M6FLT005.WPD DRAFT 1 February 18, 1998

- Draft -

Trip Length Activity Factors for Running Loss and 

Exhaust Running Emissions

Report Number M6.FLT.005

Edward L. Glover

David J. Brzezinski

U.S.EPA Assessment and Modeling Division

1.0 INTRODUCTION

MOBILE6 will compute and report (as a user option) hourly emission factors for

start, running, and evaporative emissions. These will be in addition to the standard daily

emission estimates which are currently calculated by MOBILE5. The hourly emission

factors will allow the MOBILE6 model to provide more precise output that accounts for the

time of day that vehicle emissions occur. The temporal distribution of emissions is an

important factor in the formation of diurnal evaporative and start emissions. 

The hourly emission estimates require considerable vehicle activity information and

analysis. The term “activity” refers to the vehicle’s operating mode such as running, idling,

parked (soaking), etc. The specific activity information includes soak durations, time of

soak, trip lengths, time of trip, and other information. This document (M6.FLT.005)

discusses the issue of vehicle run time as it pertains to running loss emissions and exhaust

emissions. Other activity estimates needed to develop daily emission factors for hot soak,

diurnal, start or resting loss emissions will be documented in other MOBILE6 documents

listed with the report numbers “M6.FLT.XXX.”
M6FLT005.WPD DRAFT 2 February 18, 1998

2.0 DATA SOURCES USED

The primary data source for this analysis is an EPA instrumented vehicle study

conducted in Baltimore and Spokane. In these studies, instrumentation to monitor vehicle

usage was installed with the motorists’ permission on 168 randomly selected vehicles while

they were tested at an Inspection / Maintenance (I/M) station. The motorists returned one

or two weeks later to have the instrumentation removed. Information from more than

8,500 vehicle-trips was recorded. The raw data collected from the study were processed

by the Radian Corporation under EPA contract to create a “trip characteristics” file. This

processed file was used to develop the hourly soak time distributions. For more details

regarding the instrumented vehicle study and the data processing, please refer to the

document “Travel Trip Characteristics Analysis” Final Report under EPA Contract 68-C1-

0079 WA 2-05 with Sierra Research. 

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.

3.1 Definitions

3.1.1 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.

3.1.2 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
M6FLT005.WPD DRAFT 3 February 18, 1998

will be referred to as the “trip distance length”.

3.2 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 6:00AM 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 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 1. 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 1

Hourly Ranges

Group Name Hourly Range Time

6 6 - 7 6 am to 7 am

7 7 - 8 7 am to 8 am

8 8 - 9 8 am to 9 am

9 9 - 10 9 am to 10 am

10 10 - 11 10 am to 11 am

11 11 - 12 11 am to noon

12 12 - 13 noon to 1 pm

13 13 - 14 1 pm to 2 pm

14 14 - 15 2 pm to 3 pm

15 15 - 16 3 pm to 4 pm

16 16 - 17 4 pm to 5 pm

17 17 - 18 5 pm to 6 pm

18 18 - 19 6 pm to 7 pm

24 19 - 24 and 24 - 5 7 pm to 6 am
M6FLT005.WPD DRAFT 4 February 18, 1998

3.3 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. 

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. 

3.4 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 2a.

Table 2a

Trips per Car per Day

Cars Trucks

Weekday Weekend Weekday Weekend

7.28 5.41 8.06 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.

3.5 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
M6FLT005.WPD DRAFT 5 February 18, 1998

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 2b

Daily Distribution of Weekday Trips by Hourly Group (in percent)

Hour VMT Based Trip Count Based

6 3.67 1.983

7 7.29 5.461

8 8.18 5.872

9 4.75 4.744

10 4.59 5.217

11 5.40 6.757

12 6.10 8.237

13 7.05 7.352

14 7.97 8.069

15 8.14 9.015

16 8.94 8.603

17 8.19 7.779

18 6.28 6.025

24 13.45 14.887

Significant differences in the VMT based and the trip count based distribution can

occur. For example, Table 2b 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.
M6FLT005.WPD DRAFT 6 February 18, 1998

Table 2c

Daily Distribution of Weekend Trips by Hourly Group (in percent)

Hour VMT Based Trip Count Based

6 0.90 0.905

7 1.70 1.962

8 2.84 3.068

9 4.57 6.590

10 6.20 6.992

11 7.62 7.998

12 8.79 10.312

13 9.98 7.294

14 7.64 8.803

15 9.19 7.294

16 9.04 7.998

17 6.88 7.042

18 6.57 6.087

24 18.07 17.656

3.6 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 MOBILE5 (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

MOBILE5 running loss activity estimates are shown in Table 2d (the percentages shown

in Table 2d add up to 100%). For additional details regarding the MOBILE5 activity
M6FLT005.WPD DRAFT 7 February 18, 1998

parameters for running losses see the EPA report EPA-AA-AQ-AB-94-01 May 94 entitled

“User Guide to MOBILE5.”

