Road traffic noise and when a barrier is justified

An acoustic barrier only works where it breaks the line of sight between the source and the receiver, and on road traffic noise that means it has to be tall enough to screen a lorry exhaust stack at the far kerb from a first-floor bedroom window. On a scheme with two-storey housing set back 15m from a road, the barrier height needed to protect the upper floor is usually unbuildable, unacceptable in townscape terms, or both.

So the first question on a road noise scheme is not what height of barrier is needed. It is whether a barrier is the right tool at all. Layout, orientation and building envelope do more for upper-floor bedrooms than any fence, and a barrier that protects gardens while doing nothing for bedrooms should be presented as exactly that.

Calculating facade noise levels from traffic

Road traffic noise prediction in the United Kingdom follows the Calculation of Road Traffic Noise method, using traffic flow, percentage of heavy vehicles, mean speed, gradient, road surface and the geometry between the road and the receiver.

Three inputs cause most of the argument. Flow data, which should come from a recent count rather than an old model and should be the annual average weekday figure the method requires. Heavy vehicle percentage, because heavy vehicles dominate the level and a figure taken from a different road type will be wrong. And road surface, since a low-noise surface course can be worth several decibels, and assuming one that the highway authority has no plans to lay is a common way to produce an optimistic result.

Then convert to the indices the assessment needs. Facade levels are usually reported as free-field or facade values depending on the criteria applied, and the two differ, so state which convention is used at every assessment position. A report that mixes them is a report that will be queried.

Validate the model against measurements. A short attended measurement at a known position, compared with the prediction for the same position and time, tells a reviewer that the model is calibrated. That single page of validation prevents a lot of correspondence.

Where a barrier works and where it cannot

Barrier performance depends on the path length difference between the diffracted route over the top of the barrier and the direct route. In practical terms, three rules follow.

A barrier close to the source or close to the receiver works better than one in the middle. At the source it screens more of the sound; at the receiver it can be lower for the same effect.

Performance falls away rapidly as the receiver rises. A 2m fence that gives a worthwhile reduction at ground-floor and garden level gives very little at a first-floor window, and almost nothing at second floor.

Low frequencies diffract more readily than high, so the perceived character of what remains changes. A barrier removes the tyre hiss and leaves the rumble, and residents describe the result as still being noisy even where the measured reduction is real.

That last point is worth saying plainly in the report. An honest statement that a barrier will reduce garden noise by a stated amount while leaving upper-floor bedrooms reliant on the building envelope is more credible than a prediction that implies the problem is solved.

Height, length and the flanking problem

A barrier that is right in section and too short in plan does very little, because sound arrives around the ends.

The working rule is that the barrier needs to extend well beyond the receiver in both directions, and the further the receiver is from the barrier, the longer it needs to be. On a residential frontage that usually means a continuous barrier along the whole site boundary, returning down the side boundaries far enough to cut the flanking paths.

Gaps defeat the whole thing. A pedestrian access, a vehicle crossing, a drainage gap under the fence, a gate that does not seal: each is a hole through which sound arrives, and a 100mm gap along the base of a timber fence can cost most of the predicted benefit. Detail the base, specify an overlap at any access, and model the barrier as it will be built rather than as a continuous plane.

Two further constraints usually govern the final height. Daylight to the dwellings behind it, since a 3m barrier close to a ground-floor window has its own amenity cost. And the highway authority's sightline requirements where the barrier is near a junction or an access.

Barrier materials, gaps and maintenance

Mass is what matters acoustically, and the threshold is lower than people expect. A superficial mass of around 10 to 15 kg/m² is generally enough for the barrier to be acoustically opaque, which a close-boarded timber fence with overlapping boards achieves and a lapped panel fence does not.

The realistic options, with their practical drawbacks:

1. Close-boarded timber with a substantial rail and post, which is inexpensive to build and has a service life measured in a couple of decades.
2. Acoustic timber panels with an absorbent core, which perform slightly better on the source side and cost more.
3. Earth bunds, which have an indefinite life, need land and can be planted, and are the option most likely to survive fifty years.
4. Masonry or concrete, which lasts and which reads as a hard urban edge that a landscape officer will resist on a rural edge.
5. Transparent acrylic, used where views or daylight must be kept, with a maintenance and bird-strike burden.

Maintenance is the question a council will ask and the one applicants answer worst. A timber barrier on the boundary of 40 gardens, with no adopted owner, will have gaps within fifteen years. Put the barrier on land in a single ownership, whether a landscape strip or a bund, and secure its maintenance through a management company. The detail of when a fence earns its place is covered in acoustic fences and when a barrier actually reduces noise.

Visual effect and highway authority consent

A barrier long enough to work is a substantial piece of townscape. On an edge-of-settlement site it will be visible from the road and from the countryside beyond, and it can become the single most prominent feature of the scheme.

Deal with it as a designed element. Show it in elevation and in the street scene, show it in the landscape drawings with the planting that will soften it, and be realistic about how long that planting takes to do anything. A bund with a hedge on top reads very differently from a 3m fence, and on a rural edge the difference decides the application.

Where the barrier sits on or near the highway, or affects visibility at an access, it needs the highway authority's agreement, and where it sits on highway land it needs a separate consent. Neither is automatic, and a barrier designed on the assumption that it can stand on the verge is a barrier that may have to move 3m into the site, which changes its height.

Alternatives: layout, bunds and envelope design

The hierarchy in the national noise guidance runs from avoiding the effect to mitigating it, and layout comes first.

Put the noise-tolerant uses at the front. A row of garages, a parking court, a commercial unit or a terrace with single-aspect flats facing away from the road all screen what sits behind. Orientate dwellings so bedrooms face away from the road and habitable rooms have an alternative aspect. Use the building line itself as the barrier by running a terrace parallel to the road.

Then the envelope. Glazing and ventilation specification can achieve internal levels well within the guidance figures, and the question becomes whether occupants can open windows and whether external amenity space is acceptable. An assessment that relies on closed windows with mechanical ventilation has to say so and has to address overheating at the same time, because the two interact.

Gardens are where barriers still earn their place. External amenity space guidance is about daytime levels at ground level, which is exactly what a barrier or bund delivers.

Justifying the choice in the acoustic report

Set out the options considered and why the chosen package was selected, rather than presenting a barrier as the only answer. Officers and inspectors respond well to a report that shows the layout was tested first.

The Noise Impact Assessment should give the traffic inputs with their source and date, the validation against measurement, predicted free-field and facade levels at each assessment position on each floor, the barrier performance with its height, length, mass and position, the residual levels internally and externally, and the ventilation and overheating consequences of any closed-window assumption. Where the road also drives an emissions question, the two assessments should share the same traffic data, as set out in air quality neutral emissions benchmarks.

Then offer the conditions: the barrier specification and position, its construction before occupation of the dwellings it protects, a maintenance mechanism, and the glazing and ventilation specification.

Enter your postcode on PlanScope to see which noise and environmental reports your scheme is likely to need.

This is general guidance rather than legal advice. Check the position with your local planning authority, the highway authority or a qualified acoustician before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.