APPENDIX E-1 

Mobile Source Highway Emissions Inventory

An Explanation of Methodology

July 2009
Appendix E-1 2

Explanation of Methodology

July 2009

TABLE OF CONTENTS

INTRODUCTION .......................................................................................................................................................3

OVERVIEW OF EMISSIONS INVENTORIES ....................................................................................................................3

HIGHWAY VEHICLE EMISSION INVENTORIES..............................................................................................................3

WHERE DOES PENNSYLVANIA OBTAIN ITS DATA?......................................................................................6

DATA USED IN MOBILE ...........................................................................................................................................6

WHAT ARE THE NECESSARY DATA INPUTS TO MOBILE?........................................................................................7

EMISSION AND SPEED RELATIONSHIPS.......................................................................................................................9

ROADWAY DATA .....................................................................................................................................................10

ADDITIONS AND ADJUSTMENTS TO ROADWAY DATA ..............................................................................................11

PRODUCING FUTURE YEAR VOLUMES .....................................................................................................................14

SPEED/EMISSION ESTIMATION PROCEDURE...............................................................................................15

VOLUME/VMT DEVELOPMENT ...............................................................................................................................15

SPEED/DELAY DETERMINATION ..............................................................................................................................18

HPMS AND VMT ADJUSTMENTS ............................................................................................................................19

VMT AND SPEED AGGREGATION.............................................................................................................................20

MOBILE EMISSIONS RUN .......................................................................................................................................21

TIME OF DAY AND DIURNAL EMISSIONS ..................................................................................................................21

PROCESS MOBILE OUTPUT ....................................................................................................................................21

RESOURCES.............................................................................................................................................................24

HIGHWAY VEHICLE INVENTORY GLOSSARY .............................................................................................25

List of Exhibits

EXHIBIT 1: EMISSION CALCULATION PROCESS FOR PENNSYLVANIA ..............................................................................5

EXHIBIT 2: MOBILE INPUTS .........................................................................................................................................7

EXHIBIT 3: VOC AND NOX -- SPEED V. EMISSIONS .....................................................................................................10

EXHIBIT 4: PENNDOT CLASSIFICATION SCHEME.........................................................................................................11

EXHIBIT 5: MOBILE6 VEHICLE TYPES .......................................................................................................................13

EXHIBIT 6: PPSUITE SPEED/EMISSION ESTIMATION PROCEDURE...............................................................................16

EXHIBIT 7: VMT/VHT AGGREGATION SCHEME ..........................................................................................................20

EXHIBIT 8: SUMMARY OF PPSUITE METHODOLOGY ..................................................................................................22
Appendix E-1 3

INTRODUCTION

The purpose of this document is to explain how Pennsylvania estimates emissions from highway vehicles 

for inclusion in its emission inventories and State Implementation Plans. 

Overview of Emissions Inventories

Pennsylvania’s inventories generally categorize emissions into four categories:

 highway vehicles

 stationary sources (major industrial, commercial and utility sources)

 area sources (smaller industrial/commercial sources, consumer products)

 nonroad mobile sources (including construction and agricultural equipment, lawn and garden 

equipment)

An evaluation of emissions from point, area, and mobile sources is needed to develop a complete 

inventory needed for a maintenance demonstration plan. This inventory provides the mobile source 

activity and emissions for base and future years that can be used to support such a demonstration. 

Of all of the sources of air pollution, only the emissions of some stationary sources are measured directly 

and continuously through instrumentation. Emissions from all other sources must be estimated in some 

fashion, including those from highway vehicles. In their very simplest form, estimates of emissions 

follow the following pattern:

Emission rate x activity level = emissions per time period (usually day or year)

Most emission rates have been developed by EPA, in cooperation with industry and states, over many 

years and are compiled and documented in a reference volume, Compilation of Air Pollution Emission 

Factors (AP-42). For example, the annual VOC emissions from residential fuel oil heating could be 

estimated by:

AP-42 emission rate X activity level = emissions

0.713 pounds/gallon X # dwelling units x % using oil x # gallons per unit # pounds of VOC 

per year

Adding up the products of the emission rates and activity levels for all sources of a given pollutant 

constitutes the emission inventory for that pollutant.

Highway Vehicle Emission Inventories

Highway vehicles contribute significantly to air pollution, particularly to ground-level ozone, which is 

the most persistent air pollutant in Pennsylvania. Ozone is not created directly but formed in sunlight 

from VOCs and NOx. Both VOCs and NOx are emitted from highway vehicles. In addition, fine 

particulates are also emitted directly by motor vehicles as a result of the combustion process (tailpipe 

emissions), re-entrained and transportation construction dust; and formed through reactions in the 

atmosphere among the precursors VOC, NOx, NH3 and SOx. 
Appendix E-1 4

Obviously, direct measurement of emission levels from all vehicles in use is impossible. In comparison 

to highway vehicles, estimating residential heating emissions is a fairly simple calculation because there 

is a constant emission rate and a fairly simple measure of activity. For highway vehicles, however, 

estimating the emission rate and activity levels of all vehicles on the road during a typical summer day is 

a complicated endeavor.

If every vehicle emitted the same amount of pollution all the time, one could simply multiply those 

emission standards (emission rate in grams of pollution per mile) times the number of miles driven 

(activity level) to estimate total emissions. But, the fact is that emission rates from all vehicles vary over 

the entire range of conditions under which they operate. These variables include air temperature, speed, 

traffic conditions, operating mode (started cold? started warm? running already warmed up?) and fuel. 

The inventory must also account for non-exhaust or evaporative emissions. In addition, the fleet is 

composed of several generations, types of vehicles and their emission control technologies, each of 

which performs differently. This requires that the composition of the fleet (vehicle ages and types) must 

also be included in the estimation algorithm. 

In order to estimate both the rate at which emissions are being generated and to calculate vehicle miles 

traveled (activity level), Pennsylvania examines its road network and fleet to estimate vehicle activity. 

For ozone-related inventories, this is done for a typical summer weekday. For CO and PM emission 

inventories, this may be done for a typical winter weekday or annual conditions. Not only must this be 

done for a baseline year, but it must also be projected into the future. This process involves a large 

quantity of data and is extremely complex. 

Computer models have been developed to perform these calculations by simulating the travel of vehicles 

on the Commonwealth’s roadway system. These models then generate emission rates (also called 

emission factors) for different vehicle types for area-specific conditions and then combine them in 

summary form. The “area-specific conditions” include vehicle and highway data, plus control measure 

characteristics and future year projections of all variables. 

