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The source adds the following information about the different monitoring types:

Roadside sites are generally within one to five meters of a busy road and usually located around adult breathing height. These sites give the best estimate of public exposure along busy roads. Roadside sites are useful for identifying air quality hotspots due to traffic that may have potential health impacts - especially those frequented by large numbers of pedestrians.

Urban background sites are located further away from sources of emissions and are not influenced by one single nearby pollution source. In London, traffic is the main source for background sites to avoid, and there are guidelines about how close background sites can be to roads. The benefit of urban background sites is they are usually representative of all other urban background locations within an area of several square kilometers. For this analysis, “Suburban” and “Urban Centre” sites have been treated as “Urban Background.