Visibility splays and the stopping sight distance calculation

A visibility splay calculation has two inputs and one output. The x distance, measured back from the give-way line along the centre of the access, and the y distance, measured along the nearside edge of the major road in each direction. Join the points and you have the triangle that must be kept clear. The y distance is the stopping sight distance for the speed vehicles actually travel on the major road, which is rarely the same as the posted limit.

Manual for Streets is the governing document for most residential and lightly trafficked situations, and it takes 2.4 m as the appropriate x distance in most cases. Everything else on the drawing follows from the speed evidence.

What a visibility splay is measuring

The splay answers a single question: can a driver waiting to emerge see far enough along the major road to pull out before an approaching vehicle reaches the junction, and can that approaching driver see the emerging vehicle in time to stop.

That is why the y distance is a stopping sight distance. It is the distance a vehicle travels during the driver's reaction time plus the distance it takes to brake to a halt, measured at the speed that vehicle is realistically doing.

Heights matter as much as distances. Manual for Streets works from a driver eye height of 1.05 m to an object height of 0.6 m, which is roughly the bonnet of a small car. Anything inside the triangle that is taller than the object height blocks the view, so a 1 m wall, a hedge, a utility cabinet or a parked car in a lay-by all count.

Vertical geometry is the part that gets forgotten. A splay can be clear in plan and useless in section, because a crest in the major road hides the approaching vehicle. Check the long section where the road rises or falls across the junction.

The x distance and where it is taken from

The x distance is measured from the give-way line back along the centre line of the access road, and it sets the position of the driver's eye when they are waiting to emerge.

Manual for Streets treats 2.4 m as appropriate in most situations, because it puts the front of the vehicle close to the carriageway edge without the driver's eye being forward of the give-way line. A reduced figure of 2.0 m can be considered in lightly trafficked, low-speed situations, and highway authorities differ on how readily they accept it, so ask before designing around the reduced value.

Longer x distances appear on busier accesses, where the intention is that a vehicle can wait clear of the footway. On a commercial access with regular deliveries the highway authority may want more, and that changes the geometry of the whole splay.

Take the measurement from the actual give-way position, not the site boundary. Where the access crosses a footway, the give-way line sits behind the footway, and starting the measurement at the kerb line overstates the visibility available.

The y distance and design speed

The y distance is the stopping sight distance appropriate to the road. Manual for Streets contains the stopping sight distance table for lower speed streets, with 43 m commonly applied on a road where vehicles travel at around 30 mph and 25 m where they travel at around 20 mph. Manual for Streets 2 covers busier streets, and the Design Manual for Roads and Bridges applies on higher speed roads and the strategic road network.

Do not mix documents within one drawing. A splay derived from Manual for Streets on a road the authority treats as subject to the Design Manual for Roads and Bridges will be challenged, and the difference between the two at 40 mph and above is large.

Measure the y distance along the nearside edge of the carriageway, or along a line offset from it where the authority requires, and in both directions unless the access is one-way.

Measured speed against posted speed limit

Manual for Streets is explicit that visibility should reflect the speeds vehicles actually travel, not the speed limit, and the conventional evidence is the 85th percentile wet weather speed.

So commission a speed survey. Automatic traffic counters placed on the major road for a full week, at a location representative of the approach to the access, away from junctions and pinch points that suppress speeds artificially. Report the 85th percentile speed in each direction separately, because they frequently differ on a gradient.

This cuts both ways, and that is the honest position. On a 30 mph road through a village where recorded speeds are 34 mph, the required y distance goes up, not down. Plenty of applicants commission a survey hoping for a reduction and receive evidence that makes the access harder to justify. Better to know before you buy the land.

Where the survey supports a reduced splay, put the raw data in the appendix. A highway officer will not accept an 85th percentile figure without the distribution behind it.

Obstructions, hedges and land control

A splay you cannot deliver is worse than no splay, and this is where applications fail. The triangle has to sit within the highway or within land the applicant owns or controls, because a condition requiring a splay over a neighbour's garden is unenforceable.

Work through the obstructions honestly:

1. Boundary walls, fences and railings above 0.6 m within the triangle.
2. Hedges, which grow. Where a hedge has to be cut back and maintained, the land under it must be within your control, and the maintenance needs securing.
3. Street furniture, signs, lighting columns and utility cabinets. Some can be relocated at your cost, and a statutory undertaker's diversion takes months.
4. Parked vehicles, where the splay crosses an existing parking bay or an informal parking area.
5. Trees, where the trunk sits inside the triangle. Note that a tree with a clear stem may be acceptable while a hedge of the same age is not.

Where the splay needs works within the highway, those are delivered under a section 278 agreement with the highway authority, which our note on section 278 agreements explains. On the strategic road network the consultee position is different again, and our piece on when National Highways objects covers that.

Forward visibility around a bend

Forward visibility is the companion calculation, and it applies along the major road rather than at the access. It asks whether a driver following the road can see far enough ahead around a curve to stop.

Measure it as a chord across the inside of the bend, from a point on the vehicle path to another point the stopping sight distance away along that path, using the same eye and object heights. The obstruction is usually a wall, hedge or building on the inside of the curve.

Two practical points. On a new access near a bend, both calculations apply and the more onerous one governs the design. And where the access itself is on the inside of a bend, the emerging driver's view in one direction may be cut by the curve rather than by any object, which no amount of hedge removal will fix.

Showing the splay on a drawing

Produce one drawing that a highway engineer can check without asking questions. Scale plan at 1:200 or 1:500 with a bar scale. Both splay triangles drawn and dimensioned, with x and y labelled. A note stating the eye height, the object height and the document the values come from. The 85th percentile speeds in each direction and the survey reference. Existing and proposed boundary treatments shown separately, with anything being removed marked up. The application red line and any blue line land, so that control of the splay land is visible on the same sheet.

Then carry the same numbers into the written evidence. A Transport Statement for a small scheme or a Transport Assessment for a larger one should quote the splay dimensions and the speed survey rather than referring vaguely to adequate visibility. Councils usually expect a suitably qualified author for transport assessment work and professional review is strongly advised.

This is general guidance rather than legal advice. Visibility standards are applied by the local highway authority and vary, so check the required values and the governing document with them before relying on any of this.

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