Federal Test Procedure Review Project:

Preliminary Technical Report

May 1993

EPA 420-R-93-007

Certification Division

Office of Mobile Sources

Office of Air & Radiation

U.S. Environmental Protection Agency

DISCLAIMER: Although the information described in this preliminary technical report has been funded in part

by the United States Environmental Protection Agency under contracts to Radian Corporation and to Sierra

Research, Inc., it has not been subjected to the Agency's peer and administrative review processes. It is

being released for information purposes only and could be used in potential regulation development. 
Table of Contents

Executive Summary ...................... 1

Chapter 1. Introduction .................. 8

Chapter 2. Background . . . . . . . . . . . . . . . . . . . 10

2.1. The Air Quality Problem . . . . . . . . . . . . . . 10

2.2. History of the Federal Test Procedure . . . . . . . 11

2.3. Statutory Provision . . . . . . . . . . . . . . . . 14

2.4. Areas of Potential Concern . . . . . . . . . . . . 14

2.4.1. Fuel . . . . . . . . . . . . . . . . . . 14

2.4.2. Temperature . . . . . . . . . . . . . . . 16

2.4.3. Altitude . . . . . . . . . . . . . . . . 17

2.4.4. Driving Behavior (Including Acceleration)

. . . . . . . . . . . . . . . . . . . . 18

2.4.5. Test Procedure Modifications . . . . . . 20

2.5 Heavy-Duty Vehicles and Engines . . . . . . . . . . 20

Chapter 3. Project Overview . . . . . . . . . . . . . . . . 23

3.1. Research on Driving Behavior . . . . . . . . . . . 24

3.2. Emission Assessment of In-Use Driving . . . . . . . 27

3.2.1. Cycle Development . . . . . . . . . . . . 27

3.2.2. Emission Simulation Model . . . . . . . . 29

3.2.3. Vehicle Testing Programs . . . . . . . . 31

3.3. Notice of Proposed Rulemaking Development . . . . . 31

Chapter 4. Driving Survey Methods . . . . . . . . . . . . . 33

4.1. Survey Options . . . . . . . . . . . . . . . . . . 33

4.1.1. Choosing Survey Approach . . . . . . . . 33

4.1.2. Selecting Survey Sites . . . . . . . . . 35

4.2. Instrumented Vehicle Approach . . . . . . . . . . . 36

4.2.1. Key Features . . . . . . . . . . . . . . 36

4.2.2. Instrumented Vehicle Field Results . . . 39

4.2.3. Atlanta Instrumented Vehicle Study . . . 42

4.3. Chase Car Approach . . . . . . . . . . . . . . . . 43

4.3.1. Key Features . . . . . . . . . . . . . . 44

4.3.2. Chase Car Field Results . . . . . . . . . 46

4.3.3. Los Angeles Chase Car Study . . . . . . . 47

4.4. Analyses of Potential Bias . . . . . . . . . . . . 47

4.4.1. Analysis of Instrumented Vehicle Method . 48

4.4.2. Analysis of Chase Car Method . . . . . . 52

4.5. Selection of Principal Data Set . . . . . . . . . . 55
FTP Preliminary Report: March 27, 1998

4.5.1. Comparison of the Two Surveys Results . . 55

4.5.2. Rationale for Choosing 3-parameter

Instrumented Vehicle data . . . . . . . . 58

Chapter 5. Analytical Methods and Considerations . . . . . . 60

5.1. Driving Behavior Variables . . . . . . . . . . . . 60

5.1.1. Speed-Based Measures of Driving Behavior

. . . . . . . . . . . . . . . . . . . . 60

5.1.2. Trip-based Measures . . . . . . . . . . . 62

5.1.3. Vehicle-Based Measures . . . . . . . . . 65

5.2. Descriptive Methods . . . . . . . . . . . . . . . . 66

5.3. Statistical Accuracy . . . . . . . . . . . . . . . 68

5.4. Computer Resource Issues . . . . . . . . . . . . . 69

5.5. Driving Conditions . . . . . . . . . . . . . . . . 70

5.5.1. Engine and Catalyst Cooldown . . . . . . 70

5.5.2. Road Grade . . . . . . . . . . . . . . . 75

Chapter 6. Discussion and Analysis of Driving Survey

Results . . . . . . . . . . . . . . . . . . . 77

6.1. Summary of In-Use Driving Results for Four Cities

. . . . . . . . . . . . . . . . . . . . . . . . . 77

6.1.1. Driving Behavior - Second-by-Second

Analysis . . . . . . . . . . . . . . . . 77

6.1.2. Driving Behavior - Trip Analysis . . . . 82

6.1.3. Choosing Representative Survey Data . . . 83

6.2. Analysis of Baltimore Driving Conditions . . . . . 87

6.2.1. Speed-based Measures of Driving Behavior

. . . . . . . . . . . . . . . . . . . . 87

6.2.2. Trip Measures . . . . . . . . . . . . . . 98

6.2.3. Vehicle-based Measures . . . . . . . . . 108

6.2.4. Vehicle Soak . . . . . . . . . . . . . . 109

6.2.5. Trip Start Driving Activity . . . . . . . 115

6.3. Comparisons to the Federal Test Procedure . . . . . 127

6.3.1. Speed-based Measures . . . . . . . . . . 127

6.3.2. Trip Comparison . . . . . . . . . . . . . 140

6.3.3. Vehicle Soak . . . . . . . . . . . . . . 143

6.3.4. Trip Start Driving Activity . . . . . . . 145

6.3.5. Road Grade . . . . . . . . . . . . . . . 154

Chapter 7. Test Cycle Development Methods and Approach . . . 155

7.1. Test Cycle Methods . . . . . . . . . . . . . . . . 155

7.2. Method Types . . . . . . . . . . . . . . . . . . . 156

7.2.1. Segment-splicing Methods . . . . . . . . 156

7.2.2. Monte Carlo Simulation . . . . . . . . . 157
FTP Preliminary Report: March 27, 1998

7.2.3. Engineering Cycles . . . . . . . . . . . 158

7.3. Cycle Criteria . . . . . . . . . . . . . . . . . . 159

7.4. Cycle Validation . . . . . . . . . . . . . . . . . 159

7.5. Current EPA Test Cycle Development . . . . . . . . 160

List of Appendixes . . . . . . . . . . . . . . . . . . . . . 161

Appendix A. Chase Car Method Bias Analysis: Supplementary

Tables

Appendix B. Baltimore 3-Parameter Vehicle Characteristics

Appendix C. Summary Statistics, Distributions, and Graphs

Appendix D. Baltimore Soak Periods

Appendix E. Trip Start Driving Activity
FTP Preliminary Report: March 27, 1998

1

Executive Summary

Pursuant to Section 206(h) of the Clean Air Act, as amended

in 1990 (Act, or CAA), EPA has undertaken a review of the Federal

Test Procedure (FTP) used to test light duty vehicle and light

duty truck emissions to determine whether it adequately

represents actual current driving conditions. The driving cycle

used for the FTP was adopted over twenty years ago and

accumulated research suggests that it may no longer adequately

represent overall vehicle emission control performance under

current driving conditions.

Project Overview

The principal subject of this report is driving behavior,

including acceleration and trip start patterns. After reviewing

existing research on driving behavior, the Agency determined that

new surveys were needed to assess current driving in U.S. urban

nonattainment regions. A parallel study of current technology

vehicle emissions under the full range of driving conditions is

also in progress. Results from these efforts will be combined in

determining the need for test procedure revisions.

This preliminary technical report discusses how the driving

surveys were conducted, presents analyses of the results, and

compares the data to the existing FTP. Quantitative assessments

of the emission impacts are still in progress and are not

discussed in this report.
FTP Preliminary Report: March 27, 1998

2

With support from the American Automobile Manufacturers

Association (AAMA) and the Association of International

Automobile Manufacturers (AIAM), EPA conducted surveys of driving

behavior in Baltimore, MD, and Spokane, WA. Two methods of data

collection were employed. In an instrumented vehicle study, 113

Baltimore and 102 Spokane vehicles were equipped with "3-

parameter" datalogger packages that recorded second-by-second

speed and two other variables during periods of operation. As

part of the same surveys, the manufacturers recruited 79 vehicles

for study using "6-parameter" instruments designed to measure

additional variables.

