United States Air and Radiation EPA420-R-01-013
Environmental Protection April 2001
Agency M6.FLT.005
vvEPA Trip Length Activity
Factors for Running
Loss and Exhaust
Running Emissions
> Printed on Recycled Paper
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EPA420-R-01-013
April 2001
Trip for
Edward L. Glover
David J. Brzezinski
Assessment and Standards Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
NOTICE
This technical report does not necessarily represent final EPA decisions or positions.
It is intended to present technical analysis of issues using data which are currently available.
The purpose in the release of such reports is to facilitate the exchange of
technical information and. to inform the public of technical developments which
may form the basis for a final EPA decision, position, or regulatory action.
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1.0 INTRODUCTION
The original analysis of trip length distribution factors for running emissions was based on
a detailed analysis of the EPA instrumented vehicle database. In this analysis individual
distributions based on the EPA data were created for individual hours. Unfortunately, this ultimately
created a problem in the MOBILE6 model because the corresponding running loss emission factors
were never updated from MOBILES, and these older factors were not on an hourly basis. As a result,
the new MOBILE6 trip length activity estimates were discarded and the original MOBILES running
loss trip distribution was inserted into MOBILE6 so that consistency could be maintained between
the activity information and the emission information.
This document is structured so that it includes the new running loss trip length analysis
description and the results as well as the original MOBILES results which will be used in MOBILE6.
This inclusion is made so as to preserve a record of the analysis for possible future use should the
emission estimates get updated.
2.0 DATA SOURCES USED
The original analysis of running loss emissions soak distributions were based on the EPA
instrumented vehicle study conducted in Baltimore and Spokane. This data set is described briefly
in M6FLT003, M6FLT004 and M6FLT006. The revised analysis and running loss emission soak
time distributions are based on the same datasets used to develop the daily numbers for MOBILES.
3.0 METHODOLOGY FOR RUNNING LOSS EMISSION ACTIVITY
This section describes the basic methodology to develop the trip length activity estimates
used to calculate running loss emissions. The process consisted of several steps. These are
discussed below.
1.1 Definitions
1.1.1 Running Loss Emissions
Running loss emissions are evaporative emissions which have escaped from a vehicle while
the engine is operating. These emissions may appear from numerous locations on a vehicle such as
the evaporative canister, the fuel inlet, the top of the gas tank, and other spots where the integrity of
the evaporative system has broken down or the purge system has become inoperative. Running loss
emissions may be an artifact of a particular evaporative system design or the result of poor
maintenance. In severe cases the emissions can become quite high as the result of large thermal
gradients around a vehicle's fuel and evaporative system. In addition, because of greater heating of
the fuel and evaporative system on longer trips, running loss emissions are not constant throughout
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a trip. Instead, the rate is assumed to continually increase as a function of trip length until it reaches
a plateau at a trip length of about 50 to 60 minutes.
1.1.2 Running Loss Trip
For the purpose of activity estimates for running loss emissions, a trip is defined as the
duration of time or distance between a vehicle key on and key off (excluding engine stalls, which
were removed from the database). Throughout this document the time duration of a trip will be
referred to as the "trip time length". The duration of a trip in miles will be referred to as the "trip
distance length".
1.2 Hourly Intervals and Weekday versus Weekend
The running loss emission calculation result will be reported on an hourly and weekday
versus weekend basis in MOBILE6. The results will vary by hour and weekday / weekend status
because of a different number of trips per day for weekday and weekend (See Sections 1.3 and 1.4).
1.3 Trips per Car-Day
The first necessary parameter in the model is the estimate for trips/car-day. Four different
estimates were developed. There is one estimate for each combination of car or truck and weekday
or weekend. These are average values obtained from the instrumented vehicle database. The values
are shown in Table 2a.
Table 2a
Trips per Car per Day
Cars
Weekday
7.28
Weekend
5.41
Trucks
Weekday
8.06
Weekend
5.68
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1.4 Daily Trip Distribution by Hourly Group
Table 2b contains distributions of vehicle trips by hourly group. One of the distributions is
based on vehicle miles travelled (VMT) of the trips, and the other is based on the distribution of trip
counts. Distance estimates and count-based estimates are provided for each of the fourteen groups,
and separate estimates are provided for weekends and weekdays. The trip count distribution is
shown for comparison purposes, since it is expected that the MOBILE6 model activity distribution
for running losses will be based on trip distances in miles. The distance-based distribution will be
used because it is directly proportional to the VMT accumulated during the hourly group. This is
important because in the MOBILE6 model running loss emissions are a function of miles, and are
presented in units of grams / mile.
