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inputs, and default values are different, as discussed below. For many calculation inputs, detailed
(e.g. average vehicle speed by roadway segment) or average (e.g., average vehicle speed over all
roadway segments) data can be used. In each case, the detailed data inputs produce more accurate
results. The following lists only a few of the many calculation inputs.
G-11
G2.1 Exhaust Emission Indices
There are HC, CO, NOx, PM-10, and SO2 exhaust emission indices applicable to GAVs. In
addition, there are fugitive dust emission indices that represent reentrained road dust, which
include exhaust emissions as well as brake-wear and tire-wear, applicable to GAVs. Due to a
predominance of unleaded and diesel fueled vehicles, of which the lead content of the fuel is
negligible, it is assumed that the lead emissions also are negligible. Vehicle emission indices are
based on many inputs to the emissions calculation (e.g., cold/hot start percent, speed, study year)
not referenced above in the simplified methodology. The EPA specifies that the MOBILE motor
vehicle emissions model should be used to develop highway vehicle emission indices for HC,
CO, and NOx. The emission indices incorporate both moving and normal idling operational
modes. The EPA specifies that the PART5 model should be used to develop highway vehicle
emission indices for PM-10, SO2, and fugitive dust. Some of the key emission index inputs are
listed below.
G2.1.1 Cold/Hot Start Percent
The cold start percent refers to the percent of vehicle miles traveled while operating in “cold
start” mode and not yet warmed-up. Any car that is turned off for more than 1 hour operates for a
period of time while the vehicle is still cold. This period is typically defined as the first 8-10
minutes of vehicle travel. Conversely, the hot start percent is the percent of vehicle miles
traveled in “hot start” mode, when a car is turned off for less than 1 hour. The percent of vehicle
miles traveled in cold or hot mode affects emission indices. The cold or hot start occurs at the
point of origin for roadways (e.g., a passenger’s home) and in the parking lot when restarting the
vehicle to exit.
G2.1.2 Speed
The rate of emissions is very sensitive to the vehicle speed. Average speeds must be determined
separately for roadways and parking lots. An average speed for each roadway and parking lot
results in a more accurate emission estimate.
G2.1.3 Year
The calendar year for which the emissions calculation is being performed defines which emission
indices are to be calculated. There are different basic emission rates associated with each model
year group, which are defined on the basis of applicable emission standards and emission control
technologies. EPA’s motor vehicle emissions models have the ability to model emission indices
for the years 1960 through 2020.
G2.2 Evaporative-Related Emission Indices
EPA’s motor vehicle emissions model MOBILE5a calculates GAV HC evaporative-related
emission indices. Similar to exhaust emission indices, evaporative-related emission indices are
based on many inputs to the emissions calculation (e.g., ambient temperature, fuel
characteristics) not referenced above in the simplified methodologies.
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G2.3 Vehicle Trips
The number of vehicle trips refers to round trips. For roadways, this is from the point of origin to
an airport location and then on to the destination, which may be the same as the point of origin.
A total number of vehicle roadway trips or total trips per roadway segment can be used
depending on other inputs to the calculation. In parking lots a round trip is from the entrance of
the parking lot to a parking space to the parking lot exit. As with roadways, the number of
vehicle trips can be determined by individual parking lot or for all parking lots depending on
other inputs to the calculation. Instead of using an average number of vehicles entering a parking
lot, the average number of spaces filled for a parking lot can be used. Since not all vehicles
accessing the airport enter a parking lot, the total number of trips input into the calculation is
different for roadways and parking lots.
G2.4 Distance
Distance is the distance traveled by vehicles, in miles. For a more accurate emission estimate, the
average distance is needed by EPA vehicle type and GAV type (e.g., passenger, employee). If
detailed distance estimates are not available, an average for all vehicles can be used, although
emission results will be less accurate. For roadways the distance is the average miles traveled
from point of origin (e.g., an employee’s home) to the airport or air base location (e.g., parking
lot) to the point of destination (e.g., an employee’s home). As discussed above, for vehicles
　
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