Issues of poor air quality are often highly localised: the 31 non-compliant sections of the Strategic Road Network represent about 51 miles of road, out of the 4,500 managed by National Highways.13 National Highways estimates about 240 to 250 houses or properties are directly affected by these exceedances, as many are not in urban areas, and levels of NO2 drop off quickly away from the road.14 Tackling these exceedances will require interventions that may be unpopular with local residents; National Highways told us that it is looking at developing two nine-metre-high barriers in the centre of Guildford, which is the location with the most severe air quality impact from the breaches identified on the strategic road network.15 These would require planning permission, which is unlikely to be 5 C&AG’s Report, paras 3–5 6 C&AG’s Report, Figure 7 7 Q 46 8 Qq 44 - 46 9 C&AG’s Report, para 17 10 Q 46 11 Q57 12 C&AG’s Report, paras 1.21, 1.22 13 Q 19 14 Q 43 15 Q 12 10 Tackling local air quality breaches straightforward if residents are not aware of the need to tackle air quality at the location.16 Other measures face similar barriers in public understanding; for example, speed limits are introduced for a variety of reasons, and in at least four locations are driven purely by air quality concerns, but this is not always clearly communicated.17