The instrumented vehicles were observed for seven to ten day

periods. A separate chase car study collected similar speed data

in the two cities using a laser device mounted on a patrol car

that tracked in-use target vehicles. This produced relatively

short sequences of data on a much larger sample of vehicles.

The Baltimore and Spokane surveys are supplemented by data

collected in two other cities. EPA's Office of Research and

Development sponsored an instrumented vehicle study in Atlanta,

GA. Finally, the California Air Resources Board (CARB) sponsored

a chase car study in Los Angeles similar to the chase car studies

in Spokane and Baltimore.

For reasons relating to representativeness, availability,

and precision of the survey data, most of the discussion in this

report is confined to driving observed in the Baltimore 3-

parameter instrumented vehicle study. Participants in the
FTP Preliminary Report: March 27, 1998

3

instrumented vehicle study in Baltimore were recruited from two

different centralized Inspection/Maintenance stations, one in

central Baltimore and one in a suburb just outside Baltimore. 

The urban site yielded driving characteristics similar to those

in Spokane and the suburban site was similar to Atlanta and Los

Angeles. As it was not possible to obtain detailed analyses of

the Atlanta and Los Angeles data in time for this report, EPA

decided to use just the Baltimore data for purposes of this

report. This should yield a more representative picture than

also including the Spokane data. Future research will include

further analyses of data obtained from the other sources.

EPA also entered into a cooperative agreement with the New

York State Bureau of Air Research to obtain test data and

analyses of the engine and catalyst cooldown processes and

factors affecting the cooldown rates. These data are used to

assess the condition of the engine and catalyst during engine

start-up.

Preliminary Indications of Actual Driving Behavior

Speed and Acceleration

Speeds were much higher in Baltimore than are represented on

the FTP. The average speed in Baltimore was 24.5 mph (median

speed was 23.7). The speeds observed ranged to almost 95 mph;

6.4% were above 60 mph and 2.6% above 65 mph. By comparison, the

FTP has an average speed of 19.6 mph with a maximum of 56.7 mph. 

About 8.5% of all speeds in Baltimore exceeded the FTP maximum.
 Mean acceleration rates generally average to zero and are not a useful 1

measure of comparison.

 The power needed from an engine to accelerate a vehicle is proportional to 2

both the vehicle speed and the acceleration rate. Thus, neither variable, by

itself, is a good measure of the load placed on the engine during acceleration. The

joint distribution of speed and acceleration is the best measure, but it must be

examined in three dimensions, which is difficult to visualize and comprehend. While

not as good as the joint distribution of speed and acceleration, the best twodimensional measure is "specific power," which is roughly equivalent to (2 * speed

* acceleration). This measure is used extensively in this report and has the units

mph /sec. 2

FTP Preliminary Report: March 27, 1998

4

 Acceleration rates in Baltimore were also significantly

higher than those on the FTP. The acceleration rates observed

ranged up to 15 mph/sec, with a standard deviation of 1.5. The 1

FTP has a maximum acceleration rate of 3.3 mph/sec and a standard

deviation of 1.4. About 2.5% of all driving in Baltimore

exceeded 3.3 mph/sec.

Power-related measures also indicate that the observed

driving behavior was more aggressive than the FTP. Specific

power for the Baltimore sample ranged up to 558 mph /sec and 2 2

averaged 46.0, with a median of 34.7. The FTP has a maximum

power of 192, average of 38.6, and median of 21.6. An analysis

was also done of the scatter of speed-acceleration points

occurring in the Baltimore sample outside the FTP envelope of

speed and accelerations. These points represent about 18% of

total Baltimore driving time.

Driving Behavior Determinants

Vehicle Type - Speed distributions were fairly similar for

each of the three categories analyzed; trucks, sedans, and high
FTP Preliminary Report: March 27, 1998

5

performance vehicles. However, high performance vehicles

demonstrated more aggressive driving behavior than the other

classes, with over twice as much operation at power levels above

200 mph /sec. 2

Vehicle Age - Newer vehicles (1983 and later) had higher

average speeds than older vehicles (25.1 mph v. 21.2 mph), were

driven somewhat longer and farther per day, and averaged fewer

trips and slightly fewer stops per mile. The data indicate that

newer vehicles spend more time at high speeds and are used for

longer trips than older vehicles. However, analyses of the

aggressiveness of the driving behavior, as measured by

acceleration and power distributions, indicate very little

difference between older and newer vehicles.

Time of Day - Average speeds were lowest during the evening

rush period of 4-7 pm (23.0 mph) and highest during the morning

rush period of 6-9 am and night driving period of 9 pm to 1 am

(25.4 mph). Extremes of acceleration and specific power were not

highly associated with time of day. About twice as many trips

began during the evening rush period as during the morning rush

period. The morning peak period had the longest average trip

lengths (6.8 miles), while mid-day trips averaged only 4.4 miles.

Time of Week - Average weekend speeds were substantially

higher than on weekdays; 26.3 mph and 23.9 mph, respectively. 

This is due, in part, to increased high speed driving; 13.6% of

all driving during the weekend exceeded 55 mph, compared to 9.8%
 For the purposes of this report, a trip has been defined as beginning when 3

the engine is turned on and ending when the engine is shut off (although engine off

times of less than 18 seconds are ignored).

FTP Preliminary Report: March 27, 1998

6

during weekdays. Weekend driving also produced substantially

higher average time, distance, and number of trips than weekdays. 

However, acceleration and specific power measures do not indicate

that weekend driving is more aggressive than on weekdays.

Trip Patterns

Average in-use trip lengths are much shorter than the FTP,

which represents a 7.5 mile trip. The average observed trip3

covered 4.9 miles. The median value of trip distance indicates

that "typical" trips are even shorter, only 2.5 miles. One of

the in-use impacts of shorter trips is that a much higher

proportion of overall driving is done within 0.67 mile of vehicle

starts (12.0% v. 8.9% on the FTP), prior to engines and catalysts

reaching normal operating temperatures. The frequency of stops

on the FTP is also uncharacteristic of in-use trips; the average

distance between stops on the FTP is only 0.41 miles compared to

0.87 in Baltimore. Despite these differences, the FTP and

Baltimore trips disagree only slightly in the proportion of time

spent in the four operating modes: idle, cruise, acceleration,

and deceleration. 

Vehicle Soaks

The in-use data contains a large proportion of intermediate

soak periods (that is, the time between the end of a previous
FTP Preliminary Report: March 27, 1998

7

trip and the beginning of the next one) that are not reflected on

the FTP. The FTP contains soak periods of 10 minutes and 12-36

hours; almost 40% of all soak periods in Baltimore were between

10 minutes and 2 hours. As catalysts cool off much faster than

engines and most are almost completely cold in about 45-60

minutes, this is a potential emission concern. Analyses indicate

that only about 30% of all in-use starts occur with catalysts hot

enough to be immediately effective; the FTP implicitly assumes

that 57% of all starts occur with hot catalysts. On the other

hand, the FTP implicitly assumes that 43% of all starts occur

with cold engines, while less than 25% of in-use starts occur

with cold engines.

Trip Start Driving Activity

While the FTP has lower speeds and is less aggressive than

in-use driving behavior, overall, the reverse occurs for the

first few minutes after a vehicle start. The average observed

speed during the first 80 seconds of all trips (the initial idle

period was not included in this period) was only 14.4 mph,

compared to 23.1 mph for the first micro-trip on the FTP. The

average in-use speed 81-240 seconds into the trip was 22.8 mph,

compared to 29.8 for a comparable period on the FTP. The

aggressiveness of the FTP was also off substantially, with the

first micro-trip on the FTP substantially less aggressive than

in-use driving and the second micro-trip greatly overaggressive. 

Under similar ambient air temperature conditions, the

initial idle time on the FTP after a cold start is similar to

observed data. However, after a hot start the initial idle time
FTP Preliminary Report: March 27, 1998

8

on the FTP is much longer than observed data. The FTP uses an

initial idle time of 20 seconds after both cold and hot starts;

observed initial idle times after cold starts averaged 28 seconds

with a median of 9 seconds, while initial idle times after hot

starts averaged only 12 seconds with a median of 5 seconds.