Table A2b
Daily Distribution of Weekday Trips by Hourly Group (in percent)
Hour
6
7
8
9
10
11
12
13
14
15
16
17
18
24
VMT Based
3.67
7.29
8.18
4.75
4.59
5.40
6.10
7.05
7.97
8.14
8.94
8.19
6.28
13.45
Trip Count Based
1.983
5.461
5.872
4.744
5.217
6.757
8.237
7.352
8.069
9.015
8.603
7.779
6.025
14.887
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Significant differences in the VMT based and the trip count based distribution can occur.
For example, Table A2b shows that approximately 4.155 percent of the weekday daily trip VMT
occured during the period from 6:00 AM to 6:59:59 AM; however, this VMT represented 1.983
percent of the total daily trips. The data which underlie Table 2b were obtained from the
instrumented vehicle database. Each column sums to 100 percent.
Table 2c
Daily Distribution of Weekend Trips bv Hourly Group (in percent)
Hour
6
7
8
9
10
11
12
13
14
15
16
17
18
24
VMT Based
0.90
1.70
2.84
4.57
6.20
7.62
8.79
9.98
7.64
9.19
9.04
6.88
6.57
18.07
Trip Count Based
0.905
1.962
3.068
6.590
6.992
7.998
10.312
7.294
8.803
7.294
7.998
7.042
6.087
17.656
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1.4 Running Loss Trip Distance Length Distribution Within An Hourly Group
The distribution of trip lengths used in computing running loss emissions in the MOBILE6
model are shown in Table 2d. These are the percentage of VMT that each type of trip represents in
overall national driving over all times of the day. The skewing of the distribution towards the higher
time lengths (i.e., 51+ minutes) reflect the fact that the longer duration trips contain the most VMT
weighting. For example, Table 2d states that 16.775 percent of all trips (on a VMT basis) last 21 -
30 minutes.
These results were taken from MOBILES, and will be applied in MOBILE6 for all individual
hourly results. The original MOBILE6 analysis discussed in Appendix A included the development
of separate distributions by hourly group. For example, the morning rush hour trip distribution was
different than the late evening distribution. This revised trip distribution could not be used in
MOBILE6 because the basic running loss emission factors (not updated from MOBILES) were not
compatible with it.
The overall set of MOBILES running loss activity estimates are shown in Table 2d (the
percentages shown in Table 2d add up to 100%). For additional details regarding the MOBILES
activity parameters for running losses see the EPA report EPA-AA-AQ-AB-94-01 May 94 entitled
"User Guide to MOBILES."
TABLE 2d
MOBILES Running Loss Activities
Trip Duration
Range (min)
Percentage
0- 10
minutes
6.744
11-20
minutes
18.507
21 -30
minutes
16.775
31 -40
minutes
13.108
41 -50
minutes
8.335
51+
minutes
36.531
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1.5 Using the Hourly Running Loss Activities in MOBILE6
1.5.1 Hourly Running Loss Calculation
The average hourly running loss emissions are calculated from four parameters. These are:
(1) the six running loss emission values (one value for each trip duration category). These values
will not be updated for MOBILE6, and can be found in EPA document AP-42 for MOBILES. (2)
the six trip duration by hourly group running loss activity values shown in Table 2d. (3) the number
of trips per day per vehicle values shown in Table 2a, and (4) the daily distribution of trips by hourly
group (shown in Tables 2b and 2c).
MOBILE6 will multiply each of the six running loss emission values with each of the
corresponding six running loss activity values. The six products (emissions x activities) are then
summed to produce an estimate of running losses on a per trip basis. The calculation is done only
once for each of the 28 groups, and produces the same running loss emission value for each of the
28 groups.
The second calculation takes the 28 hourly running loss emission values in per trip units, and
converts them into average hourly running loss emission values. This is done by multiplying the 28
hourly per trip running loss values by the number of trips per day (shown in Table 2a) and by the
appropriate value from the daily distribution of trips by hourly group table (shown in Table 2b or 2c).
The daily distribution of trips by hourly group apportions the overall trips per day value into hourly
group values.