MOBILE. The heart of the highway vehicle emission calculation procedure is EPA’s highway vehicle 

emission factor model, MOBILE. This is a FORTRAN program that calculates average in-use fleet 

emission factors for ozone precursors for each of twenty-eight categories of vehicles under various 

conditions affecting in-use emission levels (e.g., ambient temperatures, average traffic speeds, gasoline 

volatility) as specified by the model user. MOBILE produces the “emission rates” referred to in the 

previous section. 

The model was first developed as MOBILE1 in the late 1970s, and has been periodically updated to 

reflect the collection and analysis of additional emission factor data over the years, as well as changes in 

vehicle, engine and emission control system technologies, changes in applicable regulations, emission 

standards and test procedures, and improved understanding of in-use emission levels and the factors that 

influence them. For this inventory effort, Pennsylvania utilizes MOBILE6.2 as approved by EPA. In 

addition to VOC, CO, and NOx, MOBILE6.2 produces emission factors for PM related emission 

categories as needed for the PM2.5 inventory analyses. 

PPSUITE. Pennsylvania also uses a post processor named PPSUITE (formerly named PPAQ - Post 

Processor for Air Quality), which consists of a set of programs that perform the following functions:

 Analyzes highway operating conditions
Appendix E-1 5

 Calculates highway speeds 

 Compiles vehicle miles of travel (VMT) and vehicle type mix data

 Prepares MOBILE6 runs

 Calculates emission quantities from output MOBILE6 emission rates and accumulated highway 

VMT.

PPSUITE has become a widely used and accepted tool for estimating speeds and processing MOBILE 

emission rates. It is currently being used for the New York City region, for the north and south New 

Jersey regions, and in other states including Louisiana, Virginia, and Indiana. The software is based 

upon accepted transportation engineering methodologies. For example, PPSUITE utilizes speed and 

delay estimation procedures based on planning methods provided in the 2000 Highway Capacity Manual, 

a report prepared by the Transportation Research Board (TRB) summarizing current knowledge and 

analysis techniques for capacity and level-of-service analyses of the transportation system. 

These two computer programs interact as shown in Exhibit 1. 

Exhibit 1

Emission Calculation Process for Pennsylvania

Roadway Data Source Growth Method Method to Calculate Method to Calculate

Used for Analysis to Future Years VMT and Speeds Emissions

 PENNDOT Use 2005 DOT 

data for each county study of potential PPSUITE MOBILE

 growth

 PPSUITE Input Data PPSUITE Processes 

MOBILE Output

PPSUITE Calculates

 Final Emissions
Appendix E-1 6

WHERE DOES PENNSYLVANIA OBTAIN ITS DATA?

Data Used in MOBILE 

Two major types of information are written into the MOBILE model by EPA: basic emission rates and 

travel weighting rates. EPA’s Office of Mobile Sources obtains this information from a number of 

sources, including its new vehicle certification program, in-use vehicle random sample studies and 

special studies (including information from some state I/M programs). For more information on 

MOBILE, a users guide and various documents (as well as the model itself) are available through EPA’s 

website (http://www.epa.gov/OMSWWW/models.htm). 

Basic emission rates are those which are produced under very standardized conditions. The model then 

modifies (corrects and/or weights) these rates based on other model or input parameters. Rates are 

incorporated for model year and vehicle type. MOBILE also calculates an assumed amount of increase in 

emissions as vehicles accumulate mileage.

In addition to exhaust emissions, evaporative VOC emission sources from gasoline-powered vehicles are 

also included1: 

 diurnal emissions (evaporated gasoline emissions generated by the rise in temperature over the 

course of a day when the vehicle is not being driven), 

 hot soak emissions (evaporated gasoline emissions occurring after the end of a vehicle trip, due to the 

heating of the fuel, fuel lines, fuel vapors), 

 running loss emissions (evaporated gasoline emissions occurring while a vehicle is driven, due to the 

heating of the fuel and fuel lines), 

 resting loss emissions (small but continuous seepage and minor leakage of gasoline vapor through 

faulty connections, permeable hoses and other materials in the fuel system).

Evaporative emissions are very dependent on temperature and fuel volatility as well as vehicle model 

year.

Travel Weighting Fractions. Research has found that newer cars tend to be driven more. The model 

reflects this, using state-specific vehicle age distributions from registration data. The model also contains

assumptions about trips per day and miles per day by age of the vehicle. This is important for exhaust 

emissions because these emissions are greater when the vehicle is not warmed up (cold start). Also, this 

information helps characterize evaporative emissions.

 

1

Some states use MOBILE to estimate refueling emissions (gasoline vapor emissions generated by the refueling of vehicles, 

where in the absence of controls the vapor in the vehicle fuel tank is displaced by the incoming liquid fuel and released to the 

atmosphere). Pennsylvania handles these emissions in the area source inventory.
Appendix E-1 7

What Are The Necessary Data Inputs to MOBILE? 

A large number of inputs to MOBILE are needed to fully account for the numerous vehicle and 

environmental parameters that affect emissions including traffic flow characteristics (as determined from 

the PPSUITE software), vehicle descriptions, fuel parameters, inspection/maintenance program 

parameters, and environmental variables as shown in Exhibit 2. With some input parameters, MOBILE 

allows the user to choose default values, while others require area-specific inputs.

Exhibit 2

MOBILE Inputs

 Vehicle Fuel Inspection/ Environmental

Descriptions Parameters Maintenance Variables

 Age Mix RVP Start Year Min, Max

 Model Years Temperatures

Vehicle Type Reformulated Type, 

 Mix Fuels Vehicle Types, Humidity

 Frequency

 Stringency,

 Speeds Oxygenated Waiver Rate, Cloud Cover

 (PPSUITE) Fuels Compliance

 Basic Refueling Anti-Tampering Sunrise/Sunset

Emission Rates Controls Pressure/Purge

 Tests

 Calculate Emission Factors

(Grams per Mile) for VOC and NOx

For an emissions inventory, area specific inputs are used for all of the items shown in Exhibit 2 except 

for the basic emission rates, which are MOBILE defaults. In addition, Pennsylvania uses the 

MOBILE6.2 default starts-per-day data and soak distributions that are used to calculate the number of 

starts in cold and hot start modes. EPA requires that the number of starts occurring per vehicle be 

determined from instrumented vehicle counts. Since such local data is not available, the MOBILE6.2 

national defaults are used for the Pennsylvania analyses. A vehicle will generate more emissions when it 

is first operated (cold start). It generates emissions at a different rate when it is stopped and then started 

again within a short period of time (hot start). Soak distributions are used to determine the time between 

when an engine is turned off to the next time it is restarted.
Appendix E-1 8

Vehicle Descriptions. Vehicle age distributions are input to MOBILE for each county based on 

registered vehicles reflecting July 1 summer conditions. These distributions are obtained from 

PENNDOT’s Bureau of Motor Vehicle Registration Database. Vehicle Type Mix is calculated from 

algorithms using a combination of MOBILE6.2 default percentages and PENNDOT truck percentages 

from roadway data. (See also the discussion of Vehicle Type Pattern Data in the next section.) Speeds

are discussed extensively in the next section.

