Visibility splay calculations and the standards behind them

A visibility splay calculation produces two distances and a triangle. The X distance is the set-back of the emerging driver from the edge of the main carriageway, and the Y distance is how far along that carriageway they can see in each direction. The triangle between them has to be clear of obstructions above a stated height, and it has to be on land you control.

The number that matters is Y, because it is derived from the stopping sight distance for the actual speed of traffic on the road. Not the speed limit. The measured speed, which on most rural lanes and many urban roads is different from the signed limit in one direction or the other.

What a splay has to achieve

The purpose is intervisibility. A driver waiting to emerge has to see an approaching vehicle early enough, and the approaching driver has to see the emerging vehicle early enough, for the approaching driver to stop without a collision if the emerging driver misjudges it.

That is why Y equals stopping sight distance rather than some comfort margin. Stopping sight distance is the distance a vehicle travels during the driver's perception and reaction time plus the distance it takes to brake to a stop. Increase the speed and both components grow, the braking component faster than the reaction component, which is why splay requirements rise sharply above 30 mph.

The splay is measured as the envelope between the driver's eye position at the X distance back from the carriageway edge and the nearside carriageway edge at the Y distance in each direction. Eye height and object height assumptions vary between the main design documents, and the figures to use are those in the current edition of whichever document the highway authority applies, which for residential streets is normally Manual for Streets and for higher speed roads is the Design Manual for Roads and Bridges.

Measuring actual speeds rather than limits

The speed survey is the part applicants skip and highway officers insist on. The convention is to measure the 85th percentile wet weather speed of approaching traffic, which means automatic traffic counters or radar equipment deployed for a continuous period, usually a week, in each direction and clear of the junction's own influence.

Things that invalidate a survey:

• Equipment sited within the influence of the junction, a bend or a traffic calming feature, so the recorded speeds are artificially low.
• A survey week coinciding with roadworks, a diversion or a school holiday.
• Fewer than seven consecutive days, which fails to capture the weekday and weekend difference.
• Only one direction surveyed on a two-way road.

Where measured speeds come out below the signed limit, the required Y distance falls and a tight access can become achievable. That is a legitimate and common result on narrow lanes, and it is why the survey is worth paying for before concluding that a site has no viable access. Where they come out above the limit, the requirement rises and some applicants quietly drop the survey, which an experienced officer notices.

Deriving the required distances

The X distance in residential and low-speed situations is commonly taken as 2.4 m, measured from the carriageway edge to the driver's eye, with 2.0 m accepted in some low-speed urban situations and larger set-backs applied on higher speed roads. The logic is that 2.4 m puts the driver's eye roughly at the back of the footway with the vehicle bonnet not yet projecting into the running lane.

The Y distance follows from the measured speed. An illustrative derivation, using a 1.5 second perception and reaction time and a deceleration rate of around 0.45g:

1. At 30 mph the vehicle covers roughly 13.4 m each second, so 1.5 seconds of reaction accounts for about 20 m.
2. Braking from 13.4 m per second at about 4.4 m per second squared accounts for a further 20 m or so.
3. The sum, rounded, is in the low forties of metres, which is close to the tabulated figure for 30 mph in Manual for Streets.

Use the tabulated figure rather than your own arithmetic, and state the edition. The calculation above is illustrative and included to show why the numbers move the way they do, not to replace the table. Set the derivation out in the Transport Statement with the survey data appended, or in a Transport Assessment on a larger scheme.

Obstructions within the splay

Anything within the triangle above the stated height is an obstruction, and the height threshold depends on the authority, commonly 0.6 m for low-level obstructions with a separate allowance for slender features. What counts in practice:

Trees inside the splay are the most awkward case, because the solution may be removal and the tree may be protected. Check for a Tree Preservation Order and for conservation area status before assuming a tree can go, and get the tree constraints surveyed in a Tree Survey (Arboricultural Impact Assessment) so the access design and the arboricultural case are produced from the same drawing rather than contradicting each other.

Land control over the splay area

This is the failure that cannot be conditioned away. The splay has to lie within the public highway, within the application site, or within land in which the applicant has a sufficient legal interest to keep it clear in perpetuity.

A splay crossing a neighbour's front garden hedge is not deliverable, and an authority cannot grant permission on the basis that the neighbour will probably not mind. The routes available are acquiring the land or an easement, obtaining a licence from the highway authority where the land is already highway, or dedicating land as highway under section 38 of the Highways Act 1980 so that the authority maintains it. Where works to the highway itself are needed, a section 278 agreement is the mechanism.

Check the highway extent on the definitive highway record rather than assuming the verge belongs to the council. Verges are frequently in private ownership with a public right of passage over the carriageway only.

Conditions securing retention

Expect two conditions. One requiring the splays to be provided in accordance with an approved drawing before the access is first brought into use. One requiring the splay areas to be kept permanently clear of obstructions above the stated height, often with the height written into the condition.

That second condition binds successors in title, which matters when you sell. Tell the buyer, and put the splay on the plan attached to the transfer, because the commonest enforcement complaint about a new access is a hedge that has grown.

Set the whole access case out in the Planning Statement with the survey data, the derivation, the splay drawing and the land interest evidence in that order. And carry out the speed survey before you fix the access position. Moving an access after the layout is drawn is the most expensive avoidable change on a small site.

This is general guidance, not legal advice. Design standards, eye and object height assumptions and local obstruction height thresholds vary by authority and change, so confirm the requirements with the highway authority before designing.

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