1 Spatial Modelling of the Terrestrial Environment - Georeferencial

234 Spatial Modelling of theTerrestrial Environment
Where LC 1 is the light duty vehicle correction factor, HC 1 is the heavy duty vehicle
correction factor and S is the average speed on the link.
These were then used in conjunction with the appropriate relative emission rate (E) to
compute emissions totals for each link in the transport network. Finally, the relationship
between distance (D) and concentration (C) was modelled (equation (5)) and used to estimate
concentrations for each cell in the grid on the basis of the distance to the closest point
in the transport network:
C = 0.551 − 1.14E − 2D + 8.15E − 5D 2 − 1.93E − 7D 3 + ε
R 2 = 0.93 (5)
As the analysis was concerned with just road traffic emissions, no attempt was made
to deal with background emissions levels which are normally treated as a constant at this
stage in an air quality analysis. All analyses and calculations were carried out within the
ArcView GIS version 3.2, which allowed for display and analysis of results.
11.5 Modelling Emissions Impact in Cambridgeshire
Evaluation of the relationship between planning policy decisions and emissions outcomes
for Cambridgeshire involved implementing and running the land use/transport model for
both reference and policy cases. The reference case aimed to reproduce the actual behaviour
of land use and transport during the study period and provided a basis for model validation.
The policy case then enabled a package of alternative ‘what if’ policies to be tested.
The transportation patterns associated with each case were used for mapping emissions
concentration, which was in turn overlaid onto the pattern of settlement to provide an index
of impact. Each stage in this process will now be described in turn.
11.5.1 Building the Cambridgeshire Model
Work on the construction of a model for Cambridgeshire was carried out between 1998
and 2000. The study area, key towns and road links are shown in Figure 11.2, whilst an
overview of the model structure is provided in Figure 11.3. The work depended heavily on
the extension of an existing model of the City of Cambridge to cover the 108 wards within the
Districts of Cambridge, South Cambridgeshire, East Cambridgeshire and Huntingdonshire
(see Figure 11.3). For the purposes of analysis, wards were aggregated into 67 internal
zones and further 10 zones were used to represent the external study area including the
surrounding administrative districts, London and the rest of the country. The model was
calibrated largely with 1991 Census and other survey data and operated through time in
5-year steps from a starting forecast year of 1996 through to 2006.
Calibration data sources included:
employment by ward from 1991 census Special Work Place Statistics;
households and employed residents by ward from 1991 Census Local Base Statistics;
commercial floorspace by District, interpolated from Department of the Environment
time series statistics.