Significant changes have occurred in the MOBILE6.2 model as compared to previous releases. Some of 

the information previously applied by the post processor after running MOBILE can now be input 

directly to the MOBILE6.2 model run. This includes information on the hourly distribution of VMT and 

the hourly speeds that occur during the day. Another important change in MOBILE6.2 is the influence of 

facility type on output emission factors. For example, MOBILE6.2 assumes that an average speed on a 

freeway results in a different emission factor than the same speed on an arterial roadway. Thus 

MOBILE6.2 is indirectly accounting for the accelerations and decelerations that typically occur on such 

roadways. MOBILE6.2 has four distinct facility types: Freeway, Arterial, Local, and Ramp. For any 

emission run, the input functional classes analyzed must be mapped to the above facility types. The 

following mapping scheme is used for the Pennsylvania runs:

PENNDOT Functional Classes MOBILE6.2 Facility Type

1,11,12 Freeway

2,6,7,8,14,16,17 Arterial

9,19 Local

Since ramps are not directly represented within the RMS database information, it is assumed that 8% of 

the Freeway VMT is Ramp VMT. This is consistent with the recommendations provided in EPA’s 

Technical Guidance on the Use of MOBILE6.2 for Emissions Inventory Preparation. 

Fuel Parameters. The same vehicle will produce different emissions using a different type of gasoline. 

Fuel control strategies can be powerful emission reduction mechanisms. An important variable in fuels 

for VOC emissions is its evaporability, measured by Reid Vapor Pressure. 

MOBILE allows the user to choose among conventional (used in most of Pennsylvania), federal 

reformulated (now used in the Philadelphia area), oxygenated (not used in Pennsylvania) and low Reid 

Vapor Pressure (RVP) gasoline (used in the Pittsburgh area starting in 1998). Pennsylvania chooses the 

MOBILE inputs appropriate to the year, season, and control strategy for the area being modeled. 

MOBILE also allows users to calculate refueling emissions, the emissions created when vehicles are 

refueled at service stations. Pennsylvania includes refueling emissions in its area source inventory and 

not in its highway vehicle inventory. However, that calculation uses a grams per gallon emission rate 

generated by MOBILE. 

Vehicle Emission Inspection/Maintenance (I/M) Parameters. MOBILE allows users to vary inputs 

depending on the I/M program in place for the area or, of course, choose “no I/M program.” The inputs 

include:

 program start year

 stringency level 

 first and last model years subject to the program

 waiver rates
Appendix E-1 9

 compliance rates

 program type (test-only, test-and-repair, etc.) and effectiveness

 frequency of inspection (annual, biennial)

 vehicle type coverage

 test type (idle, loaded, etc.)

 pass/fail standards or “cutpoints”

 technician training program

Some cutpoints (the emissions at which vehicles are failed) are contained in MOBILE, while others must 

be put in by the model user. Pennsylvania uses the parameters specific for the geographic area and year 

for which the modeling is being performed. 

Environmental Variables. Evaporative emissions are influenced significantly by the temperatures of 

the surrounding air. Minimum and Maximum temperatures have been compiled for each county based on 

information from EPA’s CHIEF bulletin board reflecting airport temperatures on emission violation days.

For annual and winter weekday analysis, average monthly temperatures are compiled for each county.

A summary of the MOBILE related inputs used for the inventory and future year estimates are attached 

with the submission.

Emission and Speed Relationships

Of all the user-supplied input parameters, perhaps the most important is vehicle speed. Emissions of both

VOC and NOx vary significantly with speed, but the relationships are not linear, as shown in Exhibit 3. 

While VOCs generally decrease as speed increases, NOx decreases only at the low speed range and 

increases steeply at higher speeds. Unlike ozone precursors, the MOBILE6.2 calculation of PM2.5

emission factors is not sensitive to many key input parameters including vehicle speed. 

To obtain the best estimate of vehicle speeds, Pennsylvania uses the PPSUITE set of programs, whose 

primary function is to calculate speeds and to organize and simplify the handling of large amounts of data 

needed for calculating speeds and for preparing MOBILE input files.
Appendix E-1 10

Exhibit 3

0

1

2

3

4

5

6

7

2.5 5 10 15 20 25 30 35 40 45 50 55 60 65

Speed (mph)

C

o

m

p

o

site

E

mis

sio

n

F

a

ctor (g/mi)

NOX

MOBILE6 VOC and NOx

Speed vs. Emissions

VOC

Roadway Data

The roadway data input to emissions calculations for Pennsylvania uses information from the Roadway 

Management System (RMS) maintained by PENNDOT’s Bureau of Planning and Research. PENNDOT 

obtains this information from periodic visual and electronic traffic counts. RMS data is dynamic since it 

is continually reviewed and updated from new traffic counts and field visits conducted by PENNDOT. 

Information on roadways included in the National Highway System is reviewed at least annually, while 

information on other roadways is reviewed at least biennially. 

On a triennial basis, a current “snapshot” of the RMS database is taken and downloaded to provide an upto-date record of the Commonwealth’s highway system for estimating emissions. A 2002 “snapshot” is 

used as the basis for 2002 base year inventory calculations. The current “snapshot”, in this case 2005, is 

used as the basis for future year inventory estimates. Growth rates are used to factor the 2005 volumes to 

each future analysis year.

The RMS database contains all state highways, including the Pennsylvania Turnpike, divided into 

segments approximately 0.5 miles in length. These segments are usually divided at important 

intersections or locations where there is a change in the physical characteristics of the roadway (e.g. the 

number of lane changes). There are approximately 99,000 state highway segments for the 67 

Pennsylvania counties contained in the RMS. Each of these segments contains an abundance of 

descriptive data, but only the following information is extracted for emission calculations:
Appendix E-1 11

 Lanes

 Distances

 Volumes in Average Annual Daily Traffic (AADT)

 Truck percentages

 PENNDOT urban/rural classifications 

 PENNDOT functional class codes

RMS volumes and distances are used in calculating highway VMT totals for each county. As discussed 

in the next section, adjustments are needed to convert the volumes to an average summer weekday, 

winter weekday, and monthly day. Lane values are an important input for determining the congestion 

and speeds for individual highway segments. Truck percentages are used in the speed determination 

process and are used to split volumes to individual vehicle types used by the MOBILE software. 

