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hours). Therefore, for these control strategies, the environmentally sound choice may best
be guided by optimising the resulting traffic conditions, assuming that emissions and
therefore air quality will also benefit. Again, integrated and detailed simulation of traffic
and air quality is less important.
For developing medium term strategies, the demands on integration between the traffic
model and the air quality model are highest, whilst then, also, the time scale allows for more
detail and dynamics in each of the components. Examples of such strategies at the urban
level are demand control during adverse meteorological conditions (e.g. the odd/even
numberplate mechanism used in Paris in Summer 1997), the planning of signing and
guidance during special events (City centre closures, sports events, but also medium term
bridge or sewer repairs). Integrated dynamic tools are required to quantify the effects of
alternative strategies, and to aid in choosing the most appropriate one.
1.1 Current international approaches
Because of the many dimensions of the link between traffic flows and air pollution,
modelling tools are essential when identifying management or control strategies to cope
with increasing demands on the travel system and tightening environmental standards.
Different countries have different tools available to quantify the transport-environment link
in the medium to long term, and different levels of enforcement.
In the United States, since the publication of the Transport Conformity Rule in 1993,
metropolitan planning organisations have had to extend their analyses regarding the
environmental consequences of their transport plans. The required analyses are geared
towards medium to long term planning (of the order of years) and must take place at two
levels:
• regional analyses, estimating emissions from road traffic and other sources, using large
scale traditional transport models;
• local analyses, concentrating on air quality impacts at emission hot-spots.
The regional analyses employ standard emissions models (EMFAC and MOBILE), which
are driven by total traffic volumes and average speed, with generally a standard fleet
composition. Their input is provided by static travel demand models, distinguishing the
demand for travel, its spatial distribution, and the resulting road traffic conditions in the
network, now and in alternative future scenarios. Emissions rather than immissions are at
the centre of the American approach.
In the UK, also, the emphasis in modelling air quality impacts of transport is on emissions.
The Design Manual for Roads and Bridges (Vol 11) prescribes an air quality screening
method based on traffic flows, vehicle mix and fixed emission parameters. Simple empirical
relations are used to convert these into air quality values, based on Gaussian dispersion
techniques. The analyses are limited to the immediate vicinity of the roads, generally no
further than 10m from the central axis.
In The Netherlands, for the long term National Environmental Policy Plans, traffic emission
scenarios are calculated using the FACTS and ATTACK models (see Van Wee, 1996). The
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