Case study: Southgate Road, Potters Bar

The property

A semi-detached house on Southgate Road, Potters Bar, Hertfordshire (EN6 5EA) — a residential road that is also the A111, carrying local through-traffic — with the garden's seating area set back from the road behind an existing boundary fence. We ran this address through AcoustaPLOT's full assessment: real terrain from national LIDAR data, the real building footprint, the real fence position and height, and — as of the correction below — the real measured traffic count for this stretch of road, modelled with CRTN (1988) for the road-traffic source and ISO 9613-2 for outdoor sound propagation, including real screening and reflection from the surrounding buildings.

What the assessment actually found

At the road-facing façade, the modelled exposure is 67.2 dB Lₑden. That's still a level where BS 8233 and WHO guidance both flag noise as a real factor in how habitable outdoor amenity space and ground-floor rooms facing the road will feel — particularly for anyone spending time outside, or sleeping with a window open on that side — even though it is noticeably lower than the figure we published when this case study first went live (more on that below). AcoustaPLOT's own garden-comfort benchmark sits at 55 dB Lₑden — both the façade and garden figures here sit well above it, which is why this address reads as a real, not marginal, case for mitigation.

At the garden seating area, roughly 13m back from the road and behind the existing 2m boundary fence, the modelled level comes down to 66.6 dB. On its own that looks like only a modest reduction from the façade figure — 0.6 dB — and we're not going to round that up to sound better than it is. What that comparison hides is the fence's real contribution: modelled with no fence at all, the same spot comes out at 69.8 dB, so the existing fence is doing roughly 3.2 dB of work. The façade and garden numbers end up close together not because the fence does nothing, but because the seating area sits only a little further from the road than the house itself does — being further away and having a fence are two separate effects, and here the fence is carrying almost all of the benefit.

A correction, made in the open

When this case study first went live, on 2026-09-01, the façade figure was 71.4 dB and the garden figure was 69 dB. Both numbers were real engine output — CRTN and ISO 9613-2, the real fence, the real terrain — but the traffic count feeding them was not: it was a placeholder AADT figure (40,000 vehicles a day, 8% HGVs) shared with an internal reference render, not this road's actual measured traffic. That placeholder was clearly documented as such internally, but it was never corrected before the case study was published, and the published text did not carry the caveat.

We found the Department for Transport's own count point for this exact stretch of road while researching this correction: count point 56641, on the A111, 18.4 metres from this address — and, unlike many DfT figures, a manual count rather than an estimate. It records 16,009 vehicles a day with 2.3% HGVs, both a good deal lower than the placeholder. Re-running the same engine with that real figure is what produced the 67.2 dB and 66.6 dB above — a materially lower, and now fully sourced, pair of numbers. We're publishing the correction rather than quietly editing the page, for the same reason we built AcoustaPLOT in the first place: a number you can't trace back to where it came from isn't worth much, on our own case study or anyone else's report.

Why a taller fence alone doesn't fix this

Once the traffic count was corrected, we also checked whether a taller version of the same fence, in the same position, would help — and it barely does. Modelled at 2.5m instead of 2m, the same seating spot comes out at 66.2 dB; at 3m, 66.1 dB — a few tenths of a decibel for an extra metre of fence height. That's a genuinely useful thing to know before spending money on a taller garden fence expecting a big win: at this particular geometry, height isn't where the benefit is.

That doesn't mean nothing would help. A barrier's effectiveness depends on more than height — its position relative to the source and the receiver, how far it runs along the boundary, and whether sound reaches the seating area by wrapping around its ends rather than over the top. This modelled fence has no return sections (no wings turning the corner at each end), which is exactly the kind of detail a Barrier Design report is built to explore properly, option by option, rather than guessing that “taller” automatically means “quieter”.

The honest takeaway

This is still a real assessment of a real address — the property, the fence, the terrain and the buildings around it are all exactly as they are on Southgate Road — and now the traffic count is real too. The story it tells changed once the last placeholder number was replaced with a measured one: less noise at the façade, a garden benefit that comes almost entirely from the fence rather than distance, and a taller fence in the same spot that would barely move the number. None of that was obvious from the original, non-DfT-sourced figures. That's exactly why AcoustaPLOT models a specific address with real inputs rather than offering a generic estimate — and why we're comfortable publishing this correction rather than leaving the old numbers up.

If you're weighing up an address like this

Southgate Road isn't a dramatic outlier. Our "State of UK Traffic Noise" analysis put an A-road count of this size roughly in the middle of the national distribution, not at either extreme. That's the point: most properties near a classified road land somewhere in this unremarkable middle ground, where the difference between a placeholder estimate and a real, sourced number is often the difference between “sounds concerning” and “manageable, with the right barrier decision.” A full AcoustaPLOT report runs the same real-address pipeline used here — real terrain, real fence geometry, and wherever a DfT count point exists nearby, a real measured traffic figure — for your own address, not a generic one. If your own address turns out to be quieter than this one, that's a genuinely useful thing to know before spending money on a barrier it doesn't need — and if it's louder, the same report tells you that too, with the same DfT-backed traffic figure wherever one exists nearby.

Methodology

This case study uses AcoustaPLOT's standard engine: CRTN (1988, “Calculation of Road Traffic Noise”) for the road-traffic source level, and ISO 9613-2-style line-source propagation for the spatial result, including over-the-top and lateral end-diffraction from the fence's real returns, real building screening and façade reflection from the surrounding houses (their real position and height; footprint width is a type-based estimate, not survey data), and real terrain elevation at the source, from national LIDAR data.

Traffic input is the Department for Transport's AADF count point 56641 on the A111, 18.4m from this address: a manual count (not a DfT estimate) of 16,009 vehicles a day, 2.3% HGVs, for 2025 — the same dataset behind our “State of UK Traffic Noise” article. The rear boundary length used in the model (12m) is a typical-UK-property default, not a measurement taken at this address, and has not been corrected in this pass; a full Barrier Design report for a specific address always starts from the real drawn boundary rather than this default. Figures are Lₑden (the EU day-evening-night noise index), rounded to one decimal place. We use Lden rather than a peak or average-hour figure because it is the index UK planning guidance and the Environmental Noise Directive both benchmark against — a single number that already weights evening and night-time traffic more heavily, on the basis that the same vehicle count is more disruptive outside the working day.