Emission Impact Assessment Plans

The analysis of data obtained from the driving surveys

described here indicates significant differences exist between

actual driving behavior and the FTP. The driving survey data

will serve as the primary input into programs to assess the

difference between emissions predicted by the FTP and emissions

that occur in actual driving. This assessment requires the

development of driving cycles that are more representative of the

driving behavior information obtained from the surveys. EPA, in

cooperation with the California Air Resources Board, has

investigated cycle generation alternatives and developed improved

methods. Cycles generated from the driving survey data using

these methods will be used in test programs to quantify in-use

emissions. 
FTP Preliminary Report: March 27, 1998

9

Chapter 1. Introduction

The cornerstone of the Clean Air Act is the effort to attain

and maintain National Ambient Air Quality Standards (NAAQS). 

Regulation of emissions from on-highway, area, and stationary

sources prior to enactment of the Clean Air Act Amendments (CAAA)

of 1990 has resulted in significant emission reductions from

these sources. However, many air quality regions have failed to

attain the NAAQS, particularly for ozone and CO. This is due to

many factors, including the number of vehicles on the road and a

corresponding increase in the number of miles driven by the inuse fleet which, even though single vehicles have experienced

significant emission reductions, has increased total emissions

from the motor vehicle fleet.

The Clean Air Act, as amended (CAA, or Act), contains a

large number of provisions to further improve ambient air

quality. Section 206(h) of the Act requires that EPA review its

regulations for the testing of motor vehicles and revise them if

necessary to ensure that motor vehicles are tested under

circumstances reflecting actual current driving conditions. This

preliminary technical report discusses the driving behavior

research conducted by the EPA in its review of the Federal Test

Procedure (FTP) under Section 206(h) of the Act. The report

discusses the need for the research, methods and approaches, and

analyses of the driving behavior observed. Results from previous

research are not included in this report; nor are quantitative

assessments of the emissions impact of such driving behavior

(although qualitative implications are discussed).
FTP Preliminary Report: March 27, 1998

10

Chapter 2 discusses background information, including the

history of the Federal Test Procedure and the Act's provisions

for EPA review of the FTP. Chapter 3 provides an overview of the

goals and general approach to the entire FTP project, including

assessing the emissions impact of actual driving behavior and

development of proposed rules. Chapter 4 describes the survey

methods and programs used to determine actual driving behavior. 

Chapter 5 discusses the analytical methods used to evaluate the

driving survey data. Chapter 6 presents the results of the

Agency's analyzes of driving behavior and provides comparisons to

the existing test procedures. Chapter 7 describes the

methodologies developed to generate test cycles from the survey

data.
 EPA, Office of Public Affairs, Environmental News, August 16, 1990. 4

 Nonroad Engine and Vehicle Emission Study--Report, U.S. EPA, EPA-21A-2001, 5

November 1991.

 Ibid. 6

FTP Preliminary Report: March 27, 1998

11

Chapter 2. Background

2.1. The Air Quality Problem

Motor vehicles are a well known major source of volatile

organic compounds (VOC) and oxides of nitrogen (NOx), both of

which are precursors of ground level ozone, or smog. Motor

vehicles are also a major source of carbon monoxide (CO)

emissions. While significant progress has been made over the

past two decades in controlling motor vehicle emissions, as of

August 1990, 96 air quality control regions still failed to meet

the national ambient air quality standard (NAAQS) for ozone, and

forty-one regions failed to attain the NAAQS for CO.4

As an example of the impact of motor vehicles, estimates of

ozone precursors can be used. Volatile organic compounds and NOx

interact in sunlight to form ozone. Motor vehicles are estimated

to contribute approximately 25% of VOC emissions nationally

during the summer months. Small "area sources" such as 5

bakeries, dry cleaners, and consumer solvents contribute 45% and

large point sources such as petroleum refineries contribute 10%

of VOC emissions. Additionally, motor vehicles are estimated to 6

contribute approximately 42% of nationwide CO emissions during
 Ibid. 7

 The regulations that encompass the many aspects of the FTP are generally 8

contained in 40 CFR Part 86, Subparts A and B.

 For further discussion on the development of this cycle, see: Kruse, Ronald 9

E., and Thomas A. Huls, SAE Paper #730553 "Development of the Federal Urban Driving

Schedule," 1973. A speed-time trace of this cycle is contained in 40 CFR Part 86,

Appendix I.

FTP Preliminary Report: March 27, 1998

12

the winter months. Clearly, motor vehicles are a major source 7

of ozone precursor and CO emissions in nonattainment areas.

2.2. History of the Federal Test Procedure

The FTP is the test procedure used to determine compliance

of light-duty vehicles (LDV) and light-duty trucks (LDT) with

federal emission standards. As designed, the FTP was intended 8

to represent typical driving patterns in primarily urban areas.

Preproduction vehicles are tested using the FTP as part of

the motor vehicle certification process. The certification

process is used to establish that each vehicle is designed to

comply with emission standards for its full useful life. The FTP

is also used to test production line and in-use vehicles for

compliance with appropriate emission standards.

The FTP is more than just a driving cycle; it provides a way

to consistently and repetitively measure concentrations of HC,

Nox, CO, and carbon dioxide (CO2)emissions which occur when a

vehicle is driven over a simulated urban driving trip. The 9

principal elements of the test are designed to test the
 G.C. Hass, et. al., "Laboratory Simulation of Driving Conditions in the Los 10

Angeles Area," SAE Paper No. 660546, August, 1966.

FTP Preliminary Report: March 27, 1998

13

evaporative and exhaust emissions under several simulated

situations. Evaporative emissions are tested after heating the

fuel tank to simulate heating by the sun (the diurnal test) and

again after the car has been driven and parked with a hot engine

(the hot soak test). Exhaust emissions are measured by driving

the vehicle (placed on a dynamometer) on a simulated urban

driving trip under two conditions: with a cold start designed to

represent a morning start-up after a long soak (a period of nonuse) and with a hot start that takes place while the engine is

still hot. The FTP also encompasses all factors relevant to

vehicle testing, such as fuel, vehicle preconditioning, ambient

temperature and humidity, aerodynamic loss, and vehicle inertia

simulations. In addition to evaporative and exhaust emissions,

the FTP is also used in evaluating fuel economy.

The driving cycle used for the FTP was derived to simulate a

vehicle operating over a road route in Los Angeles believed to be

representative of typical home to work commuting. The original

road route was selected in the mid-1960s by trial-and-error to 10

match the engine operating mode distribution (based on manifold

vacuum and rpm ranges) obtained in central Los Angeles using a

variety of drivers and routes with the same test vehicle.

Using an instrumented 1964 Chevrolet, recordings were made

of actual home-to-work commute trips by employees of the state of

California's Vehicle Pollution Laboratory. By trial and error, a

specific street route in the vicinity of the Lab was found that
 D.H. Dearm and R.L. Lamoureux, "Survey of Average Driving Patterns in the 11

Los Angeles Urban Area," TM-(L)-4119/000/01, February 28, 1969.

FTP Preliminary Report: March 27, 1998

14

matched the average speed/load distribution on the commute trips. 

That 12 mile route was called the "LA4."

In an effort to develop an improved Federal Test Procedure

(based on speed-time distributions rather than manifold vacuum

and rpm ranges), six different drivers from EPA's West Coast

Laboratory drove a 1969 Chevrolet over the LA4 route. The six

traces were analyzed for idle time, average speed, maximum speed,

and number of stops per trip. The total time required for the

six trips ranged from 35 to 40 minutes, with an average of 37.6

minutes. One of the six traces demonstrated much harder

acceleration rates than the other five and was discarded. The

other five traces were surprisingly similar. Of those five, the

trace with the actual time closest to the average was selected as

the most representative speed-time trace. That trace contained

28 "hills" of non-zero speed activity separated by idle periods

and had an average speed of 19.2 miles per hour.

Based on a 1969 report on driving patterns in Los Angeles,11

the average trip length was estimated to be 7.5 miles. Several

of the hills and portions of others were eliminated in order to

shorten the cycle to 7.5 miles while maintaining the same average

speed. The shortened route, designated the LA4-S3, was 7.486

miles in length with an average speed of 19.8 mph. Slight

modifications to some of the speed-time profiles were also made

in cases where the acceleration or deceleration rate exceeded the

3.3 mph/s limit of the belt-driven chassis dynamometers in use at
FTP Preliminary Report: March 27, 1998

15

the time. Mass emission tests comparing the shortened cycle to

the full cycle showed very high correlation. The final version

of the cycle was designated the LA4-S4 cycle and is 7.46 miles in

length with an average speed of 19.6 mph.

