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FAQ 41 – Effect of Roadside Barriers

How effective are physical barriers alongside busy roads in reducing pollutant concentration at adjacent areas?

There continues to be extensive research undertaken into the effectiveness of roadside barriers on reducing pollutant concentration at the leeward side (i.e. downwind) of the barrier. Such studies are driven by either monitoring or modelling approaches and applied to a wide variety of barriers varying in design and dimensions. The presence of a barrier can change pollutant concentrations on both sides of the barrier by influencing the way pollutants originating from vehicle emissions are dispersed.

A physical barrier can be constructed out of a number of different materials including vegetation (such as hedgerows to create a ‘green’ barrier). For green barriers there is further consideration to the removal of pollution through photosynthetic uptake of gases, which is detailed more fully in FAQ 105. Therefore, this FAQ focuses on physical barriers that are not based on vegetation.

In recent years, Highways England have collated evidence gained from a number of projects on the subject of physical barriers based on physical studies and computational fluid dynamics (CFD) modelling. The extent of success of a physical barrier is shown to be dependent upon the dimensions (heights varying from between 4 – 9m); the construction material and the design (e.g. angle, overhang, etc).

For straight barriers (of 4m or 6m in height), it was concluded that there was no significant difference between NO2 concentrations monitored in front and behind the installed barrier. However, emerging evidence from an 18-month study completed in Dordrecht, Holland, of a barrier with a height of 9m and a curved cantilever presented reductions in NO2 annual mean concentrations of between 2 and 5µg/m3 behind the installed barrier. Following on from the Dordrecht study, CFD modelling based upon the design of a curved 9m barrier has predicted reductions upon an averaged region behind the barrier of up to 33-36% dependent upon traffic conditions.

Research continues to be ongoing and further information can be accessed through the following links.

Consideration to coatings of physical barriers that may give rise to absorption of NOx by the coatings is discussed elsewhere in FAQ 42.

Consideration to the impact that vegetation and trees have on air pollutant concentrations is discussed elsewhere in FAQ 105.


Summary of Research Projects to Improve Air Quality on or Close to the Strategic Road Network

Released: December 2019 (PDF, 2.3 MB, 23 pages)

Hosted on www.highwaysengland.co.uk

Canopy Research and Development

Released: April 2016 (PDF, 4.2 MB, 128 pages)

Hosted on www.highwaysengland.co.uk

Air Quality Barrier Computational Fluid Dynamic (CFD) Modelling

Released: March 2019 (PDF, 3.3 MB, 53 pages)

Hosted on www.highwaysengland.co.uk

Air Quality Barrier Report – Preliminary Feasibility Study

Released: January 2019 (PDF, 4.8MB, 28 pages)

Hosted on www.highwaysengland.co.uk

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