 

Pennsylvania classifies its road segments by function, as well as whether it is located in an urban, small 

urban or rural area, as indicated below in Exhibit 4. The PENNDOT urban/rural (UR) and functional 

classes (FC) are important indicators of the type and function of each roadway segment. The variables 

provide insights into other characteristics not contained in the RMS data that are used for speed and 

emission calculations. In addition, VMT and emission quantities are aggregated and reported using both 

UR and FC codes.

Exhibit 4

PENNDOT Classification Scheme: Urban/Rural Codes and Functional Class Codes

Urban/Rural Code 1=Rural

2=Small Urban

3=Urban

Functional Class Rural Functional Classes Used Urban Functional Classes Used

For Rural Areas For Small Urban and Urban Areas

------------------------------------- ------------------------------------------

1=Rural Freeway 11=Urban Freeway

2=Rural Other Principal Arterial 12=Urban Expressway

6=Rural Minor Arterial 14=Urban Principal Arterial

7=Rural Major Collector 16=Urban Minor Arterial

8=Rural Minor Collector 17=Urban Collector

9=Rural Local 19=Urban Local

Note: Functional Classes 3,4,5,10,13,15,18 are not currently used in PENNDOT’s RMS database

Additions and Adjustments to Roadway Data

Before the RMS data can be used by PPSUITE for speed and emission calculations, several adjustments 

and additions must be made to the roadway data.

HPMS Adjustments: According to EPA guidance, baseline inventory VMT computed from the RMS 

highway segment volumes must be adjusted to be consistent with Highway Performance Monitoring 
Appendix E-1 12

System (HPMS) VMT totals. The HPMS VMT reported for Pennsylvania is a subsystem of the RMS 

established to meet the data reporting requirements of the Federal Highway Administration (FHWA) and 

to serve as PENNDOT’s official source of highway information. Although it has some limitations, the 

HPMS system is currently in use in all 50 states and is being improved under FHWA direction. 

The HPMS VMT totals are developed from the data contained in the RMS database at the time of 

reporting and serves as a “snapshot” of the RMS data for a particular year. Since the RMS database does 

not contain many local roads, a separate procedure is used by PENNDOT to estimate total local VMT for 

the HPMS system. HPMS VMT summaries are prepared each year and reported by PENNDOT 

urban/rural and functional class codes. The VMT contained in the HPMS reports are considered to 

represent average annual daily traffic (AADT).

Although the HPMS VMT and the roadway data used for an inventory emissions analysis are both based 

on data from the RMS system, differences do exist between them and include the following. First, the 

HPMS and inventory roadway data are “snapshots” of the RMS data taken at different times. Since the 

RMS is dynamic, changing constantly due to new data, differences will result between the data used for 

calculating HPMS VMT totals and the inventory data used for an emissions analysis. Second, local 

estimates of HPMS VMT are obtained through alternative procedures developed by PENNDOT. 

However, the emissions inventory makes use of those few local roads contained in the RMS system. To 

account for such differences, adjustment factors are calculated and used to adjust the inventory roadway 

data to the reported HPMS VMT totals submitted to FHWA.

Adjustment factors are calculated which adjust the base year RMS download (in this case 2005) VMT to 

the reported HPMS totals for that year. These factors are developed for each county, urban/rural code, 

and functional class combination and are also applied to all future year runs. Adjustments for the 

“higher” functional classes (e.g. Freeway, Arterials - major routes) were very close to 1.000 since HPMS 

VMT is derived from RMS information, and the only difference in the data is that the “snapshot” for the 

emission calculations is taken at a different time than for the HPMS. “Lower” classes (e.g. local roads) 

require greater adjustment since a large part of the local system is not under state jurisdiction and is not 

in the RMS database. There is, of course, a significant amount of local road mileage in the state. It is 

assumed that those local streets that are in RMS are representative of all local streets in their area with 

respect to volume and speed, so that roadway mileage adjustment is appropriate. 

 

Seasonal Adjustments to Volumes: The RMS contains AADT volumes that are an average of all days 

in the year including weekends and holidays. An ozone emission analysis, however, is based on a typical 

summer weekday. PM and CO analyses require volumes representing a typical winter weekday and 

monthly average conditions (needed for producing annual totals). Therefore, the RMS volumes must be 

seasonally adjusted. Seasonal factors were developed for each functional class and urban/rural code 

based on yearly count information prepared by PENNDOT’s Bureau of Planning and Research. These 

factors are applied to the existing RMS AADT volumes to produce the July volumes. 

Additional Network Information: The PPSUITE software system allows for many additional variables 

other than those available in the RMS database. Using these variables improves the ability of 

Pennsylvania to incorporate real roadway conditions into its estimates. The variables include 

information regarding signal characteristics and other physical roadway features that can affect a 

roadway’s calculated congested speed. PPSUITE’s ability to estimate congested speeds by road segment 

improves Pennsylvania’s emissions inventories because of the overwhelming role speed plays in 

emission rates. If specific information regarding these variables is known or obtained for areas, this 
Appendix E-1 13

information can be appended to the RMS database. Otherwise, default values are assumed based on 

information provided by the PPSUITE input speed/capacity lookup data as described below. 

Speed/capacity lookup data provides PPSUITE with initial (free-flow with no congestion) speeds and 

capacities for different urban/rural code and functional class groupings. The initial speeds and capacities 

are used by PPSUITE in determining the final congested speed for each roadway segment. Speeds can 

also be greatly impacted by signals and other roadway features. As a result, this data provides default 

signal densities (average number of signals per mile for different functional classes) as well as default 

values for variables that determine the decay of speed with varying levels of congestion. As discussed 

above, values from the speed/capacity data can be overridden for specific links by directly coding values 

to the roadway database segments. The speed capacity data was developed from a combination of 

sources including the following:

 Information contained in the 2000 Highway Capacity Manual

 PENNDOT information on speeds and signal densities

 Engineering judgment

24-hour Pattern Data: Speeds and emissions vary considerably depending on the time of day (because 

of temperature) and congestion. Therefore, it is important to estimate the pattern by which roadway 

volume varies by hour of the day. The 24-hour pattern data provides PPSUITE with information used to 

split the daily roadway segment volumes to each of the 24 hours in a day. Pattern data is in the form of a 

percentage of the daily volumes for each hour. Distributions are provided for each county and functional 

class grouping. This data was developed from 24-hour count data compiled by PENNDOT’s Bureau of 

Planning and Research.

Vehicle Type Pattern Data: Basic emission rates may differ by vehicle type. These types are listed 

below in Exhibit 5.