This cycle is now commonly referred to as the "LA4" or the

Urban Dynamometer Driving Schedule (UDDS). It has been the

standard driving cycle for the certification of LDVs and LDTs

since the 1972 model year. Beginning with the 1975 model year,

the cycle was modified to repeat the initial 505 seconds of the

cycle following a 10 minute soak at the end of the cycle. This

allows emissions to be collected on a "hot" start (the engine is

still warm) as well as after a cold start and during operation. 

The test then provides a more accurate reflection of typical

customer service than running just one 7.46 mile cycle from a

cold start.

2.3. Statutory Provision

Section 206(h) of the Act directs that EPA "review and

revise as necessary" the regulations pertaining to the testing of

motor vehicles to "insure that vehicles are tested under

circumstances which reflect the actual current driving conditions

under which motor vehicles are used, including conditions

relating to fuel, temperature, acceleration, and altitude."

This preliminary technical report documents technical

aspects of EPA's review process to date. This study does not

attempt to determine or justify the need for revisions to the
 40 CFR §86.113-94. 12

 Evaporative emissions include diurnal, hot soak, refueling, and running 13

losses.

FTP Preliminary Report: March 27, 1998

16

FTP. Such a justification would be part of any regulatory

decision making that EPA may conduct on FTP modifications.

2.4. Areas of Potential Concern

Section 206(h) of the Act specifically requires that EPA

consider actual driving conditions under which motor vehicle are

used, including conditions relating to these four areas: fuel,

temperature, altitude, and acceleration. Following is a

discussion of these conditions, plus a discussion of other

driving conditions of potential concern when reviewing the

adequacy of the FTP.

2.4.1. Fuel

Gasoline - The composition of the gasoline used for the FTP

(commonly referred to as indolene) was established by regulation

over 20 years ago. While it was representative of in-use fuel 12

at the time, commercial or in-use fuel properties have changed

significantly since then, in some cases having a major impact on

vehicle emissions, both tailpipe and evaporative. Studies 13

conducted during the 1980s indicated that vehicles tend to have

higher emissions during operation on commercial gasoline than on

indolene, particularly through evaporative losses.
 55 FR 23658 (June 11, 1990). 14

 57 FR 19535 (May 7, 1992). 15

FTP Preliminary Report: March 27, 1998

17

To address this concern, EPA has established volatility

limits for gasoline and alcohol blended fuels. These regulations

capped the allowable Reid vapor pressure for commercial gasoline

during the summer months. The second phase of these controls

became effective in the summer of 1992. As a result of these 14

actions, the emissions of a vehicle fueled with indolene are more

representative of the emissions from vehicles fueled with

commercial gasoline.

Diesel fuel - The Agency has taken steps to reduce the

sulfur content of in-use diesel fuel. Regulations published on

May 7, 1992, to reduce the sulfur content in diesel fuel, are

scheduled to take effect on October 1, 1993.15

Alcohol and Other Fuels - The Agency promulgated

regulations in 1989 which established emission standards and test

procedures for vehicles fueled with methanol and proposed similar

regulations in 1992 for vehicles fueled with natural gas and

liquefied petroleum gas. At this early stage of alternative fuel

development, it is impossible to know what the real-world fuel

compositions will be for any of these fuels when used in

automotive applications. In each of these rulemakings, EPA has

avoided adoption of narrow fuel specifications, specifying

instead that test fuels be representative of typical in-use

fuels.
 An engine start is considered to be a "cold" start if it is preceded by a 16

long uninterrupted soak, such as those starts that occur after an overnight soak.

 57 FR 31888 (July 17, 1992). 17

FTP Preliminary Report: March 27, 1998

18

2.4.2. Temperature

The FTP is conducted between 68 F and 86 F and includes a o o

cold start in its driving cycle. As ambient air temperatures 16

decrease, cold start emissions increase because a richer air/fuel

mixture must be employed to ensure the presence of sufficient

fuel vapor for combustion. In addition, colder temperatures lead

to longer warm-up times.

This is not a major concern for ozone, which is primarily a

summertime phenomenon, but it is for CO. Most CO exceedances

occur from December to March and over half occur at temperatures

below 45 F. o

To reduce the emissions generated from motor vehicles during

cold temperature operation, EPA recently issued 20EF CO emission

standards and test procedures. These regulations were issued on

July 17, 1992, and are scheduled for phase-in beginning with the

1994 model year. The regulations also establish interim 17

temperature defeat device criteria to maintain proportional CO

emission control between the 20 F standard and the warm o

temperature standards. These regulations ensure that the

Agency's test procedures properly reflect the impact of

temperature on CO emissions. As the cold CO regulations will

prevent emission step-functions just below 68 F that could also o
 58 FR 16002, March 24, 1993. 18

 56 FR 25724, June 5, 1991. 19

FTP Preliminary Report: March 27, 1998

19

impact HC emissions, they will also ensure that the FTP is

representative of HC emissions at colder temperatures.

At warmer temperatures the primary emission concern is

increased fuel evaporation. The Agency has recently published

regulations revising its evaporative test procedures to address a

number of concerns, including temperature. The final 18

regulations are expected to specify ambient test temperatures of

95 F. These new test requirements should ensure that vehicles o

can control evaporative emissions for most in-use events.

2.4.3. Altitude

It has long been recognized that without compensation for

the lower air density at high altitude locations engines tend to

operate at rich air/fuel mixtures more frequently and, therefore,

have excessive HC and CO emissions. Virtually all LDVs have been

required to meet emission standards at both low and high

altitudes without adjustment or modification since the 1984 model

year. Light-duty trucks and light-duty vehicles have had

separate high altitude standards since the 1982 model year. 

Regulations published on June 5, 1991, will require LDTs to meet

emission standards at both low and high altitudes without

adjustment or modification beginning with the 1997 model year. 19

The cold temperature CO regulations require that both LDVs and

LDTs meet the cold temperature CO standard at both low and high
FTP Preliminary Report: March 27, 1998

20

altitudes without modification. The FTP does not specify an

altitude range in which the test must be conducted. In effect,

the regulations allow the FTP to be conducted at any altitude and

this, in fact, occurs.

2.4.4. Driving Behavior (Including Acceleration)

Current technology vehicles have achieved impressive

reductions in emissions during normal operation, primarily due to

catalyst technology development. Catalyst conversion

efficiencies (that is, the rate at which HC and CO are oxidized

into carbon dioxide (CO ) and water vapor, or the rate at which 2

oxides of nitrogen (Nox) are reduced to nitrogen and oxygen) in a

modern, properly operating, warmed-up vehicle can simultaneously

exceed 98% for HC, 99% for CO, and 90% for Nox. This includes

typical transient operation in urban traffic situations, such as

that represented by the FTP. However, these simultaneous

catalyst conversion efficiencies are only achievable by a threeway catalyst in a very narrow range of air/fuel ratios around the

minimum theoretical air requirement for complete combustion

(called stoichiometry). Thus, modern, properly operating

vehicles are designed to operate at stoichiometry as much as

possible during the FTP.

Two types of operation make it difficult to operate an

engine at stoichiometry. The first type of operation is a cold

start. Fuel must be vaporized with air to combust properly. When

the engine is cold, not enough heat is available to properly

vaporize the fuel, requiring the addition of more fuel for proper
 Engine-out NOx emissions decrease under rich operation, but NOx reduction 20

efficiencies in the catalyst also drop. Overall, there may be a slight increase in

tailpipe NOx emissions under rich operation, but the effect is relatively minor and

varies from vehicle to vehicle.

FTP Preliminary Report: March 27, 1998

21

operation. Cold start emissions are also increased due to the

lack of conversion activity in the catalyst until it heats up

(little catalyst activity occurs below roughly 600 F). Thus, o

emission rates during cold starts can be 20 to 100 times the

emission rates during stoichiometric operation. In fact, the

majority of emissions from modern, properly operating vehicles

operating over the FTP occur during the first 10% of the test,

before the engine and catalyst have warmed up. This raises a

concern within the Agency as to whether or not the cold start

portion of the FTP properly reflects the proportion of time

vehicles actually spend in the warm-up mode.