TABLE 2d

MOBILE5 Running Loss Activities

Trip Duration 0 - 10 11 - 20 21 - 30 31 - 40 41 - 50 51+

Range (min) minutes minutes minutes minutes minutes minutes

Percentage 6.744 18.507 16.775 13.108 8.335 36.531

3.6.1 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 twentyfive 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 labelling 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
M6FLT005.WPD DRAFT 8 February 18, 1998

category number. Running loss emissions from trips are also measured in 10 minute

intervals, and are keyed to the upper duration of the trip.

Table 3

Trip Duration Categories

Category Number Trip Duration Range (in Minutes)

1 0 - 10 minutes

2 11 - 20 minutes

3 21 - 30 minutes

4 31 - 40 minutes

5 41 - 50 minutes

6 51+ 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 /weekdayweekend group. From the sums, percentages contributions were calculated for each

category within a group. Tables 4a and 4b show these percentages for weekdays and

weekends.
TABLE 4a

Hour 0-10 Min 11-20 Min 21-30 Min 31-40 Min 41-50 Min 51+ Min

Weekday 6 14.89% 22.70% 29.44% 20.76% 12.22% 0.00%

Weekday 7 16.06% 31.05% 40.00% 9.13% 3.75% 0.00%

Weekday 8 14.56% 33.48% 18.37% 18.49% 3.01% 12.10%

Weekday 9 27.40% 32.71% 22.20% 4.75% 0.00% 12.94%

Weekday 10 28.47% 42.99% 17.06% 7.92% 3.56% 0.00%

Weekday 11 32.94% 32.78% 14.96% 7.53% 5.34% 6.47%

Weekday 12 33.41% 39.64% 15.19% 11.76% 0.00% 0.00%

Weekday 13 28.89% 46.80% 13.93% 9.27% 1.10% 0.00%

Weekday 14 27.08% 39.31% 18.29% 3.18% 1.38% 10.76%

Weekday 15 26.79% 41.84% 24.28% 3.90% 2.20% 0.99%

Weekday 16 24.91% 40.78% 18.79% 10.87% 1.76% 2.89%

Weekday 17 21.09% 34.84% 29.41% 9.23% 5.42% 0.00%

Weekday 18 26.80% 32.23% 25.10% 11.98% 3.05% 0.84%

Weekday 24 20.95% 37.26% 24.51% 7.52% 6.37% 3.38%

TABLE 4b

Hour 0-10 Min 11-20 Min 21-30 Min 31-40 Min 41-50 Min 51+ Min

Weekend 6 18.98% 60.63% 20.39% 0.00% 0.00% 0.00%

Weekend 7 26.89% 42.40% 30.71% 0.00% 0.00% 0.00%

Weekend 8 20.60% 41.68% 18.82% 0.00% 18.90% 0.00%

Weekend 9 29.83% 38.26% 23.09% 8.82% 0.00% 0.00%

Weekend 10 25.57% 42.08% 16.58% 15.77% 0.00% 0.00%

Weekend 11 27.61% 36.42% 13.25% 10.31% 3.38% 9.03%

Weekend 12 25.85% 35.51% 28.61% 3.54% 6.49% 0.00%

Weekend 13 18.37% 19.41% 22.48% 9.86% 4.32% 25.56%

Weekend 14 23.71% 39.19% 12.26% 14.93% 9.91% 0.00%

Weekend 15 21.16% 25.39% 25.46% 4.07% 0.00% 23.92%

Weekend 16 25.66% 25.90% 12.34% 6.60% 15.03% 14.48%

Weekend 17 24.31% 37.71% 27.30% 3.91% 0.00% 6.76%

Weekend 18 18.01% 38.43% 18.27% 20.83% 4.46% 0.00%

Weekend 24 15.69% 27.42% 15.74% 9.57% 2.58% 28.99%

M6FLT005.WPD DRAFT 9 February 18, 1998

TABLE 4a and 4b 

Mileage (VMT) Distribution by Trip Length (in time)

3.6.2 Results

Tables 4a and 4b show the running loss activity estimates for all 28 groups and 6

categories. Each of the cells in Tables 4a and 4b 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.
M6FLT005.WPD DRAFT 10 February 18, 1998

3.7 Using the Hourly Running Loss Activities in MOBILE6

3.7.1 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 MOBILE5. (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 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 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 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. 

3.7.2 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 2b and 2c. The number of

trips per day per vehicle from Table 2a is used to calculate the final daily running loss

emissions.

4.0 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.
M6FLT005.WPD DRAFT 11 February 18, 1998

 4.1 Definition

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."

4.2 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 2a. The trip

distribution by hourly group allocates the trips per vehicle over all 14 hourly groups. It is

shown in Tables 2b and 2c.
M6FLT005.WPD DRAFT 12 February 18, 1998

COMMENTS

Comments on this report and its proposed use in MOBILE6 should be sent to the attention

of the author, and submitted electronically to mobile@epamail.epa.gov, or by fax to

(313)741-7939, or by mail to MOBILE6 Review Comments, US EPA Assessment and

Modeling Division, 2565 Plymouth Road, Ann Arbor MI 48105. Electronic submission

of comments is preferred, since we will make any comments available on our web site. In

your comments, please note clearly the document that you are commenting on including the

report title and the code number listed. Please be sure to include your name, address,

affiliation, and any other pertinent information.