1.5.2 Daily Running Loss Calculation
An overall daily running loss average is computed by weighting together the average running
loss values of the 28 groups. The weighting factors are the daily distribution of trips by hourly group
values shown in Tables 2b and 2c. The number of trips per day per vehicle from Table 2a is used
to calculate the final daily running loss emissions.
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Appendix A
New Methodology for Running Loss Emission Activity
Some of this material is duplicative of the material in the Main Section of the document. It was
retained so that this section would appear to the reader as cohesive and complete.
Al.O METHODOLOGY FOR RUNNING LOSS EMISSION ACTIVITY
This section describes the basic methodology to develop the trip length activity estimates
used to calculate running loss emissions. The process consisted of several steps. These are
discussed below.
Al.l Definitions
Al.1.1 Running Loss Emissions
Running loss emissions are evaporative emissions which have escaped from a vehicle while
the engine is operating. These emissions may appear from numerous locations on a vehicle such as
the evaporative canister, the fuel inlet, the top of the gas tank, and other spots where the integrity of
the evaporative system has broken down or the purge system has become inoperative. Running loss
emissions may be an artifact of a particular evaporative system design or the result of poor
maintenance. In severe cases the emissions can become quite high as the result of large thermal
gradients around a vehicle's fuel and evaporative system. In addition, because of greater heating of
the fuel and evaporative system on longer trips, running loss emissions are not constant throughout
a trip. Instead, the rate is assumed to continually increase as a function of trip length until it reaches
a plateau at a trip length of about 50 to 60 minutes.
Al.l.2 Running Loss Trip
For the purpose of activity estimates for running loss emissions, a trip is defined as the
duration of time or distance between a vehicle key on and key off (excluding engine stalls, which
were removed from the database). Throughout this document the time duration of a trip will be
referred to as the "trip time length". The duration of a trip in miles will be referred to as the "trip
distance length".
A1.2 Hourly Intervals
The 24 hour day was divided into 14 different hourly groups. Thirteen of these groups have
a duration of one hour. These start at 6:OOAM and run through 7:59:59PM. The fourteenth group
contains the remaining nighttime hours as one interval. Collapsing these hours into one group was
done for three reasons: (1) the emissions contributed during the night have a relatively smaller
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impact on daily ozone or CO formation than those contributed during the morning or day, (2) there
were relatively little data for these time periods, and (3) what data were available produced results
which showed very little hour to hour variance. The hourly intervals are shown in Table Al. In
addition to their use for the running loss activity estimates presented here, the same hourly group
intervals are used in the calculation of activity estimates for start emissions, running emissions, hot
soak emissions, resting loss emissions, and diurnal emissions.
Table Al
Hourly Ranges
Group Name
6
7
8
9
10
11
12
13
14
15
16
17
18
24
Hourly Range
6-7
7-8
8-9
9- 10
10- 11
11 - 12
12- 13
13- 14
14- 15
15- 16
16- 17
17- 18
18- 19
19 -24 and 24 -5
Time
6 am to 7 am
7 am to 8 am
8 am to 9 am
9 am to 10 am
10 am to 11 am
1 1 am to noon
noon to 1 pm
1 pm to 2 pm
2 pm to 3 pm
3 pm to 4 pm
4 pm to 5 pm
5 pm to 6 pm
6 pm to 7 pm
7 pm to 6 am
A1.3 Weekday Versus Weekend
For a number of the activity parameters a significant difference existed between the value for
the weekday and the value for the weekend. Conceptually this make sense since most motorists have
different usage patterns for their vehicles on weekdays than on weekends. Differences may also exist
for the various days of the week; however, the database was too small to reliably discern these
differences.
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The MOBILE6 model will distinguish between weekend and weekday in terms of activity
and emissions, and a user input will be required to tell the model which one is to be reported. The
default will likely be the "weekday." Also, since the default MOBILE6 hourly activity estimates are
based exclusively on 168 vehicles, and cannot possibility reflect all geographical areas, times, or
other variables, the user will have the option of providing running loss activity data into the
MOBILE6 model from an external file.
A1.4 Trips per Car-Day
The first necessary parameter in the model is the estimate for trips/car-day. Four different
estimates were developed. There is one estimate for each combination of car or truck and weekday
or weekend. These are average values obtained from the instrumented vehicle database. The values
are shown in Table A2a.
