Sample Calculation of Emission
Reductions and Fuel Savings from a
Carpool Program
The amount of pollution that a vehicle emits and the rate at which
it consumes fuel are dependent on many factors. The U.S. Envi-
ronmental Protection Agency (EPA) has developed a series of computer
models that estimate the average emissions for different types of high-
way vehicles. This fact sheet is one of a series on highway vehicle
emission factors. It illustrates the use of national average annual
emissions and fuel consumption for gasoline-fueled light-duty vehicles
(passenger cars) and light-duty trucks (pickup trucks, sport-utility
vehicles, and the like) in estimating the benefits of carpool and ride-
sharing programs.
c/:
Introduction
This fact sheet presents an example calculation illustrating how the emissions reduc-
tions and fuel savings resulting from implementation (or expansion) of a carpool or
ride-share program can be estimated. The basic concept is to estimate the reduction
in vehicle miles traveled (VMT) that will occur as a result of the program, and to
combine that with estimates of average emissions and fuel consumption per vehicle
per mile traveled, in order to obtain an estimate of the total fuel savings and emis-
sions reductions.
The most current version of the computer model that EPA uses to estimate average
in-use emissions from highway vehicles is MOBILE6.2. EPA, the States, and others
use this model to estimate total emissions of pollutants generated by highway vehicles
in various geographic areas and over specific time periods. The emission rates (or
"emission factors") presented in this fact sheet are based on national average data
representing the in-use fleet as of July 2008,
United States
Environmental Protection
Agency
Office of Transportation and Air Quality
EPA420-F-08-028
September 2008
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The emission rates for hydrocarbons (HC), carbon
monoxide (CO), carbon dioxide (CO2), nitrogen ox-
ides (NOx), and particulate matter (PM10 and PM25)
are presented in the following tables. The HC num-
bers include both tailpipe and evaporative emissions,
whereas the rates for the other pollutants are for tail-
pipe emissions only. They assume an average, properly
maintained vehicle operating on typical gasoline on
a warm summer day. Emission rates can be higher in
very hot weather (especially HC) or very cold weather
(especially CO),
The carbon dioxide (CO2) emission factor used in the
following tables is based on the average fuel economy
of the in-use fleet of gasoline-fueled light-duty vehicles,
24.1 miles per gallon (mpg) for passenger cars, and 17.3
mpg for light trucks (e.g., pickup trucks and SUVs),
This corresponds to a fuel consumption rate of (1/24.1
= ) 0.042 gallons per mile driven (gal/mi) for cars and
0.058 gallons per mile for light-duty trucks. An aver-
age of 19.4 lb (8.81 kg) of CO2 is produced per gallon
of gasoline consumed.
More refined estimates of these benefits can be devel-
oped through use of the MOBILE6.2 highway vehicle
emission factor model to estimate emission factors that
are specific to the geographic area and time period
of interest. However, for a first-order approximation
of the benefits of such programs, use of the national
average values as provided in this sample calculation
should be sufficient.
Abbreviations and Acronyms
Used
CO:
C02:
HC:
NOx:
PM10:
PM
2.5
SUV:
VMT:
VOC:
Carbon monoxide; a regulated
pollutant
Carbon dioxide; the primary
byproduct of all fossil fuel
combustion
Hydrocarbons; molecules
formed of hydrogen and car-
bon that constitute gasoline,
diesel, and other petroleum-
based fuels; a precursor emis-
sion for ground-level ozone
("smog") formation, which is
a regulated pollutant
Nitrogen oxides; a regulated
criteria pollutant
Particulate matter under 10
microns diameter; a regulated
pollutant
Particulate matter under 2.5
microns diameter, sometimes
referred to as "fine particu-
late"
Sport-utility vehicle
Vehicle miles traveled
Volatile organic compounds;
equivalent to THC plus al-
dehydes minus both methane
and ethane
Sample Calculation
The first step in calculating the benefits of a new or expanded carpool or ride-share program is to
estimate the reduction in total vehicle miles traveled (VMT) that will result from such a pro-
gram. This value is the product of the number of people (vehicles) no longer commuting alone