The second type of operation that makes it difficult to

operate an engine at stoichiometry is high engine loads. High

loads on an engine running at stoichiometry can dramatically

increase engine and catalyst temperatures. These elevated

temperatures increase engine-out NOx emissions and can cause

engine knock and/or damage to the catalyst. The performance and

driveability of an engine under high load can be improved by

running with a richer air/fuel mixture. Thus, to prevent overtemperature damage to the catalyst, and to ensure the best

possible driveability and performance, vehicles are often

designed to operate rich under high engine loads. While such a

design has little effect on NOx emissions, it increases HC and 20

CO by almost the same 20 to 100 times factor as cold start
FTP Preliminary Report: March 27, 1998

22

operation. This also raises a concern within the Agency as to

whether or not a significant amount of high engine load operation

occurs in-use that is not properly reflected on the FTP. Due to

the nonlinear nature of the emission rates, this amount of

driving could actually be fairly small and still have a

significant emission impact.

A wide variety of in-use factors impact the amount of time

vehicles spend in either a warm-up or high-load mode. Factors

related to warm-up include distributions of trip length, time

between trips (referred to as "soak time"), ambient air

temperature, initial idle time, and driving behavior. Factors

that can cause high loads on an engine include high acceleration

rates, high speeds, positive road grades, air conditioning

operation, or some combination of factors (such as moderate

acceleration up a moderate grade). Complicating the assessment

is that different vehicles have very different calibration

strategies. Thus, the impact on emissions of the exact same

driving behavior may vary widely from vehicle to vehicle.

2.4.5. Test Procedure Modifications 

For repeatability, it is necessary to conduct certification

and enforcement testing in a laboratory. This gives rise to a

number of procedures designed to simulate the actual forces and

conditions a vehicle would experience on the road. The most

significant of these is the dynamometer itself, which must

simulate the inertial mass of the vehicle (which doesn't actually

move in the laboratory), aerodynamic losses, and duplicate tire-
 Light-duty is defined as less than or equal to 8500 pounds gross vehicle 21

weight.

FTP Preliminary Report: March 27, 1998

23

to-road traction and rolling losses. Other potential areas of

concern are the type and amount of external air provided for

vehicle cooling, the manner in which air conditioning losses are

simulated, and manual transmission shift points.

The impact of these test procedure factors on emissions has

been investigated at various times in the past. As a result of

these investigations, the Agency has already begun the process of

converting from small turn-roll waterbrake dynamometers to large

single-roll electric dynamometers in its own laboratory in Ann

Arbor, Michigan.

2.5 Heavy-Duty Vehicles and Engines

This preliminary report discusses the test procedures for

light-duty vehicles and light-duty trucks, but does not address

the very different procedures employed for testing heavy-duty

engines. The Agency believes this is consistent with the 21

intent of Congress, as expressed in the text of the Act and its

legislative history. Section 206(h) requires that EPA review

test procedure regulations issued under Section 206 for a

specific purpose - to insure that vehicles are tested under real

world conditions. While the test procedure for light-duty

vehicles and trucks involve vehicle testing, heavy-duty engines

are tested differently, using an engine only test. Since the

purpose of this provision is to review and, if necessary, revise

EPA's regulations for testing motor vehicles, it is reasonable
 See Senate Report No. 101-228, 101st Congress, 1st Session 106-107 (1989). 22

 In a recent rulemaking EPA addressed whether the current heavy-duty engine 23

test procedure reflected actual operating conditions for heavy-duty engines used in

urban buses, and determined there was no need to change the test procedures. [58

FR 15781, March 24, 1993]

 48 FR 52170, November 16, 1983. 24

FTP Preliminary Report: March 27, 1998

24

for EPA's review to address light-duty vehicle test procedures

and not heavy-duty engine test procedures. The legislative

history confirms that the review required under Section 206(h) is

to address EPA's test procedures for light-duty vehicles and

trucks. This interpretation is confirmed by the fact Congress 22

separately addressed the adequacy of EPA's test procedures for

urban bus engines, a subset of heavy-duty engines. Section

219(e) requires that test procedures used for urban bus emission

standards "reflect actual operating conditions." If Section

206(h) addressed both light and heavy-duty test procedures this

provision would amount to surplusage.23

EPA of course has the discretion to review the test

procedure for heavy-duty engines, and in fact did conduct such a

review in the recent past. EPA conducted a large-scale, in-depth

study over several years of heavy-duty vehicle driving conditions

in the late 1970s and early 1980s. Based on that study, EPA

revised the heavy-duty engine test procedures and driving cycle

in 1983. It was not considered either necessary or feasible to 24

re-examine the heavy-duty test procedures in this review process,

given the deadlines imposed by the CAAA and the fact that it

previously took over 8 years to assess actual heavy-duty driving

behavior and revise the heavy-duty engine test procedures.
FTP Preliminary Report: March 27, 1998

25
FTP Preliminary Report: March 27, 1998

26

Chapter 3. Project Overview

It is a basic premise that motor vehicle emission levels

determined through the FTP should adequately reflect in-use

vehicle emissions. If in-use driving modes exist that generate

significant amounts of emissions that are not reflected on the

FTP, then the anticipated benefits from motor vehicle standards

are not being fully achieved.

It is also generally agreed that no test procedure can

reasonably duplicate all in-use conditions. The overall goal of

the Agency's review of the FTP is to aid in determining whether

or not the FTP should be modified to reflect in-use conditions

not currently found in the test and, if so, what modifications

should be made. To meet this goal it is not enough to simply

examine factors such as ambient temperature ranges, in-use fuel

characteristics, or driving patterns. For example, qualitative

evidence has existed for years that certain types of actual

driving behavior are not represented on the FTP, such as high

acceleration rates. However, it would be counterproductive to

modify the FTP unless two conditions are met. First, the driving

behavior or other condition not represented properly by the FTP

should contribute a significant amount to motor vehicle

emissions. If it does not, then modifying the FTP could incur

substantial costs and disruption with little or no air quality

benefit. Second, any modification to the FTP should be expected

to promote design improvements to vehicles and thereby create

real improvements in controlling in-use emissions. If the

current FTP is already effective in reducing emissions during
FTP Preliminary Report: March 27, 1998

27

non-FTP type driving or other conditions, then modifying the FTP

could again incur substantial costs with little or no air quality

benefit. Even if off-cycle emissions exist that are not properly

controlled by the FTP, it is critical to ensure that FTP

modifications will actually promote the proper design

improvements. The Agency believes this approach is a reasonable

way to implement the requirements of Section 206(h) of the Act.

This chapter of the report outlines the various research

programs undertaken by the Agency to assess the FTP. The purpose

is to provide a context for the driving survey data and analyses

presented in Chapters 4-6. Section 3.1. discusses the reasons

behind the Agency's decision to conduct large-scale surveys of

in-use driving behavior. Section 3.2. outlines the methods and

test programs being developed by the Agency to assess the

emission impact of the driving survey information presented in

this preliminary report. Section 3.3 outlines some of the issues

and options that need to be addressed as part of the rulemaking

development process.

3.1. Research on Driving Behavior

Based upon the logic outlined in Section 2.4., EPA compared

conditions on the FTP and in-use for fuels, temperature,

altitude, and driving behavior and determined that driving

behavior was the most important area on which to focus the FTP

review. EPA has also already initiated work on dynamometer

improvements necessary to correct the inherent limitations of the

small twin-roll waterbrake dynamometer currently in use. This
 As acceleration is the rate of change of vehicle speed, highly accurate and 25

frequent measurements of vehicle speed are necessary to properly compute

acceleration.

FTP Preliminary Report: March 27, 1998

28

work to upgrade the Agency's dynamometers is currently in

progress. 

Shortly after enactment of the 1990 amendments to the CAA,

EPA evaluated existing data on actual driving behavior. Because

one of the primary concerns with the FTP is fuel enrichment, as

discussed in Section 2.4.4., quantitative assessments of

acceleration rates were a high priority. It quickly became

apparent that very little information existed that could be used

to assess the frequency and magnitude of high acceleration

rates. The only existing sources of second-by-second speed 25

measurements were from the Operational Characterization Study

(OCS) conducted by EPA in Columbus, Ohio, in 1983, the European

Drive Project in six European cities, and work conducted by H. C.

