2 Methodology

2.1 SELECTION OF ROAD LINKS IN THE SURVEY
Estimates of roadside NO2 concentrations in 2005 have been taken from the national modelling work which was conducted for the review of the National Air Quality Strategy (DETR et al, 1999). National modelling results indicated that a total of 761 major urban road links would be at risk of exceeding the annual mean objective of 21 ppb in 2005. 395 of these links are in inner London, 100 in outer London and 266 in other cities (Stedman et al, 1998). This pilot study has focused on approximately 100 links with the highest estimated concentrations outside London and also on the North Circular Road (A406) in London. Details of these links are shown in Appendix 1, for outside London, and in Appendix 2 for the A406. The survey in each city did not include all the road links with estimated concentrations of over 21 ppb, because of time constraints. For those locations outside London, a core set of the top 60 road links were surveyed and road links from a secondary set of the next 40 were covered in similar locations, where possible. Those road links not included in the survey, but in the top 100, were in Coventry, Derby, Bradford, Frimley and Nottingham. The road links that have been surveyed are marked in the tables in Appendices 1 and 2.

Detailed information on the split between built-up and non-built-up road links was not available for the major roads in Northern Ireland at the time of the national modelling used in this report . The only non-motorway in Northern Ireland links with estimated concentration greater than 21 ppb in 2005 are the A12 West Long and A55 University Road/Malone Road in Belfast. The likelihood of housing close to these roads has not been assessed. However, further GIS analysis is being undertaken as part of the European Union Daughter Directive Article Five Assessment. This provides an approximate split of built-up and non-built-up roads based on land cover information and will be used to improve future air quality modelling in Northern Ireland.

2.2 FIELD SURVEY
For each road link visited, various information was recorded using the questionnaire shown in Table 1 below. This included noting the presence of houses, number of houses and distances of the building facade from the kerb, along with other descriptive information about the road link such as number of lanes of traffic and the nature of other buildings. Video and digital photographs were also taken to provide further contextual information.

2.3 GIS ANALYSIS
For those road links where houses were found within 10 m of the kerb, GIS analysis has been used to combine road link and postcode delivery point data to derive additional estimates of numbers of dwellings along these links. A combination of field results and GIS analysis was required because the national road network, postcode and population GIS data sets are generally only accurate to within several hundred metres.

Postcode information has been obtained for each of the road links where houses were reported in the field survey. These various postcode points were then located in the Unit Postcode database. This database contains details of the numbers of delivery addresses within each unit postcode area. For example, part of York Road in Leeds has the postcode LS9 9DN. Within this unit there are 31 residential delivery points and 14 business or other non-residential delivery points. The database also contains grid references for the centre of each unit postcode area, although the level of positional accuracy of these points is on average within 100 metres. It has therefore not been possible to rely on the GIS alone to make an assessment of the numbers of residences close to the roads of concern.

Table 1 Questionnaire used in survey of road links
Road link ID from GIS database                                                
City  
Name of road  
Road No. e.g. A40  
Length (m)  
Identified correctly? Give any corrections here  
Any houses within 10 m? Y/N  
Description of aspect. Buildings both sides?  
Continue survey? Y/N  
Number of houses along road or block  
Distance of building facade from kerb (m)  
Hospital or School on road? Y/N (details)  
Number of road lanes  
Location of photos and videos taken  
Date / time  
Initials or surveyor.  

2.4 TRAFFIC FLOW DATA
The national model of roadside NO2 concentrations uses national traffic flow information. Locally-derived traffic flow information is also available for some locations, as well as modelled flows generated by other research work. The project team has collected some of these data, for comparison with the national modelling, for Manchester, Salford and Birmingham.

Analysis has been undertaken to compare the concentrations of NO2 that have been predicted using the national modelling and those using the above additional traffic flow data.

Introduction          Results
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