in their own vehicles and the average distance in miles that those people normally commute. In
this sample calculation, the following assumptions are made:
• The average commute is a 30 mile round trip
• There are a total of 1000 employees that will participate in the program
• Each of those employees currently commutes alone in their own vehicle
• After implementation of the program, each vehicle will carry two occupants
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Thus, without a carpool program, the total number of miles this group of employees travels each
business day is 30,000:
(1) 1000 employees x 1 vehicle/employee = 1000 vehicles
(2) 1000 vehicles x 30 miles/vehicle = 30,000 miles
With a carpool program that has two employees in each vehicle, the total number of miles is
reduced to 15,000 each business day:
(3) 1000 employees x (1 vehicle / 2 employees) = 500 vehicles
(4) 500 vehicles x 30 miles/vehicle = 15,000 miles
Note that if the average vehicle occupancy under the carpool or ride-share program is greater
than two, that average occupancy can be substituted for (persons/vehicle) in equation (3). For
example, if it is estimated that the average occupancy of each vehicle under the program will
increase to 2.5 persons (equivalent to two vehicles in use per five employees), then equations
(3) and (4) become:
(3a) 1000 employees x (1 vehicle / 2.5 employees) = 400 vehicles
(4a) 400 vehicles x 30 miles/vehicle = 12,000 miles
To calculate the reduction in VMT resulting from the carpool program, subtract the new VMT
(in equation 4) from the original VMT (in equation 2):
(5) 30,000 miles (daily VMT before program) - 15,000 miles (daily VMT after program) =
15,000 miles (daily VMT reduction)
(5a) 30,000 miles (daily VMT before program) - 12,000 miles (daily VMT after program) =
18,000 miles (daily VMT reduction)
The second step in calculating the benefits of a carpool program is to combine the estimated
reduction in VMT with the emission factors (in grams per mile [g/mi]) and fuel consumption (in
gallons per mile [gal/mi]) rates for the average vehicle.
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Average Emission Reductions and Fuel Savings
Per Day Per Vehicle for Gasoline Passenger Cars
Pollutant/
Fuel
VOC
CO
NOx
PM10
PM9,
2..J
C02
Gasoline
Consumption
Emission
Factor
1.034g/mi
9.400 g/mi
0.693 g/mi
0.0044 g/mi
0.0041 g/mi
368.4 g/mi
0.042 gal/mi
Reduction
inVMT
15,000 miles
15,000 miles
15,000 miles
15,000 miles
15,000 miles
15,000 miles
15,000 miles
Calculations
1.034 g/mi x
15,000 mix
1 lb/454 g
9.400 g/mi x
15,000 mix
1 lb/454 g
0.693 g/mi x
15,000 mix
1 lb/454 g
0.0044 g/mi x
15,000 mix
1 lb/454 g
0.0041 g/mi x
15,000 mix
1 lb/454 g
368.4 g/mi x
15,000 mix
1 lb/454 g
0.042 g/mi x
15,000 mix
1 lb/454 g
Bene.t
(Emission
Reduction &
Fuel Savings)
34.2 Ib VOC
SlllbCO
22.9 Ib NOx
0.141bPM10
O.MlbPM,,
2..J
12, 172 Ib OX
2.
630 gallons
gasoline
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Average Emission Reductions and Fuel Savings
Per Day Per Vehicle for Gasoline Light Trucks
Pollutant/
Fuel
VOC
CO
NOx
PM10
PM25
C(X
L
Gasoline
Consumption
Emission
Factor
1 .224 grams (g)
11.84g
0.95 g
0.0049 g
0.0045 g
513.5 g
0.058 gallons (gal)
Reduction
inVMT
15,000 miles
15,000 miles
15,000 miles
15,000 miles
15,000 miles
15,000 miles
15,000 miles
Calculations
1.224g/mix
15,000 mi x
1 lb/454 g
11.84g/mix
15,000 mi x
1 lb/454 g
0.95 g/mi x
15,000 mix
1 lb/454 g
0.0049 g/mi x
15,000 mi x
1 lb/454 g
0.0045 g/mi x
15,000 mi x
1 lb/454 g
513.5 g/mi x
15,000 mi x
1 lb/454 g
0.058 g/mi x
15,000 mi
Benefit
(Emission
Reduction & Fuel
Savings)
40.4 Ib VOC
39 lib CO
31.41bNOx
0.161bPM10
0.151bPM9,
2..J
16,966 Ib CO9
2.
870 gallons
gasoline
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For More Information
The other fact sheets in this series and additional information are available on the Office of
Transportation and Air Quality's Web site at:
Emission factor fact sheets: www.epa.gov/otaq/consumer.htm
Modeling and estimating vehicle emissions: www.epa.gov/otaq/models.htm
Fuel economy: www.epa.gov/fueleconomy
www.fueleconomy.gov
Improving fuel economy and reducing emissions: www.epa.gov/epahome/trans.htm
www.fueleconomy.gov/feg/drive.shtml
Finding the "greenest" vehicle: www.epa.gov/greenvehicles
Voluntary emission reduction programs: www.bestworkplacesforcommuters.org
www.italladdsup.gov
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