Watson and his associates in Melbourne, Australia. The European

and Australian work, while interesting and informative, would not

be considered directly representative of how vehicles are driven

in the U.S.A.

The OCS study was more relevant, but contained some inherent

limitations. The study installed instrumentation on several

vehicles which were then loaned to study participants. The

driving patterns of 47 private citizens in Columbus, Ohio, were

monitored covering a total of 251 days of vehicle operation. 

However, only three different loaner vehicles were used (a

Chevrolet Impala, a Ford Fairmont, and a Chevrolet Chevette),
 Testing by the California Air Research Board on 10 vehicles over different 26

high acceleration modes had demonstrated that the mode generating the highest

emissions varied greatly from vehicle to vehicle.

FTP Preliminary Report: March 27, 1998

29

making it difficult to match the participant's personal vehicle. 

Also, some doubts are raised as to whether loaner vehicles would

be driven the same as the driver's personal vehicle. These

potential problems with the loaner vehicles used in the study

cast some doubt on how well the results represent overall driving

behavior. More importantly, significant problems in data

collection were encountered during the study, leading to concerns

regarding data validity.

These problems with the OCS data, combined with concerns

about using 8-year old data and the ability of Columbus, Ohio, to

represent major nonattainment areas throughout the country, led

EPA to conclude that it would not be appropriate to evaluate

potential revisions to the FTP based upon the OCS results. 

However, this left the Agency with choosing between three

undesirable options. The first option was to proceed quickly

with a new cycle and standard, based upon best engineering

judgment, designed to control high acceleration enrichment. 

However, without knowledge of actual acceleration behavior, this

approach had a high risk of both forcing excessive controls for

driving that rarely occurs (increasing vehicle costs without

attendant emission benefits) or of missing acceleration modes

that cause significant in-use emissions. In addition, this 26

approach would not allow investigation of other factors that

could potentially affect in-use emissions, such as driving

behavior during the vehicle warm-up period and catalyst cooldown
FTP Preliminary Report: March 27, 1998

30

after the vehicle has been turned off. Because of the risk of

overlooking important test procedure modifications and of causing

excessive costs, the Agency concluded that this would not be an

appropriate route to take.

Another option was to take the position that there was no

available information upon which the Agency could justify

revisions to the FTP and, therefore, that no revisions were

necessary. The Agency quickly rejected this option because it

appeared to violate the spirit of Section 206(h)'s requirements

and of EPA's general goal to provide a healthy environment.

The third option was for EPA to conduct basic research into

actual driving behavior and conditions prior to deciding whether

to initiate rulemaking. The Agency concluded that this was the

only feasible alternative to simply ignoring any potential

modifications to the FTP and was important since the relative

impact of off-cycle emissions are likely to increase as emissions

represented by the FTP are reduced.

Other CAA mandates, such as the Tier I emission standards

and longer useful life, should reduce the baseline emissions

derived through FTP testing. If adopted, Tier II emission

standards would have a like effect. Thus, any off-cycle

emissions will become relatively more important in the future. 

The research program undertaken by the Agency is designed both to

quantify any emission impacts from off-cycle driving behavior,

and to provide information needed to determine whether or not EPA

should make regulatory changes to the FTP.
FTP Preliminary Report: March 27, 1998

31

The program developed by the Agency to evaluate driving

behavior contains three basic components. First, to determine

how vehicles are actually driven, the Agency monitored an

extensive amount of actual vehicle operation. This is described

in the Chapter 4 of this report, "Driving Survey Methods and

Approach." Second, the data from the vehicle monitoring has been

analyzed to determine cycle and trip information and the impacts

of different factors on driving behavior. These analyses are

discussed in Chapter 6, "Discussion and Analysis of Driving

Survey Results." The Agency has also worked toward developing

new driving cycles that represent the complete range of actual

driving behavior. This part of the program is described in

Chapter 7, "Test Cycle Development Methods and Approach."

3.2. Emission Assessment of In-Use Driving

3.2.1. Cycle Development

To assess the impact of non-FTP driving, it is necessary to

estimate the difference between emissions predicted by the FTP

cycle and emissions that occur in actual driving. Using either

computer models or dynamometer testing, this assessment requires

the development of one or more driving cycles that are

representative of the real world. The driving survey data

discussed in Chapters 4 through 6 will serve as the primary input

to this component of the project.

Several approaches to cycle development have been used in

the past. These vary considerably in level of subjectivity. One
FTP Preliminary Report: March 27, 1998

32

approach, used in developing the current FTP, is to splice

together segments of real speed patterns that are selected from

the survey data. A final cycle is obtained by matching summary

features of the resulting speed-time trace with those of the full

sample. A virtue of this and related approaches is their basis

in real driving experience that can be reproduced in dynamometer

testing. The choice of segments and matching criteria are

potential difficulties.

The speed-time trace used for the heavy-duty engine test

cycle was generated using Monte Carlo simulation. With this

method, each second-by-second value is chosen according to

statistical criteria derived from the survey data. Cycles are

subjected to matching criteria in order to screen out

unsatisfactory candidates. This is likely to be a more efficient

method of producing different cycles, but these cycles are wholly

"unreal" in comparison to the splicing approach described above

and have some potential of yielding driving segments that could

arguably be unrealistic.

Because of the concerns over the available cycle generation

methods, the Agency, in cooperation with the California Air

Resources Board, initiated a program to investigate cycle

generation alternatives and develop improved methods. The

results of this work are discussed in Chapter 6.
FTP Preliminary Report: March 27, 1998

33

3.2.2. Emission Simulation Model

In analyzing data from the in-use driving surveys it is

essential to consider the emissions impact of the real world

driving patterns that are not represented by the current FTP

driving cycle. As discussed in Section 2.4.4., this requires

assessment of a wide range of driving behavior, factors

influencing emissions, and manufacturer calibration strategies. 

In order to perform these large scale assessments, EPA is

developing a computer model which simulates vehicle emissions

over any desired driving cycle.

EPA is using the modeling approach because it affords

flexibility in analyzing the emission impact of the driving

survey data. Even if EPA had adequate time to do all the

emission assessments with vehicle testing, many analyses can be

done much more efficiently and quickly with a computer simulation

model. A simulation model will allow the emission assessment of

a number of unedited and/or composite driving cycles over a large

number of vehicles with relative ease. Thus, while the Agency

intends to use vehicle test results as the core for emission

impact assessments, the model will be used to fill in holes in

the test data, assess factors that were not included in the test

program, and conduct quick assessments of potential changes to

the cycles and factors after the testing has been completed. 

Vehicle testing will also be used during the course of the

emission assessment effort in order to validate the results of

the computer simulation model.
FTP Preliminary Report: March 27, 1998

34

The simulation model computes instantaneous fuel and

emission rates based on instantaneous vehicle speed. This model

is currently being developed as two components, known as VEHSIM

(short for "VEHicle SIMulation") and VEMISS (short for "Vehicle

EMISSions").

The VEHSIM component was originally developed by GM and

later revised by the Department of Transportation. The VEHSIM

model takes instantaneous (generally second-by-second) vehicle

speed inputs and calculates instantaneous engine speed and load.

These calculations of engine speed and load are performed

utilizing vehicle information regarding vehicle aerodynamics,

drivetrain, transmission, and engine accessories stored in a

database known as a "part library".

The second component of the model, VEMISS, was developed by

EPA to provide fuel and emission rate calculations based on the

engine speed and load inputs produced by VEHSIM. VEMISS uses a

series of lookup tables, known as engine maps, to simulate the

fuel and emission rates for a particular vehicle. An engine map

contains fuel and emission rates over a matrix of engine speed

and load. VEMISS implements an interpolation method with the

engine map in order to calculate fuel and emission rates for an

instantaneous engine speed and load.

Cold start emission simulations also need to be developed to

estimate the impact of cold start driving behavior and soak time.