Table A2a
Trips per Car per Day
Cars
Weekday
7.28
Weekend
5.41
Trucks
Weekday
8.06
Weekend
5.68
These estimates are based on the instrumented car data, and are subject to revision pending
completion of a thorough analysis of national trip data by an EPA contractor. The values could
potentially vary considerably from those presented here.
A1.5 Daily Trip Distribution by Hourly Group
Table A2b contains distributions of vehicle trips by hourly group. One of the distributions
is based on vehicle miles travelled (VMT) of the trips, and the other is based on the distribution of
trip counts. Distance estimates and count-based estimates are provided for each of the fourteen
groups, and separate estimates are provided for weekends and weekdays. The trip count distribution
is shown for comparison purposes, since it is expected that the MOBILE6 model activity distribution
for running losses will be based on trip distances in miles. The distance-based distribution will be
used because it is directly proportional to the VMT accumulated during the hourly group. This is
important because in the MOBILE6 model running loss emissions are a function of miles, and are
presented in units of grams / mile.
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Table A2b
Daily Distribution of Weekday Trips by Hourly Group (in percent)
Hour
6
7
8
9
10
11
12
13
14
15
16
17
18
24
VMT Based
3.67
7.29
8.18
4.75
4.59
5.40
6.10
7.05
7.97
8.14
8.94
8.19
6.28
13.45
Trip Count Based
1.983
5.461
5.872
4.744
5.217
6.757
8.237
7.352
8.069
9.015
8.603
7.779
6.025
14.887
Significant differences in the VMT based and the trip count based distribution can occur.
For example, Table A2b shows that approximately 4.155 percent of the weekday daily trip VMT
occured during the period from 6:00 AM to 6:59:59 AM; however, this VMT represented 1.983
percent of the total daily trips. The data which underlie Table A2b were obtained from the
instrumented vehicle database. Each column sums to 100 percent.
10
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Table A2c
Daily Distribution of Weekend Trips bv Hourly Group (in percent)
Hour
6
7
8
9
10
11
12
13
14
15
16
17
18
24
VMT Based
0.90
1.70
2.84
4.57
6.20
7.62
8.79
9.98
7.64
9.19
9.04
6.88
6.57
18.07
Trip Count Based
0.905
1.962
3.068
6.590
6.992
7.998
10.312
7.294
8.803
7.294
7.998
7.042
6.087
17.656
A1.6 Running Loss Trip Distance Length Distribution by Hourly Group
A distribution of running loss trip distance lengths was developed for each of the 28 hourly
group / weekday-weekend groups. The methodology used to analyze the data, and the results of the
analysis are presented below. This particular methodology for estimating running loss activity was
chosen because it fits the running loss emission values that are available in MOBILES (these will
not be updated in MOBILE6). The only enhancement for the new model is that these activity
parameters will be available for each hourly group and for weekday/weekend categorization (28
groups) rather than just an overall average set which represents all hours and days of the week. The
overall set of MOBILES running loss activity estimates are shown in Table A2d (the percentages
shown in Table A2d add up to 100%). For additional details regarding the MOBILES activity
parameters for running losses see the EPA report EPA-AA-AQ-AB-94-01 May 94 entitled "User
11
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Guide to MOBILES."
TABLE A2d
MOBILES Running Loss Activities
Trip Duration
Range (min)
Percentage
0- 10
minutes
6.744
11-20
minutes
18.507
21 -30
minutes
16.775
31 -40
minutes
13.108
41 -50
minutes
8.335
51+
minutes
36.531
Al.6.1 Methodology
The analysis consisted primarily of categorizing each of the 8,500 vehicle trips in the
database into a particular hourly / weekday-weekend group. This was done by examining the day
of the week of the trip, and the starting and ending time of each trip. A vehicle trip was classified
as a weekday trip if it started on Monday through Friday. It was a weekend trip if it started on
Saturday or Sunday.
A vehicle trip was classified into a particular hourly group if any part of the trip duration was
in a given hourly group. A given vehicle trip could potentially be classified into one, two, or even
three different hourly groups depending on the duration of the trip, and how many group interval
boundaries it crossed. This classification could fit two general cases which are best illustrated with
an example. (1) If a vehicle trip was from 8:20 AM to 8:40 AM it was classified as a twenty minute
trip in Hourly Group 8. (2) If the trip was from 7:51 to 8:15, its contribution had to be split between
two hourly groups (7 and 8 hour). Thus, a nine minute trip was assigned to the Hour 7 group, and
a nine to twenty-five minute trip was assigned to the Hour 8 group. The nine to twenty-five minute
label was assigned to the hour 8 group rather than a fifteen minute label, because the vehicle had
been operating for nine minutes prior to the 8th hour, and would nominally have higher running
losses than a vehicle which was just starting its trip. This is reflected in the MOBILE6 model
because running losses are a function of the duration of the trip.
