answers the two refusal reasons that appear most often on small access applications: inadequate visibility, and a splay not within land controlled by the applicant. Both are evidential rather than matters of judgement, which makes them winnable.

Where highway works follow the permission, the construction of the access itself is dealt with separately, and the note on section 278 agreements explains how that sits alongside the planning consent.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope can draft the written transport documents for a small scheme from your project details, with flags where the survey evidence still has to be inserted. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.