As the cold start simulation will calculate emissions using the

warm engine-out emission maps as the starting point, development
FTP Preliminary Report: March 27, 1998

35

of the cold start module is being sequenced behind the warm

component of the model. Once the warm component is

satisfactorily validated, the cold operation component will be

developed based on existing test data, integrated into the model,

and validated.

Upon completion of VEHSIM and VEMISS, these components will 

be linked to produce a fully functioning model capable of

simulating instantaneous fuel and emission rates for an entire

vehicle trip based on an inputted speed/time trace.

3.2.3. Vehicle Testing Programs

EPA is in the process of developing a joint testing program

with the California Air Resources Board and the vehicle

manufacturers. This program will provide a base of test data

addressing the emission impacts of high speeds and acceleration

rates. The primary goals of the program are to quantify the

increase in in-use emissions caused by high speed/acceleration on

modern technology vehicles, investigate potential certification

cycles for control of high speed/acceleration emissions, and

investigate the feasibility of reducing the amount of enrichment

allowed during high speed/acceleration conditions. A secondary

goal of the test program is to investigate the emission benefits

and feasibility of insulating catalysts to decrease the rate at

which the catalyst cools off after the vehicle has been shut off. 

This testing is expected to begin around May 15, 1993 and be

completed by the end of July, 1993. 
FTP Preliminary Report: March 27, 1998

36

3.3. Notice of Proposed Rulemaking Development

The Agency will use the analyses described in the preceding

section when determining whether or not the driving cycle or

other aspects of the FTP should be revised to properly represent

vehicle emissions during actual driving conditions. However, in

any proposal to revise the FTP, EPA would also need to consider

various other issues, including:

- Technology assessment

 - Type of revision needed

- Lead time

- Cost and cost effectiveness analyses

The technology assessment includes determining the changes

needed for manufacturers to reduce emissions during the

identified off-cycle condition, the level of reduction achievable

with different technologies and/or calibration strategies, and

the feasibility of making the technology changes. Closely

related issues are cost and lead time, as greater levels of

technology change or added component requirements will increase

the cost of the regulation and, possibly, increase the lead time

needed for manufacturers to implement the changes.

The type of revision to the FTP would also have to be

considered. For example, revisions to the existing cycle would

impact the usefulness of the vast array of historical data and

would require assessments of the impact on Corporate Average Fuel

Economy requirements and on the stringency of the emission

standards. It will likely be much more cost effective, instead,
FTP Preliminary Report: March 27, 1998

37

to establish a new cycle and standard (much as was done for cold

temperature CO emissions, where a new 20 degree cycle was

established with a separate standard). If the emission benefits

prove to be relatively minor, or if it appears that emissions

could be effectively reduced without standards, it might be

desirable to simply promulgate stronger defeat device

requirements. The basic strategy to improve the FTP would have

to be evaluated, as well as the impacts on costs and emission

benefits.

Evaluation of this wide array of control strategies,

technology requirements, standard stringency, costs, and benefits

is a complex task. Due to the time constraints the Agency is

under, several potential new test cycles to control high

speed/acceleration emissions are being investigated as part of

the vehicle testing program described in Section 3.2. The level

of complexity will be significantly impacted by the results of

the study in regards to the level of off-cycle emissions and the

type of driving generating the emissions.
 Radian Corporation,"Evaluation of Driving Pattern Measurement Techniques: 27

Technical Note," Draft Report to U.S. EPA, June 17, 1991, p. 4-14. 

FTP Preliminary Report: March 27, 1998

38

Chapter 4. Driving Survey Methods

The process for selecting driving survey methods and sites

is reviewed in Section 4.1. A discussion of each method and how

it was implemented follows in Sections 4.2. and 4.3. Also

provided are results obtained at each survey site. Section 4.4.

reviews bias issues for the two survey methods. The chapter

concludes with a discussion of the principal data set which

serves as the basis for the analyses in the remainder of this

report.

4.1. Survey Options

4.1.1. Choosing Survey Approach

Prior to collecting data on in-use driving behavior, EPA had

two contractors, Radian Corporation and Sierra Research, review

previous efforts to obtain such information. Contractor reports

evaluated four alternative approaches to gathering driving

patterns data and included recommendations for this study. Table

4-1 presents the principal features of these approaches.27

In exploring survey method options, EPA sought input from

interested parties and researchers working on the subject. To

this end, EPA's Office of Research and Development (ORD)

sponsored a two-day meeting in Atlanta, GA, in June of 1991 to

discuss alternatives for collecting the in-use driving behavior
FTP Preliminary Report: March 27, 1998

39

data. Experts from industry, government, and academia critically

evaluated the approaches presented by EPA's two contractors. As

a result of the meeting, EPA's Office of Mobile Sources (OMS)

concluded that two complementary approaches--chase car and 
Table 4-1.

Alternative In-Use Driving Survey Methods

Instrumented Data Logger on Driving Chase StationaryPrivate Car Private Car Diary Car Observer 

SOURCES OF BIAS Driver does not know he is being monitored - - - + + Representative of vehicle population - - - + + Representative of drivers - - - + + Nonintrusive installation of data logging equipment - - ++ + Representative of all types of trip segments + + +- -TECHNICAL Follows each car all the time - moving or not + + - - - Contains high frequency information + P - + - Measures average speeds + P + + - Measures instantaneous speeds + P - + - Measures accelerations + P - + - Measures number of cold starts + P + - + Measures during cold starts + P - - - Measures number of warm starts + P + - + Measures during warm starts + P - - - Measures distance travelled + P + - - Contains load information + - - - - Permits various data analysis techniques + - + + + Correct observables not needed at beginning of study + - + + + Generates small quantity of data - + - - - Can measure drive train operation + - - - -COST Low procurement cost - - - + + Low installation cost per car - - + + + High capacity storage medium not required - + + - + Data recovery does not require personnel at vehicle - - + + - Does not require data keypunching + + - + -P = Measurable if pre-processor is programmed appropriately
FTP Preliminary Report: March 27, 1998

41
FTP Preliminary Report: March 27, 1998

42

instrumented vehicles--were needed to address issues critical to

the FTP study.

The Atlanta meeting also helped produce an extraordinary

level of cooperation among the participants. The California Air

Resources Board (CARB) and EPA's Office of Mobile Sources (OMS)

joined together to develop the chase car approach. The auto

industry, represented by the American Automobile Manufacturers

Association (AAMA, formerly MVMA) and the Association of

International Automobile Manufacturers (AIAM), committed to

providing technical and financial support for the implementation

of the instrumented vehicle method. And finally, EPA's ORD

agreed to sponsor an instrumented vehicle study in Atlanta, GA.

4.1.2. Selecting Survey Sites

After settling on survey methods, the next step was to

select the cities where the surveys would be conducted. EPA's

budget for the driving surveys necessitated limiting the study to

two sites. EPA gave consideration to several factors in

selecting the survey sites: geographical representation,

population of nonattainment area, and the type of nonattainment

area (CO or ozone). The site selection process was complicated

by special requirements of the two survey methods. The

instrumented vehicle approach required that the survey site have

a centralized Inspection-Maintenance (I/M) program; an up-to-date

urban transportation network model was a requisite of the chase

car method. 
 Radian Corporation, "Light-duty Vehicle Driving Behavior: Private Vehicle 28

Instrumentation, Volume 1: Technical Report," Draft Report to U.S. EPA, August 24,

1992.

FTP Preliminary Report: March 27, 1998

43

EPA's criteria and the survey method requirements quickly

limited the list of candidate cities. Baltimore, MD, was chosen

to represent a Northeast, medium-sized, ozone nonattainment area. 

For the second site, a suitable midwest city was desired, but

could not be found. The best alternative was Spokane, WA. This

Northwest city is fairly small, but owing to its geographical

features, it is typical of a CO nonattainment area. In addition

to these two cities, two other cities had already been selected

for related studies. CARB was sponsoring a chase car study in

Los Angeles, CA, and Atlanta, GA, was the site of an instrumented

vehicle study sponsored by EPA's ORD. Overall, EPA felt these

four cities provided a good representation of urban nonattaiment

areas.

4.2. Instrumented Vehicle Approach

This section discusses the principal features of the

instrumented vehicle approach and presents the field results. 