Exhibit 5

MOBILE6.2 Input Composite Vehicle Classes

1. LDV - Light-Duty Vehicles (Passenger Cars)

2. LDT1 - Light-Duty Trucks 1 (<6,000 lbs)

3. LDT2 - Light-Duty Trucks 2 (<6,000 lbs, LVW=3,751-5,750)

4. LDT3 - Light-Duty Trucks 3 (6,001-8,500 lbs)

5. LDT4 - Light-Duty Trucks 4 (6,001-8,500 lbs, LVW>5,751)

6. HDV2B - Class 2b Heavy Duty Vehicles

7. HDV3 - Class 3 Heavy Duty Vehicles 

8. HDV4 - Class 4 Heavy Duty Vehicles 

9. HDV5 - Class 5 Heavy Duty Vehicles 

10. HDV6 - Class 6 Heavy Duty Vehicles 

11. HDV7 - Class 7 Heavy Duty Vehicles 

12. HDV8A - Class 8a Heavy Duty Vehicles 

13. HDV8B - Class 8b Heavy Duty Vehicles 

14. HDBS - School Buses

15. HDBT - Transit and Urban Buses

16. MC - Motorcycles
Appendix E-1 14

MOBILE summary reports by vehicle type are also useful in knowing what kinds of vehicles generate 

emissions. The vehicle type pattern data is used by PPSUITE to divide the hourly roadway segment 

volumes to the sixteen MOBILE6.2 (MOBILE5 had eight) vehicle types. Similar to the 24-hour pattern 

data, this data contains percentage splits to each vehicle type for every hour of the day. The vehicle type 

pattern data was developed from several sources of information:

 Hourly distributions for trucks and total traffic compiled by PENNDOT’s Bureau of Planning and 

Research, according to 2005 Pennsylvania Traffic Data

 PENNDOT truck percentages from the RMS database

 MOBILE6.2 default vehicle type breakdowns for the analysis year

The vehicle type pattern data is developed for each county and functional class combination. First, RMS 

truck percentages are averaged for all roadways within a county, functional class grouping. Using this 

percentage data, the total roadway volume for any segment could be divided to both auto and truck 

vehicle type categories. However, these percentages do not yet enable volumes to be divided to each of 

the sixteen MOBILE6.2 vehicle types. As a result, MOBILE6.2 default vehicle type breakdowns are 

then used to divide the auto and truck percentages, calculated above, to each specific MOBILE6.2 

vehicle type. Note that the defaults used vary by analysis year; as a result, each forecast year will utilize 

a unique vehicle mix distribution. PENNDOT hourly distributions for trucks and total traffic are then 

used to create vehicle type percentage breakdowns for each hour of the day.

Vehicle Type Capacity Analysis Factors: Vehicle type percentages are provided to the capacity 

analysis section of PPSUITE to adjust the speeds in response to trucks. That is, a given number of larger 

trucks take up more roadway space than a given number of cars, and this must be accounted for in the 

model. Capacity is adjusted based on the factors provided in this data. Values are developed from 

information in the 2000 Highway Capacity Manual and are specific to the various facility types. 

Producing Future Year Volumes 

Growth factors are used to project future highway volumes from the volumes provided in the RMS 

database. In the past, separate factors were derived for each county and highway functional class from an 

analysis of historic global update factors provided by PENNDOT’s Bureau of Planning and Research, 

coupled with estimates of population and employment growth from the U.S. Department of Commerce’s 

Bureau of Economic Analysis (BEA). The factors were then applied to base year traffic volumes (in this 

case 2005) on each highway segment in the RMS network database. 

For this submission, county-specific growth rates from a PENNDOT study completed in 2005 have been 

used to forecast volumes. As part of that study, a statewide traffic growth forecasting system was 

developed that incorporates traffic data from PENNDOT’s Traffic Information System and 

socioeconomic forecasts. The forecasting system was developed to support Department research 

endeavors related to transportation conformity and air quality planning.

The study, as documented in the report “Statistical Evaluation of Projected Traffic Growth, Traffic 

Growth Forecasting System: Final Report, March 14, 2005”, included the development of VMT 

forecasts and growth rates for four functional classifications in each Pennsylvania county. The forecasts 

used statistical relationships based on historic HPMS VMT trends and future county socioeconomic 

projections from Woods and Poole. 
Appendix E-1 15

SPEED/EMISSION ESTIMATION PROCEDURE 

The previous sections have summarized the input data used for computing speeds and emission rates for 

Pennsylvania. This section explains how PPSUITE and MOBILE use that input data to produce emission 

estimates. Exhibit 6 on the following page summarizes PPSUITE’s analysis procedure used for each of 

the 99,000 highway segments in the state.

Producing an emissions inventory with PPSUITE requires a process of disaggregation and aggregation. 

Data is available and used on a very small scale -- individual 1⁄2 mile roadway segments 24 hours of the 

day. This data needs to first be aggregated into categories so that a reasonable number of MOBILE 

scenarios can be run, and then further aggregated and/or re-sorted into summary information that is 

useful for emission inventory reporting.

One of the major enhancements of MOBILE6.2 is the increased detail of traffic that can be input to the 

emissions model. The PPSUITE post processor calculates hourly speeds for each roadway segment. 

Since previous versions of MOBILE only allowed one average speed as input for each scenario, the postprocessed speeds had to be aggregated and run through MOBILE with scenarios representing four 

separate time periods. MOBILE6.2 allows for direct input of the 24 hourly speeds as well as options to 

account for each link’s speed separately. These added features utilize the full extent of the information 

output from the speed processing programs and provide for more accurate emission estimates of the 

available traffic data. 

Volume/VMT Development

Before speeds can be calculated and MOBILE run, volumes acquired from RMS must be adjusted and 

disaggregated. Such adjustments include factoring to future years (not needed for 2005), seasonal 

adjustments, and disaggregating daily volumes to each hour of the day and to each of the sixteen 

MOBILE6.2 vehicle types.

Future Year Volumes: The RMS database contains up-to-date current year volumes. However, to 

conduct a future year analysis, these volumes must be factored to the year being analyzed. Growth 

factors have been prepared for each county and functional class grouping. These growth factors are 

applied to the 2005 RMS volumes to obtain future year estimates that can be utilized by PPSUITE.

Example:

A typical freeway link in the RMS database is I-80 segment 2500 in Luzerne County, 

Pennsylvania. This link has an urban/rural code=1 which indicates the link is in a rural area, and a 

functional class=1 indicating a rural freeway. The average annual daily traffic (AADT) from the 

RMS database for this link in 2002 is 12,077 vehicles/day.