After labeling each of the trips in the database using the method above, each trip was
classified into one of six trip duration categories based on trip duration in minutes (see Table 3). For
example, the trip which lasted from 8:20 to 8:40 AM would fall into category #2 and hour 8. The
trip which lasted from 7:51 to 8:15 AM would be treated as two trips. The first phase (nine minutes)
would fall into the category #1 and hour 7. The second phase would fall into category #3 and hour
8. Category #3 is used because the upper duration of the trip is 25 minutes, and the maximum length
of the trip is used to determine category number. Running loss emissions from trips are also
measured in 10 minute intervals, and are keyed to the upper duration of the trip.
12
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Table A3
Trip Duration Categories
Category Number
1
2
3
4
5
6
Trip
0 -
11 -
21 -
31 -
41 -
51+
Duration Range (in Minutes)
10 minutes
20 minutes
30 minutes
40 minutes
50 minutes
minutes
After classifying the trips and phases of trips by hourly / weekday-weekend group, and by
category number, the duration in miles of each trip and trip phase was found. In cases where the trip
contained only one phase the trip distance in miles was readily available. In cases where two phases
were present, the mileage was split according to the length of the trip in time. This assumes that the
average speeds in both phases were equal.
After obtaining the mileage for each trip and trip phase, the mileages were summed for each
hourly / weekday-weekend group and for each category within a hourly /weekday-weekend group.
From the sums, percentages contributions were calculated for each category within a group. Tables
A4a and A4b show these percentages for weekdays and weekends.
13
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TABLE A4a and A4b
Mileage (VMT) Distribution by Trip Length (in time)
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekday
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Weekend
Hour
6
7
8
9
10
11
12
13
14
15
16
17
18
24
Hour
6
7
8
9
10
11
12
13
14
15
16
17
18
24
0-10 Min
14.89%
16.06%
14.56%
27.40%
28.47%
32.94%
33.41%
28.89%
27.08%
26.79%
24.91%
21 .09%
26.80%
20.95%
0-10 Min
18.98%
26.89%
20.60%
29.83%
25.57%
27.61%
25.85%
18.37%
23.71%
21.16%
25.66%
24.31%
18.01%
15.69%
TABLE 4a
11 -20 Min
22.70%
31 .05%
33.48%
32.71 %
42.99%
32.78%
39.64%
46.80%
39.31 %
41 .84%
40.78%
34.84%
32.23%
37.26%
21-30 Min
29.44%
40.00%
18.37%
22.20%
17.06%
14.96%
15.19%
13.93%
18.29%
24.28%
18.79%
29.41 %
25.10%
24.51 %
TABLE 4b
11 -20 Min
60.63%
42.40%
41 .68%
38.26%
42.08%
36.42%
35.51 %
19.41%
39.19%
25.39%
25.90%
37.71 %
38.43%
27.42%
21-30 Min
20.39%
30.71%
18.82%
23.09%
16.58%
13.25%
28.61 %
22.48%
12.26%
25.46%
12.34%
27.30%
18.27%
15.74%
31 -40 Min
20.76%
9.13%
18.49%
4.75%
7.92%
7.53%
1 1 .76%
9.27%
3.18%
3.90%
10.87%
9.23%
1 1 .98%
7.52%
31 -40 Min
0.00%
0.00%
0.00%
8.82%
15.77%
10.31%
3.54%
9.86%
14.93%
4.07%
6.60%
3.91%
20.83%
9.57%
41-50 Min
12.22%
3.75%
3.01 %
0.00%
3.56%
5.34%
0.00%
1.10%
1 .38%
2.20%
1 .76%
5.42%
3.05%
6.37%
41-50 Min
0.00%
0.00%
18.90%
0.00%
0.00%
3.38%
6.49%
4.32%
9.91%
0.00%
15.03%
0.00%
4.46%
2.58%
51+ Min
0.00%
0.00%
12.10%
12.94%
0.00%
6.47%
0.00%
0.00%
10.76%
0.99%
2.89%
0.00%
0.84%
3.38%
51+ Min
0.00%
0.00%
0.00%
0.00%
0.00%
9.03%
0.00%
25.56%
0.00%
23.92%
14.48%
6.76%
0.00%
28.99%
Al.6.2 Results
Tables A4a and A4b show the running loss activity estimates for all 28 groups and 6
categories. Each of the cells in Tables A4a and A4b show the percentage of vehicle miles travelled
(VMT) for a given hourly / weekday-weekend group which falls in a given trip duration category.
