Topographical Surveys for Visibility Splays

Designing a safe access to a highway hinges on getting the visibility splay right. Whether you are preparing a planning submission or detailed access design, the quality of the topographical survey behind your drawing will decide how quickly highways officers can sign it off. At Bury Associates we produce road and site surveys across the UK, capturing everything you need to evidence splays accurately and in line with local authority guidance, Manual for Streets, and DMRB principles. If you are new to the topic, our overview of services is here: Topographic Surveys UK.

What is a visibility splay and why it matters

A visibility splay is the clear triangular area at a junction or site access that allows drivers to see approaching traffic. It is defined by an X distance set back from the give way line and Y distances along the nearside edge of the major road in both directions. That envelope must be free of obstructions within a prescribed height band so that drivers have adequate stopping sight distance. Planning officers expect your drawing to show the splay limits precisely referenced to the road and to ground levels, with any features that could interrupt the sightline clearly identified.

What your road topographical survey must show for a visibility splay drawing

To create a compliant and defensible splay drawing, your survey should capture the following - with coordinates on national grid and levels to a known datum:

• Carriageway geometry - edges of carriageway, kerb lines, channel lines, centreline and measurable radii at the junction or proposed access.
• Spot levels at tight intervals along kerbs, the give way line, centreline and verge - gradients and crossfalls affect sightlines.
• Footways, verges and embankments - widths, back of footway lines and any slopes that could raise or lower the sightline.
• All potential obstructions with position, height and, where relevant, canopy or top height: walls, fences, hedges, trees, street furniture, signs, lighting columns, guardrails, utility boxes and cabinets, bus shelters, parapets and boundary structures.
• Vegetation detail - hedge centrelines and widths, tree trunks and canopies with ground level and height to canopy underside and crown top.
• Existing accesses and junctions nearby - geometry and levels, which may influence how the splay is measured and marked.
• Road markings, speed limit terminals and traffic signs - useful for context and for design speed assumptions.
• Drainage and surface features - gullies, manholes and covers, which help verify kerb lines and finished levels.
• Utilities evidence where it could constrain clearance works - combine with an Underground Services Tracing survey if clearance or relocation is likely.
• Sufficient survey extent along the major road to cover both Y distances appropriate to the design speed in each direction - often far beyond the red line boundary.

Extent, accuracy and height bands

Highways reviewers will look for robust evidence that the splay envelope is free of obstructions between typical assessment heights, commonly around driver eye level to object height. That means heights matter as much as positions. We capture 3D point data and levels at the features that govern visibility, using total stations, laser scanning for complex streetscapes and vegetation, and GNSS control for reliable georeferencing. If the required Y distances are long, we extend the survey accordingly so your drawing is not rejected for insufficient coverage. For planning context on why this precision helps, see our Topographical Survey and Planning Permission guide.

From survey to visibility splay drawing - our process

1. Confirm design speed and applicable guidance with the client or highways officer, then set X and Y parameters accordingly.
2. Establish control and datum, then survey the access, junction and road corridor to the full required Y distances in both directions.
3. Record the height and extent of potential obstructions within the likely splay envelope, including vegetation canopy heights.
4. Create a clean CAD base - kerbs, centrelines, edges and levels - then plot the X set back and Y lines with precise tangency to the road edge or centreline as required.
5. Check vertical constraints using spot levels and, where useful, long sections to confirm ground or object heights.
6. Annotate all obstructions.
7. Issue scaled drawings with layers for splays, road geometry, obstructions and notes, along with a coordinate list if required.

Deliverables you will receive

You will get CAD and PDF drawings showing the road topography and visibility splay envelopes. Where appropriate we can add long section plots along the nearside carriageway and a simple 3D surface for checking vertical intervisibility. If aerial context helps, we can add drone imagery captured under our Drone Inspections and Surveys service. For a deeper dive into survey methods, our Topographical Survey Equipment article explains how we collect precise road data.

Common pitfalls - and how we help you avoid them

Typical reasons for highways comments include insufficient survey extent along the road, missing heights for hedges and walls, incorrect referencing to grid and datum, and not showing the give way line precisely. Gradients are often underestimated - a small crest can block sightlines even when the plan triangle looks clear. Our workflows address these issues by extending coverage to full Y distances, capturing obstruction heights, checking vertical geometry and delivering a layered drawing that reviewers can assess quickly. If you are comparing options, see our guide on Topographical Survey cost and our broader Complete Guide to Topographic Surveys.

Ready to progress your planning application

If you need a road topographical survey for a visibility splay drawing, we can mobilise quickly and deliver drawings that meet highways expectations first time. Ask for a quote here: Quote for a Topographical Survey. You can also review typical pricing on our Survey Prices page. For complex sites - particularly where clearance involves utilities - consider combining with our Underground Utilities options so your design can move straight to mitigation. Accurate data, clear drawings and fast approvals - that is what we deliver.