Growth factors have been developed to factor the 2002 volume to future years. For example, to 

factor the 2002 volume to the year 2014, a growth factor of 1.282 would yield:

2014 volume = 12,077 vehicles/day x 1.282 = 15,483 vehicles/day

 
Appendix E-1 16

Exhibit 6

PPSUITE Speed/Emission Estimation Procedure

Data From PPSUITE Input Files PPSUITE Analysis Process Data from Roadway Source (RMS)

 The Following is Performed For

 Each RMS Roadway Segment

Percent Pattern Distributions Expand to 24 hourly volumes RMS AWDT Adjusted Volumes

Adjust Volumes for Peak Spreading

Vehicle Type Patterns Disaggregate to Vehicle Type RMS Truck Percentages

 Calculate Link & Signal Capacities Roadway Attributes

 (Lanes, FC code, UR code)

Speed/Capacity Lookup Table

 Calculate Link Calculate

Midblock Speed Approach Delay

 Calc VMT, Aggregate Link Speed

 Accumulate VMT, VHT

 (Aggregate by UR code, FC code,

 and time periods)

 The Following is Performed For

 Each Area, Functional Class 

HPMS VMT Adjustments Apply HPMS VMT adjustments

MOBILE Parameters Run MOBILE6.2 for Emission Factors

Vehicle Age Distributions

 

Min/Max/Ambient Temps 

 Humidity Calculate Emissions

 (VMT x Emission Factor)

Prepare MOBILE6 

Speed VMT File

(Account for each 

roadway segment’s speed 

for each hour of the day)
Appendix E-1 17

Seasonal Adjustments: PPSUITE takes the input daily volumes from RMS which represent AADT and 

seasonally adjusts the volumes to an average weekday in July. This adjustment utilizes factors developed 

for each functional class and urban/rural code. VMT can then be calculated for each link using the 

adjusted weekday volumes.

Example:

Again, assume the rural freeway link: I-80 segment 2500 in Luzerne County, Pennsylvania. The 

average annual daily traffic (AADT) for this link in 2002 is 12,077 vehicles/day. 

Seasonal factors have been developed for urban/rural code and functional class combinations. For 

an urban/rural code=1 and a functional class=1, the factor to convert from AADT to an average 

weekday in July is = 1.15

Average Weekday July Volume = 12,077 x 1.15 = 13,889 vehicles/day 

Total VMT (daily) for this link is calculated as volume x distance. The distance of this link as 

obtained from RMS is 0.286 miles.

2002 VMT = 13,889 vehicles/day x 0.296 miles = 41,111 vehicle-miles / day

Disaggregation to 24 Hours: After seasonally adjusting the link volume, the volume is split to each hour 

of the day. This allows for more accurate speed calculations (effects of congested hours) and allows 

PPSUITE to prepare the hourly VMT and speeds for input to the MOBILE6.2 model.

Example:

To support speed calculations and emission estimates by time of day, the July weekday volume is 

disaggregated to 24 hourly volumes. Temporal patterns were previously developed from 

PENNDOT count data and input to PPSUITE. For the I-80 rural freeway link with morning peak 

volumes similar to evening peak hours (neutral), the following temporal pattern is applied:

0

1

2

3

4

5

6

7

8

9

10

13579

11131517192123

Hour of Day

Percent of Daily

Using the I-80 segment for 1990, typical hourly volumes which result include:

8-9 a.m. 6.0% x (41,111 vehicle miles/ 0.296mi.) = 833 vehicles/hour (vph)

12-1 p.m. 5.0% x (41,111 vehicle .miles/ 0.296mi.) = 694 vph

5-6 p.m. 6.3% x (41,111 vehicle miles/ 0.296mi.) = 875 vph
Appendix E-1 18

After dividing the daily volumes to each hour of the day, PPSUITE identifies hours that are unreasonably 

congested. For those hours, PPSUITE then spreads a portion of the volume to other hours within the 

same peak period, thereby approximating the “peak spreading” that normally occurs in such overcapacity conditions.

 

Disaggregation to Vehicle Type: EPA requires VMT estimates to be prepared by vehicle type, 

reflecting specific local characteristics. As a result, for Pennsylvania’s emission inventory runs, the 

hourly volumes are disaggregated to the sixteen MOBILE6.2 vehicle types based on count data 

assembled by PENNDOT in combination with MOBILE6.2 defaults.

Example:

Disaggregation of the total I-80 volume (by hour) to the various vehicle types would include the 

following:

Total Volume 8-9 am = 833 vph

Vehicle Type Volume 8-9 am:

LDV 40.7% 338 vph

LDT1 7.4% 62 vph

LDT2 24.8% 207 vph

LDT3 7.6% 63 vph

LDT4 3.5% 29 vph

HDV2b 5.0% 42 vph

HDV3 0.5% 4 vph

HDV4 0.4% 3 vph

HDV5 0.3% 3 vph

HDV6 1.1% 9 vph

HDV7 1.3% 11 vph

HDV8a 1.4% 12 vph

HDV8b 5.1% 42 vph

HDBS 0.3% 3 vph

HDBT 0.1% 1 vph

MC 0.5% 4 vph

 

Speed/Delay Determination

EPA recognizes that the estimation of vehicle speeds is a difficult and complex process. Because 

emissions are so sensitive to speeds, it recommends special attention be given to developing reasonable 

and consistent speed estimates; it also recommends that VMT be disaggregated into subsets that have 

roughly equal speed, with separate emission factors for each subset. At a minimum, speeds should be 

estimated separately by roadway functional class. 

The computational framework used for this analysis meets and exceeds that recommendation. Speeds are 

individually calculated for each roadway segment and hour and include the delays encountered at signals. 

Rather than accumulating the roadway segments into area/functional groupings and calculating an 

average speed (as done in past), each individual link hourly speed is represented in the MOBILE6.2 

speed VMT file. This represents a significant enhancement in the MOBILE model since past versions 

only allowed input of one average speed for each scenario. MOBILE6.2 allows the input of a 
Appendix E-1 19

distribution of hourly speeds. For example, if 5% of a county’s arterial VMT operate at 5 mph during the 

AM peak hour and the remaining 95% operate at 65mph, this can be represented in the MOBILE6.2 

speed input file. For the Pennsylvania runs, distributions of speeds are input to MOBILE6.2 for separate 

scenarios representing county and functional class groupings; VMT is accumulated by the same 

groupings for the application of the emission factors to produce resulting emission totals.

To calculate speeds, PPSUITE first obtains initial capacities (how much volume the roadway can serve 

before heavy congestion) and free-flow speeds (speeds assuming no congestion) from the speed/capacity 

lookup data. As described in previous sections, this data contains default roadway information indexed 

by the urban/rural code and functional class. For areas with known characteristics, values can be directly 

coded to the RMS database and the speed/capacity data can be overridden. However, for most areas 

where known information is not available, the speed/capacity lookups provide valuable default 

information regarding speeds, capacities, signal densities and characteristics, and other capacity 

adjustment information used for calculating congested delays and speeds. 

Example:

The speed/capacity lookup table is used to obtain important data used for link speed calculations. 

