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Average Speed Models are used in the measurement of emission rates of a
pollutant for a given vehicle for various speeds during a trip.
Average Speed Emission models, along with the Emission factor models are widely
applied in national and regional inventories.
The emission factor in this model  is measured over a range of driving
cycle (which includes driving, stops, starts, acceleration and deceleration).
It is given in g/veh-km.
Though these models are good in measuring congestion, they have certain
disadvantages, which are explained below:
- A single emission factor is used for a value of average speed
irrespective of the vehicle operational characteristics.
- Average speed is a less reliable indicator of estimation of emissions for
the newest generation of vehicles ( as they have after treatment devices).
- The shape of an average speed function is not fundamental, but depends,
amongst other factors, on the cycle type used.
Even though each cycle used in the development of these functions represents a
real life driving condition, the real distribution of these driving conditions
is not normally taken into account.
- Average speed models do not allow for detailed spatial resolution in
emission predictions.
This model is similar to the drive mode elemental fuel consumption model.
Emission rates are explained as a function of the vehicle operation mode.
The model provides accurate emission estimates at micro level.
For each mode, emission rate is fixed for a particular type of vehicle and
pollutant.
Instantaneous traffic related data is required to estimate the fuel
consumption.
The total emission for a trip on a section of road is given by the product of
modal emission rate and the time spent in the mode.
Various emission models are available to estimate the contribution of motor
vehicle transportation to air pollution.
The major vehicular emission models in use are discussed briefly below:
- MOBILE: This model was proposed by the Environmental Protection
Agency of the United States.
The MOBILE model estimates the emission rates from on-road motor vehicles.
The outputs of the MOBILE model are emissions per unit time or distance of a
fleet or vehicle type (i.e. grams/mile or grams/hour) of
 and six toxic air contaminants such as lead.
MOBILE estimates emissions of both exhaust and evaporative emissions, and
particulate emissions from brake and tire wear.
MOBILE does not apply the vehicle operation such as distance travelled and
number of starts.
The model is designed to be able to predict emission rates from a future fleet
to understand how emissions will change over time.
Aggregate driving cycles are considered in this model.
MOBILE 6.2 is the current version of the model.
- MOVES: MOVES stands for ``Motor Vehicle Emission Simulator''.
MOVES is also a product of EPA.
This model was proposed as a replacement to their MOBILE model.
The MOVES model contains fine scale information, such as second by second
resolution emissions and driving behaviour that can now be collected with
on-board instrumentation.
Any driving pattern can be modelled.
- EMFAC: The ``Emission Factors'' model is developed by the
California Air Resources Board.
The model is similar to the MOBILE model, except that it is pertained to
California only.
The emission standards of California are different from rest of the US.
- COPERT: This model is developed by the European Environmental
Agency.
COPERT stands for ``Computer Program to calculate Emissions from Road
Transport''.
COPERT 4 is the current version of this model.
It classifies vehicles into various size and age groups as well as categories
for highway, urban and rural driving situations.
- CMEM: The ``Comprehensive Modal Emissions Model'', or CMEM, was
developed at the University of California, Riverside and is fine-scale
emissions predictions model.
CMEM 2.0 is the latest version.
The model predicts emissions based, not only on the average speed of the
vehicles, but also on the fuel consumption and power of the vehicles.
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