Ogilvie et al. BMC Public Health 2010, 10:703 http://www.biomedcentral.com/1471-2458/10/703 Page 5 of 13 Figure 2 Guided busway route. Credit: Cambridgeshire County Council. Reproduced with permission. Comparisons The likely catchment area (or zone of influence) of new transport infrastructure of this kind is also subject to considerable uncertainty. It is likely to differ between urban and rural parts of the route, because rural com- muters may be more likely to use feeder modes of trans- port such as park-and-ride and cycling to access the new public transport services. Furthermore, it does not necessarily follow that ‘exposure’ to the intervention is limited to (or greatest among) residents of neighbour- hoods or settlements immediately adjacent to the route, since commuters from other areas could intersect with busway services at transport interchanges such as Cambridge railway station. Defining ‘intervention’ and ‘control’ groups is therefore not as straightforward as it might be in the context of a typical parallel-group con- trolled intervention study. On recruitment to the study, members of the cohort are allocated to provisional ‘intervention’, ‘control’ and ‘reserve’ groups on the basis of their home postcodes. An a priori ’intervention’ area has been defined com- prising a set of unit postcodes (the smallest unit of postal geography in the UK) that fall within, or encroach upon, either (a) a 600 metre road network distance buf- fer around the stops along the urban sections of the route or (b) larger areas encompassing the towns and villages along the rural sections of the route. The urban buffer reflects the pedestrian access distance used to model the transport impacts of the busway at the plan- ning stage, while the rural buffers are intended to reflect the potential use of other feeder modes including the car. An a priori ’control’ area has also been defined: this comprises two areas of Cambridge city (Barnwell and Romsey Town) with similar aggregate socioeconomic and spatial characteristics to those of the urban parts of the ‘intervention’ area but with no direct access to the busway, as well as a 180° sector of the rural portion of the study area on the opposite side of the city to the northwestern radial axis of the main section of the bus- way. All members of the cohort living within the study area but not within the ‘intervention’ and ‘control’ areas are assigned to the ‘reserve’ group. The influence of the busway will not, in practice, be limited by any arbitrarily-selected distance buffer. The