For the I-80 link with an urban/rural code = 1 (rural) and a functional class = 1 (freeway), the 

lookup table provides information including the following:

freeflow speed = 65 mph

capacity = 1800 vph per lane

number of signals = 0

This information is used along with the physical characteristics of the roadway to calculate the 

delay (including congestion) to travel this link during each hour of the day:

For example: The I-80 link is calculated to have a travel time, including delay of 17.76 seconds 

for the 8-9am hour

Total travel time, in vehicle hours, for the 8-9am hour is calculated as:

VHT (8-9am) = 17.76 seconds x 833vph / 3600 sec/hr = 4.12 vehicle hours

The result of this process is an estimated average travel time for each hour of the day for each highway 

segment. The average time can be multiplied by the volume to produce vehicle hours of travel (VHT).

HPMS and VMT Adjustments

Volumes must also be adjusted to account for differences with the HPMS VMT totals, as described 

previously. VMT adjustment factors are provided as input to PPSUITE, and are applied to each of the 

roadway segment volumes. These factors were developed from 2005 data; however, they are also 

applied to any future year runs. The VMT added or subtracted to the RMS database assumes the speeds 

calculated using the original volumes for each roadway segment for each hour of the day.
Appendix E-1 20

Example:

Using the Luzerne County I-80 rural freeway link example, the daily assigned volume is adjusted 

to account for reconciliation with the HPMS VMT. RMS VMT (in AADT) for Luzerne County 

rural freeways totals 962,559 vehicle miles in 2005. HPMS VMT (in AADT) as supplied by 

PENNDOT and reported to FHWA totals to 990,088 vehicle miles for the rural freeways. A 

factor is developed by dividing the HPMS VMT by the RMS VMT:

HPMS adjustment factor for Luzerne County rural freeways = 990,088 / 962,559 = 1.029

This factor is held constant in all future years. As an example, this adjustment is made to the 

I-80 freeway link VMT for the 8-9am hour after speed calculations are made, and produces the 

final July weekday VMT for this hour used for Ozone runs.

I-80 Link VMT (8-9am) = 833vph x 0.296 miles x 1.029 = 254 vehicle miles

VMT and Speed Aggregation

As discussed in previous sections, MOBILE6.2’s ability to handle input distributions of hourly speeds 

has eliminated the need to aggregate speed data. For Pennsylvania runs, PPSUITE has been set up to

automatically accumulate VMT and VHT by geographic areas and highway functional class. The speed 

files input to MOBILE6.2 for each scenario contain the actual distribution of roadway speeds for that 

aggregation group. Exhibit 7 illustrates the scenario aggregation scheme used with MOBILE6.2.

Exhibit 7

VMT/VHT Aggregation Scheme

County 67 entries

Urban/Rural Code 1=Rural

2=Small Urban

3=Urban

Functional Class 1=Rural Freeway 11=Urban Freeway

2=Rural Other Principal Arterial 12=Urban Expressway 18 entries

6=Rural Minor Arterial 14=Urban Principal Arterial

7=Rural Major Collector 16=Urban Minor Arterial

8=Rural Minor Collector 17=Urban Collector

9=Rural Local 19=Urban Local

1,206 potential

combinations

Geographic aggregation is performed by urban, small urban, and rural areas of each county. Functional 

class aggregation is according to PENNDOT’s eighteen standard functional classes, respecting urban, 
Appendix E-1 21

small urban and rural definitions. For an individual county, this creates a potential for 18 possible 

combinations, each of which becomes an input MOBILE6.2 scenario. This allows each MOBILE6.2 

scenario to represent the actual VMT mix and speed for that geographic / highway combination. 

Altogether then, there are potentially 1,206 combinations for which speeds and VMT are computed and 

emissions are calculated with MOBILE.

MOBILE Emissions Run

After computing speeds and aggregating VMT and VHT, PPSUITE prepares input files to be run in 

EPA’s MOBILE6.2 program which is used to produce VOC and NOx emission factors in grams of 

pollutant per vehicle mile. The process uses an unmodified version of the MOBILE program that was 

obtained directly from EPA.

 

The MOBILE input file prepared by PPSUITE contains the following:

 MOBILE template containing appropriate parameters and program flags

 Temperature data specific to the county and season being run

 Vehicle age and diesel sales fraction data for the county being run

 Scenario data - contains VMT mix, speed distributions specific to scenario as produced by PPSUITE

Example:

A MOBILE input file is created by PPSUITE for Luzerne County. This file contains separate 

scenarios for each urban/rural code, functional class. A scenario represents a separate MOBILE 

run with different emission factors calculated and output for each run.

For this example, Luzerne County rural freeways will be run as a scenario with a specific VMT 

mix file and a speed distribution file accounting for all the roadway speeds within the grouping. 

Time of Day and Diurnal Emissions

Unlike in the past using MOBILE5, VMT and speeds are no longer aggregated as separate scenarios 

representing time periods. This was done in the past to account for the unique speeds encountered during 

each time period in the day. Since MOBILE6.2 allows for hourly roadway speeds to be represented in 

the speed VMT file, such a process is no longer needed. MOBILE6.2 will internally account for the 

emissions during each hour in the day and make the necessary diurnal calculations. 

Process MOBILE Output

After MOBILE has been run, PPSUITE processes the MOBILE output files and compiles the emission 

factors for each scenario. Using the above methodology, it allocates daily diurnal emissions to each of 

the time periods. Using the MOBILE emission factors, PPSUITE calculates emission quantities by 

multiplying the emission factors by the aggregated VMT totals. PPSUITE then produces an emissions 

database summarizing VMT, VHT, VOC, and NOx emissions as shown in Exhibit 8.
Appendix E-1 22

Exhibit 8

Summary of PPSUITE’s Methodology in Producing Emissions Summary

 PPSUITE Computes Speeds PPSUITE Aggregates VMT and VHT

RMS Roadway Data (120,000 records) VMT & VHT Aggregated By:

 * County (67)

 PPSUITE Computes VMT & Speed * Functional Class and 

 by Hour and Vehicle Type urban/rural codes (18)

PPSUITE Runs the MOBILE Program

 MOBILE Run for each County

 with UR, FC Scenarios

PPSUITE Processes MOBILE Output

 

Multiply VMT x Emission Rates

PPSUITE Produces Emission Database

Fields Exist For:

 VOC

 CO By Vehicle Type & Total

 NOx

By County, UR, FC
Appendix E-1 23

Example:

Luzerne County rural freeways were run as a scenario in MOBILE. Based on the input 

information, MOBILE6.2 outputs emission factors by vehicle type for this scenario as shown 

below:

Composite Emission Factors (grams/mile) from MOBIL6 output

Vehicle Type: LDGV LDGT1 LDGT2 LDGT3 LDGT4 HDDV7 ..... For all 28 M6 types

VOC: 1.22 1.86 2.42 3.68 0.36 1.13

NOX: 2.41 3.16 3.66 7.14 1.84 5.84

PPSUITE reads these emission factors from the MOBILE6.2 output file and multiplies them by the 