For example, 14.89 percent of the VMT in the 6 AM to 7 AM time interval on a weekday occurs in
trips which are 0 to 10 minutes in duration. Also, each of the rows in the tables sums to 100 percent.
14
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A1.7 Using the Hourly Running Loss Activities in MOBILE6
Al.7.1 Hourly Running Loss Calculation
The average hourly running loss emissions are calculated from four parameters. These are:
(1) the six running loss emission values (one value for each trip duration category). These values
will not be updated for MOBILE6, and can be found in EPA document AP-42 for MOBILES. (2)
the six trip duration by hourly group running loss activity values shown in Table 4a or 4b. (3) the
number of trips per day per vehicle values shown in Table A2a, and (4) the daily distribution of trips
by hourly group (shown in Tables 2b and 2c).
MOBILE6 will multiply each of the six running loss emission values with each of the
corresponding six running loss activity values. The six products (emissions x activities) are then
summed to produce an estimate of running losses on a per trip basis. The calculation is done once
for each of the 28 groups, to produce a value for each of the 28 groups. These 28 hourly running
loss emission values are per trip estimates, and are the result of an intermediate calculation in the
MOBILE6 model.
The second calculation takes the 28 hourly running loss emission values in per trip units, and
converts them into average hourly running loss emission values. This is done by multiplying the 28
hourly per trip running loss values by the number of trips per day (shown in Table A2a) and by the
appropriate value from the daily distribution of trips by hourly group table (shown in Table A2b or
A2c). The daily distribution of trips by hourly group apportions the overall trips per day value into
hourly group values.
Al.7.2 Daily Running Loss Calculation
An overall daily running loss average is computed by weighting together the average running
loss values of the 28 groups. The weighting factors are the daily distribution of trips by hourly group
values shown in Tables A2b and A2c. The number of trips per day per vehicle from Table A2a is
used to calculate the final daily running loss emissions.
A2.0 METHODOLOGY FOR RUNNING EXHAUST EMISSION ACTIVITY
This short section describes the basic methodology to develop the trip length activity
estimates used to calculate running exhaust emissions. The process and the results are virtually
identical to those shown in the running loss activity estimate sections, and are shown here for
completeness.
A2.1 Definition
15
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Exhaust running emissions are the emissions which exit a vehicle's tailpipe while the vehicle
is operating in a warmed up condition. Excluded from the definition of exhaust running emissions
are those emissions which occur during vehicle start (cranking the engine) and warm-up.
Standardized test procedures such as the Federal Test Procedure (FTP), and test equipment have
been extensively developed to measure exhaust emissions. The exhaust running emissions in the
MOBILE6 model are based on such FTP data.
Detailed information on the definition of start emissions, the methodology used to develop
the start emission estimates, and the projected MOBILE6 light duty vehicle start emission factors
are provided in the EPA reports M6.STE.003 entitled "Determination of Start Emissions as a
Function of Mileage and Soak Time for 1981-1993 Model Year Light-Duty Vehicles", and
M6.STE.002 entitled "The Determination of Hot Running Emissions from FTP Bag Emissions."
Also, detailed information on the determination of running exhaust emission factors for MOBILE6
can be found in EPA report M6.EXH.001 entitled "Determination of Running Emissions as a
Function of Mileage for 1981-1993 Model Year Light-Duty Vehicles."
A2.2 Activity Estimates
The activity estimates for exhaust running emissions required for the MOBILE6 model are
minimal, since we assume exhaust running emissions are not a function of the trip length or the
preceding soak period. Thus, the only activity estimates needed to calculate hourly running exhaust
emissions in MOBILE6 are the trips per day per vehicle estimates and the daily trip distribution by
hourly group. The trips per day per vehicle value is an estimate of overall daily vehicle usage. It is
shown in Table A2a. The trip distribution by hourly group allocates the trips per vehicle over all 14
hourly groups. It is shown in Tables A2b and A2c.
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