3 Results

3.1 FIELD SURVEY RESULTS
Appendix 3 contains a table of the questionnaire results for the 103 roads surveyed. Table 2 shows the cities in which there are road links predicted to have the highest exceedances of the 21 ppb objective for 2005 and those that have been surveyed. Table 3 contains the survey results for those 23 road links where houses were found within 10 m of the kerb.

Where a range of distances from the kerb has been noted in the survey, all houses have been included in the results even though some are slightly over 10m. There is some uncertainty over this distance and therefore these houses have been included in order to prevent an underestimate of exposure.

Table 2 Summary of Survey Results

In general, the road links that have been surveyed are inner ring-roads in cities or major radial routes into the city centres. Many are dual-carriageways alongside commercial areas. Video images have been taken on the majority of the roads surveyed (not including those in London or Manchester). A video has been produced showing these roads.

It is important to note that some of the road links have been found to be open in aspect. These do not conform to the assumption that urban roads are in canyons which results in less efficient pollution dispersion and higher exposure. This may mean that the NO2 concentrations predicted for these road links are over-estimates, but this error is difficult to quantify from the phtograph and video evidence.

Table 3 shows that Birmingham, Liverpool, Manchester and London have four or more road links where residential buildings are close to road links expected to have roadside concentrations of NO2greater than 21 ppb in 2005. These cities will therefore be considered in more detail below. In most of the other cities included in the survey, none of the road links had houses close to the kerb or there were very few, as in Hull, Leicester and Portsmouth. Leeds is an exception, where over 50 houses were found on one road (York Road). This particular road will therefore also be considered in more detail in this report.

Table 3. Details of road links where houses were found close to the kerb

3.2 ANALYSIS OF POSTCODE DATA
Table 4 shows the estimated numbers of houses found in the field survey compared with the numbers of residential delivery points in the Unit Postcode database. The Unit Postcodes database includes all residential delivery points and not just those within 10 m of the kerb. These numbers will tend to over estimate the total population exposed to high NO2 concentrations. However, there are also further uncertainties relating to the positions of these houses. The field data collected may correspond, in some cases, to a longer stretch of road than that defined as the road link in the model. This could be as a result of a misinterpretation of maps of the road links in the road network database, which do not all correspond to individually named roads. Hence, there may be some errors the field data. Additionally, houses adjacent to the road may have addresses, and therefore postcodes, that are in fact on other roads, such as at corner locations, in blocks of flats or where houses or gardens back on to the main road. The latter point is exemplified on road link 7055 on the North Circular Road in London. In this case, houses were found but they did not occur in the postcode database. With further analysis it was found that the address at that point is Waverley Gardens and this address has been used to find the residences by postcode.

The above factors explain some of the differences in the two estimated total numbers of houses shown in Table 4. Having considered these caveats, it can be seen that these roads have been correctly identified as having housing on them, and the estimated total numbers of houses on individual road links are broadly similar with only a few exceptions.

Figures 1 - 20 show the locations and photographs of the road links surveyed in Birmingham, Liverpool, Leeds, London and Manchester. The maps also include the locations of the postcode points in relation to the road links that have been found to have houses on. These are the postcode points that have been used to calculate the totals in Table 4. The points on the maps are not always adjacent to the road links because the positional accuracy of both of these data sets is only to about 100 m. Photographs of the road links where houses are present within 10 m of the kerb are presented in this section, where available. A full set of photographs are provided in Appendix 5 of the paper copy of the report. No photographs are available for Liverpool, Hull, Doncaster, Newcastle, or Portsmouth.


