A40 road noise: how it affects nearby properties
The A40 changes character dramatically along its length — from the heavily trafficked Western Avenue in west London to quiet rural stretches through Oxfordshire, Gloucestershire and towards Wales. This makes a location-specific assessment especially important: an A40 address in Northolt is a completely different noise proposition to one near Burford.
Traffic on the A40: what the 2025 DfT counts show
Western Avenue dates from the 1930s, and the A40 has been quietening, mile by mile, as it runs west towards Goodwick near Fishguard. The 182 DfT count points tell the same tale. Their median is only 13,700 vehicles a day, 58 points record fewer than ten thousand and a quarter sit under 8,100. Yet 32 points pass fifty thousand, eight pass a hundred thousand, and the busiest tenth start at 89,300, topping out at 116,893. A median is a poor guide on a road of about 261 miles that begins in the City of London. It describes the long rural majority, not the London frontage.
Read the A40 from the Welsh end and its history runs backwards. In Pembrokeshire a new 6km section between Llanddewi Velfrey and Redstone Cross opened on 2 May 2025, costing £60m, and the western third (38 points) has a median of just 10,000 and a peak of 43,969. Between Brecon and Carmarthen the road is single carriageway along the Usk and Tywi valleys, while twin tunnels beside the Wye Valley AONB near Monmouth are the only ones on its whole length. The middle third is similar, at 11,100, peaking at 59,132. Only the eastern third, with 89 points, changes the picture: a median of 22,500, a top count of 116,893, and the Westway's mostly six lanes.
Lorries are a modest presence on most of the A40, with a median share of 3.7 per cent across the 182 points and one point in ten above 7.6. Yet the readings run from 0.5 per cent to 15.8, which means the single assumption used in the table can be well off at either extreme. The indicative figures use 3.7 per cent, so a site with a share near the top of the range would sit above them.
| Quarter of points carry under | 8,100 vehicles/day |
|---|---|
| Median point | 13,700 vehicles/day |
| Three-quarters carry under | 31,200 vehicles/day |
| Busiest tenth carry over | 89,300 vehicles/day |
| Busiest single point | 116,893 vehicles/day |
The A40 on the ground
- Runs from the City of London to Goodwick near Fishguard in Pembrokeshire, about 261 miles. en.wikipedia.org
- Western Avenue (1930s) is motorway-standard dual carriageway from Paddington to the M40, apart from two signalled junctions; the Westway section is mostly six lanes. en.wikipedia.org
- Beside the Wye Valley AONB near Monmouth, the A40 passes through twin tunnels, which are the only tunnels on its whole length. en.wikipedia.org
- Between Brecon and Carmarthen the A40 is single carriageway along the Usk and Tywi valleys; Carmarthen to St Clears is dual for about ten miles. en.wikipedia.org
- The A40 Over roundabout at Gloucester was improved in 2018 with extra approach lanes and a third circulating lane, following a 2015 first phase. gloucestershire.gov.uk
- A new 6km A40 section between Llanddewi Velfrey and Redstone Cross, Pembrokeshire, was officially opened on 2 May 2025; cost £60m. gov.wales
How loud is the A40, and how fast does it fall away?
A40 homes are measured against two very different roads. For the typical section, with 13,700 vehicles a day, a house 20 m from the nearest carriageway is indicated at 66 dB Lden, with 62 dB at 50 m, 60 dB at 100 m and 57 dB at 200 m. Where flow is as heavy as the 89,300 reached by the busiest tenth of points, the figures are 73, 70, 67 and 65 dB. Both rows assume open flat ground with no screening, and a steady 50 mph that much of the A40 will not match, from signalled junctions on Western Avenue to single carriageway valleys. An address with barriers, cuttings or buildings nearby will differ from these round figures.
| Flow | 20 m | 50 m | 100 m | 200 m |
|---|---|---|---|---|
| Typical section (13,700 vehicles/day) | 66 | 62 | 60 | 57 |
| Busiest tenth of sections (89,300+ vehicles/day) | 73 | 70 | 67 | 65 |
Areas near the A40
Passing through: Greater London · Buckinghamshire · Oxfordshire · Gloucestershire · Herefordshire.
In London, dense residential frontage sits directly on the A40, often with existing noise barriers. Through Oxford and High Wycombe, flows remain substantial. AcoustaPlot assesses the specific stretch nearest your property rather than treating the whole A40 as one road, which matters a great deal on a route this varied.
Recent on the A40
A committed scheme will upgrade a 4.5-mile stretch of the A40 between the new Eynsham Park & Ride and Wolvercote in Oxfordshire, with construction due to start in late 2025/early 2026 and the first phase completing in summer 2028.
How AcoustaPLOT assesses A40 noise for your property
With about 342 km between its two ends, the A40 cannot be assessed from a single figure. AcoustaPLOT searches for one of the 182 DfT count points on the A40 within 8 km along the road from the address and uses that point's measured flow and lorry share, whether it is a quiet Welsh valley or the heavily used eastern end. It then works from the actual distance, terrain, ground, buildings or barriers and any other roads or rail nearby. It reports the day-evening-night (Lden) and night-time (Lnight) levels at your facade, and reads Defra's mapped value for any railway noise where available.
Check my property near the A40 →
A40 noise: common questions
Is the eastern A40 really that much busier than its western end?
On these counts, yes. The eastern third, running towards London, has 89 points, a median of 22,500 and a maximum of 116,893. The western third, 38 points, has a median of 10,000 and a maximum of 43,969. Eight points on the whole road exceed a hundred thousand and the busiest tenth start at 89,300, so the heaviest flows are concentrated in places rather than general.
What was done to the A40 Over roundabout at Gloucester?
It was improved in 2018 with extra approach lanes and a third circulating lane, following a first phase in 2015. Changes like these can alter where traffic queues and flows, which is one reason an address is judged from the nearest 2025 count point on the A40 rather than from older assumptions about the road.
Are there tunnels on the A40 that change noise levels?
There are twin tunnels beside the Wye Valley AONB near Monmouth, and they are the only tunnels on the whole length of the road. A home near them is outside what an open-ground table describes. Everywhere else, from Pembrokeshire to Western Avenue, the open-ground figures are a starting point only, and a nearby count point sets the flow.
What will the Oxfordshire upgrade between Eynsham and Wolvercote change?
A committed scheme will upgrade a 4.5-mile stretch of the A40 between the new Eynsham Park & Ride and Wolvercote. Construction was due to start in late 2025 or early 2026, with the first phase completing in summer 2028. Today's DfT counts describe the road as it is now, so they will not show the effects of that scheme.