Luzerne County rural freeway VMT to obtain emission totals for this scenario. (Note: emissions 

shown in kg/day which is converted to tons/day in SIP narratives)

PPSUITE computes emissions as follows for this scenario:

Emission Factors (g/mi) Emissions (kg/day)

Veh Type VMT VOC NOX VOC NOX

LDGV 84,344 x 1.22 2.41 = 102.9 203.3

LDGT1 30,713 x 1.86 3.16 = 57.1 97.1

LDGT2 21,515 x 2.42 3.66 = 52.1 78.7

LDGT3 4,209 x 3.68 7.14 = 15.5 30.1

LDGT4 3,586 x 0.36 1.84 = 1.3 6.6

HDDV7 7,483 x 1.13 5.84 = 8.5 43.7

..... Repeated for all 28 MOBILE6.2 vehicle types

-------------------------------------------------------------------------------------------------------

Total 155,903 244.6 482.0

The emissions for this scenario are reported and stored in an output database file which contains a 

record for each scenario with fields containing VMT, VHT, VOC emissions, and NOX emissions. 

Fields exist for each vehicle type and for the total of all vehicle types as shown below.

Reported by Vehicle Type 1-28 and Total --- Repeated for VHT,HC,NOX

Cnty UR FC VMT1 VMT2 VMT3 VMT4 VMT5 VMT6 VMT7 VMT8 ... VMT28

Luze 1 1 84,344 30,713 21,515 4,209 3,586 2,806 7,483 1,248

VHT1 VHT2 VHT3 VHT4 VHT5 VHT6 VHT7 VHT8 ... VHT28

1,298 473 331 65 55 43 115 19

VOC1 VOC2 VOC3 VOC4 VOC5 VOC6 VOC7 VOC8 ... VOC28

102.9 57.1 52.1 15.5 1.3 1.5 8.5 5.7

NOX1 NOX2 NOX3 NOX4 NOX5 NOX6 NOX7 NOX8 ... NOX28

203.3 97.1 78.7 30.1 6.6 11.6 43.7 10.9
Appendix E-1 24

RESOURCES 

MOBILE model

Modeling Page within EPA’s Office of Mobile Sources Website 

(http://www.epa.gov/omswww/models.htm) contains a downloadable model, MOBILE users guide and 

other information. It also contains documents relating to the next version of MOBILE (MOBILE6.2) 

expected in 1999.

"AP-42" document, "Compilation of Air Pollutant Emission Factors, Volume II: Mobile Sources," as 

updated by Supplement A (January 1991), available in hard-copy only. This material is also in the 

process of being revised and updated. Contact AP-42 Project, Test and Evaluation Branch, EPA, 2565 

Plymouth Road, Ann Arbor, MI 48105.

Highway Vehicle Emission Estimates (June 1992) and Highway Vehicle Emission Estimates II (May 

1995) discusses how EPA obtains data for MOBILE and some of the shortcomings in earlier models. 

Similar discussions of the present version’s shortcomings are discussed in papers available at the 

website. 

User’s Guide to MOBILE6.1 and MOBILE6.2, Mobile Source Emission Factor Model, EPA Office of 

Air and Radiation, EPA420-R-03-010, dated August 2003.

Technical Guidance on the Use of MOBILE6.2 for Emission Inventory Preparation, US EPA Office of 

Transportation and Air Quality, August 2004.

Policy Guidance on the Use of MOBILE6 for Emission Inventory Preparation, US EPA Office of Air and 

Radiation, January 18, 2002.

Traffic Engineering

2000 Highway Capacity Manual, Transportation Research Board, presents current knowledge and 

techniques for analyzing the transportation system.

Traffic Data Collection and Factor Development Report, 2001 Data, Pennsylvania Department of 

Transportation, Bureau of Planning and Research. 

Traffic Data Collection and Factor Development Report, 2002 Data, Pennsylvania Department of 

Transportation, Bureau of Planning and Research. 
Appendix E-1 25

Highway Vehicle Inventory Glossary 

AADT: Average Annual Daily Traffic, average of ALL days.

AWDT: Average Weekday Daily Traffic

Basic emission rates: MOBILE emission rates based on the applicable Federal emission standards and 

the emission control technologies characterizing the fleet in various model years.

Cold start: parameter in MOBILE that accounts for additional emissions resulting from a cold-started 

engine.

Diurnals: the pressure-driven evaporative HC emissions resulting from the daily increase in temperature

Emission rate or factor: expresses the amount of pollution emitted per unit of activity. For highway 

vehicles, usually in grams of pollutant emitted per mile driven. 

FC: Functional code, applied in data management to road segments to identify their type (freeway, local, 

etc.)

Fuel volatility: The ability of fuel components to evaporate, thus entering the atmosphere as pollution. 

Fuel volatility is usually measured as Reid Vapor Pressure (RVP) in pounds per square inch. The lower 

the RVP, the less volatile the fuel.

Growth factor: Factor used to convert volumes to future years

HPMS: Highway Performance Monitoring System, PENNDOT’s official source of highway information

and a subset of RMS.

I/M: Vehicle emissions inspection/maintenance programs ensure that vehicle emission controls are in 

good working order throughout the life of the vehicle. The programs require vehicles to be tested for 

emissions. Most vehicles that do not pass must be repaired.

MOBILE: The model EPA has developed and which Pennsylvania uses to estimate emissions from 

highway vehicles.

Pattern data: Extrapolations of traffic patterns (such as how traffic volume on road segment types varies 

by time of day, or what kinds of vehicles tend to use a road segment type) from segments with observed 

data to similar segments.

Program flag: In MOBILE, a numeric code which tells the program such things as how data will be 

provided by user (or whether default will be used) or how to tailor outputs.

PPSUITE: Post-Processor for Air Quality, a set of programs that estimate speeds and processes 

MOBILE emission rates.

RMS: Roadway Management System, a database maintained by PENNDOT from traffic counts and field 

visits
Appendix E-1 26

Scenario: a MOBILE run with a specific set of geographical, time period, highway facility and control 

strategy assumptions. 

Segment: (referred to as link) division of roadway in the PENNDOT Roadway Management System. 

Usually represents 0.5 mile segments of roadway. 

UR: Urban/rural code, applied in data management to identify whether a road segment is urban, small 

urban or rural.

VHT: vehicle hours traveled.

VMT: vehicle miles traveled. In modeling terms, it is the simulated traffic volumes times link length.

Vehicle Type: One of eight types, distinguished primarily by fuel type and/or weight, used in MOBILE 

modeling.