1 Spatial Modelling of the Terrestrial Environment - Georeferencial

232 Spatial Modelling of theTerrestrial Environment
zones. A modal split procedure is used to allocate flows of goods and people from the land
use model to each of the travel types available. A discrete choice, log linear regression
framework is then used to allocate flows to modes followed by an assignment procedure
which converts the flows to vehicles and allocates them to routes based on the shortest
paths. This assignment defines an initial demand for use of the transport network which
can be compared with a measure of supply based on travel time, cost and congestion. A
further iterative procedure is used to adjust the pattern of flows on the transport network
until an equilibrium pattern is reached in which utility is maximized.
As the model iterates through time, feedback effects between the land use and transport
elements are incorporated. Land use adjustments take place according to the pattern of rents
and transport adjustments are controlled by journey costs and accessibility. The explicit
treatment of land use and transport interaction is a particularly powerful feature of the
modelling framework.
Given this structure the land use and transport model offers two important features for
monitoring the environmental impact of traffic emissions:
First, it can generate estimates of traffic flow by vehicle type and speed for each link in the
regional transportation network. These variables represent primary data for calculating
emissions.
Second, it offers an immensely flexible and powerful tool for predicting the effects of
planning policy packages. Changes in transport policy such as new network links or
improvements to capacity can be evaluated by modifying the network structure and
attributes in the model. Policy changes affecting land use (e.g. permission for new settlements,
industrial installations or controls on growth) can similarly be evaluated by
adjusting the structure of the input-output matrix.
The addition of an emissions impact framework to the land use and transport model
thus provides a basis for evaluating the air quality outcomes of different planning policy
packages.
11.4 The Emissions Model
The increasing importance of meeting air quality standards has led to a rapid growth in
the number of models available for estimating emissions concentration and dispersion.
Around 20 different modelling systems are identified by DETR (2000), giving rise to the
need for central government guidance on the basic principles, properties and performance
of these models. The important differentiating features appear to be level of detail, extent of
data, computational demands, complexity and, inevitably, cost. Validation of model results
is a major issue and as yet, there seems to be no guarantee that more detailed, complex
models will give better results in all situations. The conventional wisdom for air quality
modelling is thus to use a multi-stage approach in which low-cost, relatively simple models
are used for screening. Where application of these models reveals potential air quality
problems, more detailed, sophisticated models are used to make accurate assessments.
Given the policy-based nature of the work conducted here it was felt appropriate to
employ a modified version of the emissions screening methodology currently published in
the Design Manual for Roads and Bridges (DMRB, 1999). This approach was developed