1 Spatial Modelling of the Terrestrial Environment - Georeferencial

Modelling the Impact of Traffic Emissions on the Urban Environment 233
by the UK Transport and Road Research Laboratory and has been widely used since the
early 1980s for assessment of vehicle emissions in relation to air quality standards. It has
the major advantages of being straightforward to implement, computationally efficient and
being robust in a wide range of situations. Furthermore, the underlying assumptions that
limit the performance of the model have been clearly evaluated by Hickman et al. (2000).
The DMRB methodology enables evaluation of the emissions impact for specific road
alteration/upgrade schemes and makes provision for calculation of concentration estimates
at selected receptor locations in the vicinity of the scheme which might be important in
terms of their environmental sensitivity or population impact. It also provides a basis for
estimating the net contribution of a transport infrastructure scheme to regional and global
totals. It involves:
Separation of peak hour traffic flows into light duty and heavy-duty components for the
relevant link(s) in the transport network.
Calculating a ‘relative emission rate’ for each type of traffic which takes into account
the national composition of the vehicle fleet and reflects changes in engine and fuel
technology which are tending to reduce emissions levels through time. The basis for this
calculation is a graphical relationship between relative emission rate and year, which is
in turn based on a broad analysis of average fleet composition, vehicle type and age.
Calculating a speed correction factor to account for the variation of emissions relative
to the average speed on the link. This is based on an empirical understanding of average
vehicle performance at different speeds.
Calculating an emissions concentration for the receptor point based on the relative emissions
rate, the speed correction factor for each vehicle type and the distance of the receptor
location from the source. This calculation uses a distance decay relationship specific to
each pollutant derived from a Gaussian dispersion model. It assumes a constant wind
speed of 2 m/s with wind directions being evenly distributed around the points of the
compass.
Conversion of the peak period emissions values to annual totals based on an empirical
relationship.
The basic model described above is designed to be used for a small number of links and
receptors. If necessary, it enables emissions estimates to be made using ‘pencil and paper’
methods in conjunction with readings taken from graphs of the underlying relationships
employed.
In this study wide area estimates of emissions concentrations for a regional transport
network were required. The procedure was thus modified to treat every cell in a 25-m
resolution grid positioned over the Cambridgeshire study area as a receptor point. Taking
CO as an example, regression relationships were established for the light (equation (3))
and heavy duty (equation (4)) speed correction factors:
LC 1 = 14.6 − 0.4718S + 5.29E − 03S 2 − 1.9E − 05S 3 + ε
R 2 = 0.98 (3)
HC 1 = 4.3 − 0.102S + 7.47E − 04S 2 − 6.33E − 07S 3 + ε
R 2 = 0.99 (4)