Table 4. Comparison of field survey and postcode data estimates of numbers of residential dwellings along road links with predicted annual mean NO2 concentrations of >21 ppb in 2005 and houses within 10 m of the kerb

!The following pages contain images. They will be slow to load up!
Figure 1 Birmingham: Locations of road links
Figures 2-6: Birmingham
Figures 7-8: Leeds
Figure 9 London A406: Locations of road links
Figures 10-15: London
Figure 15: Liverpool
Figures 16-20: Manchester

3.3 ESTIMATE OF EXPOSED POPULATION
From the data that have been collected in this study, it is possible to make an estimate of the total number of people exposed to exceedances of the NO2 annual average objective along those roads surveyed. In this survey, 23 of the 103 road links have been found to include housing within 10 m of the kerb. Estimates of about 1300 and 2600 households on these road links have been derived from the field survey and postcode database respectively. The average size of a UK household is 2.7 people, giving a total of about 3510 to 7020 people exposed along these road links (assuming that the population in 2005 will be the same as in 1999).

3.4 TRAFFIC FLOW DATA
Air quality modelling is being undertaken by a number of institutions in the UK, using a variety of different methods and data sources. A secondary aim of this study was to analyse the effects on the results of the national modelling of annual mean NO2 for 2005 of changing the traffic flow data for a small sample of road links.

There are two primary sources of road traffic data currently in use for preparing urban traffic emission inventories: traffic surveys and transportation models. Information from traffic surveys is attractive because it relates to real traffic on real roads whereas transportation models are a computerised reflection of the actual conditions. However, traffic surveys have the disadvantage that they only provide information relating to the specific survey points, rather than area wide information. On the other hand, transportation models are available for most of the urban area and are comprehensive (Hutchinson and Clewley 1996).

Local measured traffic flow data has been obtained for Manchester and Salford and modelled traffic flows are available for Birmingham. On those links where the national modelling predicted NO2 concentrations in 2005 to be 21 ppb or greater, NO2 concentrations have been calculated again using the local data.

Comparisons of the Manchester and Salford data with those used in the national modelling are shown in Table 5 and Table 6. Those links where NO2 concentrations in 2005 have been previously predicted to be 21 ppb or greater are shown in bold. In most cases the local data are higher than the national data, with the total flows being closer than those for LGV and HGV. These differences are consistent with the expected variations in measurements of this sort.

Modelled flows for roads in Birmingham have been obtained from Cambridge Environmental Research Consultants (CERC). These data were produced by the London Research Centre and represent flows in 1990/1991 (Hutchinson and Clewley 1996). A comparison with the national traffic flow data is shown in Table 7.

A few caveats should however be considered in relation to the comparison for Birmingham. First, the CERC data have been compiled using a different road link network and therefore a direct comparison between road links is not possible in some cases. The CERC network is more detailed, therefore average flows have been calculated where multiple CERC links that make up a national dataset link (those marked with * in the table have been calculated in this way). Second, the national dataset consists of measured flows and the CERC data are modelled flows. This difference in methods results in considerable differences in the flows on the individual links and the total for all links. Third, the base year of the two datasets is different - 1990/1991 for the CERC data and 1996 for the national data. The differences between the CERC and national data are greater on individual links than those for Manchester and Salford, but differences between the total flows are similar across these three examples.

The above data sets have been used to make a comparison of the NO2 concentrations predicted for those links where there are houses at less than 10m from the kerb, using the national data and newly obtained measured and modelled data. This comparison is shown in Table 8.
This comparison shows a broadly similar pattern to the previous comparison of traffic flows, with more variation on the Birmingham links than in Manchester and Salford. 15 (71%) of the links have concentrations within a 10% range of the national modelled NO2 concentration.

There are three key factors that have influenced the recalculated concentrations:
The relative numbers of vehicles has been kept constant for the links in Birmingham, where only modelled total flows were available, but in Manchester and Salford data were available for LGVs and HGVs. A comparison of these numbers is given in Tables 5 and 6.

The background NOx concentration used in the modelling has been kept constant for the calculation of roadside NO2 concentration using local traffic flow data. This background portion of NOx contributes roughly 30 - 50% to total roadside NOx. Therefore, on links where the differences are large for the traffic flows, smaller differences are found in NO2 concentrations. Furthermore, the relationship between roadside NOx and roadside NO2 is non-linear, and therefore this accounts for some of the further differences between the two sets of results.
Methodology         Conclusions
Report and site prepared by the National Environmental Technology Centre, part of AEA Technology, on behalf of the UK Department of the Environment, Transport and the Regions