The reader should note that a draft report prepared by the

contractor, Radian Corporation, provides a much more

comprehensive review of the instrumented vehicle study. 28

4.2.1. Key Features

EPA chose the method of instrumenting privately-owned

vehicles for several principal reasons. First, this approach
FTP Preliminary Report: March 27, 1998

44

collects second-by-second driving behavior data. Real-time

information is crucial for studying the full range of vehicle

operation. The instrumented vehicle method also obtains

important information on the start and finish time of trips,

driving behavior following the start of a trip, and the amount of

time vehicles are shut off between trips (soak time). 

EPA had concerns with certain features inherent to an

instrumented vehicle approach. The cost and time required to

recruit and instrument privately-owned vehicles necessarily

limited the number of vehicles to be instrumented. Equally

important, the driver's knowledge of the instrumentation and the

possible impact it could have on driving behavior was a concern.

The contractor, Radian Corporation, took these concerns into

account in the design of the actual project plan for collecting

instrumented vehicle data. 

The basic features of EPA's instrumented vehicle study can

be summarized as follows. Participants were recruited from

centralized I/M stations. A monetary incentive was offered to

drivers for participating in the study. While the car owner

waited, mechanics installed a small datalogger under the hood of

the vehicle to collect vehicle speed, engine RPM, and manifold

vacuum. The participants were truthfully informed on the nature

of the instrumentation; however, the contractor did not

explicitly discuss the monitoring of vehicle speed. The

dataloggers remained on the vehicle for about one week. At the

end of the week, the driver returned to the I/M station for

removal of the datalogger.
FTP Preliminary Report: March 27, 1998

45

The contractor installed two types of instrumentation

packages: a three-parameter datalogger and a 6-parameter

datalogger. The latter was sponsored by a group of domestic and

foreign auto manufacturers. In fact, the auto industry's

participation was of great value in the overall instrumented

vehicle study. Through the Ad Hoc Panel on FTP, comprised of

AAMA and AIAM members, the industry worked together with EPA in

the development and implementation of the study. Financial

support from participating auto manufacturers significantly

increased the number of vehicles instrumented in Spokane and

Baltimore. 

3-parameter Base Program

Under EPA contract, Radian Corporation custom built 55

compact 3-parameter dataloggers, each capable of recording 54

hours of second-by-second data. Initially, the contractor

manufactured 10 prototype dataloggers for testing. After testing

and making some refinements, the remaining dataloggers were

produced. The dimensions of these 45 datalogger boxes were 5" x

7" x 1.5"; the 10 prototypes were slightly larger. All of the

logger components were tested to 85 degrees Centigrade. The

compact size and durability of the dataloggers allowed mechanics

to install them under the hood of the vehicles, completely out of

the sight of the driver. The dataloggers recorded vehicle speed,

engine RPM, and manifold vacuum each second of vehicle operation. 
FTP Preliminary Report: March 27, 1998

46

In addition, the logger recorded the date and time on a real-time

basis. 

Driver Recruitment

Centralized I/M stations were selected as recruitment sites

in order to obtain the most representative sampling of drivers

and vehicles possible. In Spokane, all gasoline-powered lightduty cars and trucks are required to be tested at the sole I/M

station. Baltimore requires testing of all vehicles less than

20 years old. There are a number on I/M stations in and around

Baltimore. EPA selected two stations for recruitment: a

suburban site (Rossville station) and an urban site (Exeter

station).

EPA's contractor solicited the driver after the vehicle

passed emission testing. A monetary incentive of up to $100 was

offered to drivers to compensate for their time and

inconvenience. If the driver passed an initial screening and

agreed to participate, the contractor mechanics installed the

logger at the I/M station. Typically, the installation of the

datalogger took between 30 minutes and an hour.

In order to try and minimize sampling bias due to refusals,

the recruitment procedure included a driver\vehicle replacement

strategy. The solicitor recorded information on vehicle age,

vehicle type, vehicle's country of origin (foreign or domestic),

and driver age for all solicited drivers. If a driver refused to
FTP Preliminary Report: March 27, 1998

47

participate, a replacement driver\vehicle was found which matched

the four recorded characteristics.

Supplemental 6-parameter Program

The program of installing 6-parameter dataloggers on

manufacturer-sponsored vehicles solicited from I/M stations was

conducted in parallel with the 3-parameter study. The purpose of

this supplemental program was to collect additional vehicle

operating information. Like the 3-parameter datalogger, speed,

RPM, manifold vacuum were recorded, but, in addition, the 6-

parameter dataloggers obtained coolant temperature, throttle

position, and air/fuel ratio collected downstream of the

catalyst. Each participating auto manufacturer developed

customized interfaces for the 6-parameter dataloggers. 

Typically, the loggers were installed in the vehicle's trunk. 

Two major differences existed between the 3-parameter and 6-

parameter program. Only late-model vehicles were eligible for

the 6-parameter program and candidates were further limited to

high-volume models among the participating manufacturers.

4.2.2. Instrumented Vehicle Field Results

EPA's instrumented vehicle study was the largest study of

real-time, in-use driving patterns ever conducted in the U.S. 

Nearly 300 vehicles were instrumented in Baltimore and Spokane.

The dataloggers recorded over 6 million seconds of driving

behavior in the two cities.
FTP Preliminary Report: March 27, 1998

48

EPA's contractor carried out a one week pilot study in

Spokane in January, 1992. The pilot study tested alternative

recruitment strategies and logger installations in the field. 

The success of the pilot study permitted the start of the full

Spokane Instrumented Vehicle Study on February 3. Datalogger

retrieval in Spokane was completed by the first week in March. 

At that time, the contractor began work in Baltimore. The

contractor finished all field work by the first week of April.

In the two cities, a total of 730 drivers were solicited for

participation in the program. 331 of the solicited drivers

agreed to participate, and from that group, 294 vehicles were

successfully instrumented. Table 4-2 gives a summary of the

field study results.
FTP Preliminary Report: March 27, 1998

49

Table 4-2.

Summary of Instrumented Vehicle Field Results

City Datalogger Vehicles Screen Drivers Participa- Complete

Type Solicited Passes Participating tion InstalRate (%) lations

Both 3 and 6 727 571 331 58 294

cities

3 479 374 226 60 215

6 248 197 105 53 79

Spokane 3 and 6 246 222 168 76 144

3 161 144 111 77 102

6 85 78 57 73 42

Baltimore 3 and 6 481 349 163 47 150

3 318 230 115 50 113

6 163 119 48 40 37

The driver participation rate was substantially higher in Spokane

than in Baltimore. About three-quarters of the solicited Spokane

who passed the screening agreed to participate in the driving

patterns study. In contrast, the participation rate in Baltimore

was only 47 percent.

After completing the field work, the contractor began the 

processing of this immense data set. All data were passed

through a rigorous quality control software program to check for

errors in the data. This procedure identified 216 error-free

vehicle data sets and 78 "suspect" vehicle data sets. A

subsequent review of the "suspect" vehicles resulted in the

recovery of a number of vehicles. The final count of vehicles
FTP Preliminary Report: March 27, 1998

50

for which there was good data was 217, including 168 3-parameter

and 59 6-parameter vehicles. 

4.2.3. Atlanta Instrumented Vehicle Study

Atlanta, GA, is the site of an intensive emission inventory

study by EPA's ORD. The objective of the instrumented vehicle

study in Atlanta was to provide important vehicle operation data

for ORD's mobile source emission inventory analysis, while also

serving to supplement OMS's instrumented vehicle database.

Under a cooperative agreement with ORD, the Georgia

Institute of Technology conducted the instrumented vehicle study

in the summer of 1992. The 3-parameter dataloggers were

installed on a fleet of privately-owned vehicles. The Atlanta

study followed the procedures used in Spokane and Baltimore,

whenever possible. The most significant difference in Atlanta

was the method for recruiting drivers. Atlanta's use of a

decentralized I/M program made it impossible to use I/M stations

as a driver recruitment location. However, Atlanta does have a

centralized driver's license renewal system which requires

drivers to renew their licenses in person at one of a limited

number of Driver's License Stations. The recruitment method

utilized three such stations to recruit a representative crosssection of Atlanta drivers.

Georgia Institute of Technology conducted a successful pilot

study in the first week in July of 1992. The full study was
 Austin, Thomas C., Karl H. Hellman, and C. Don Paulsell, "Passenger Car Fuel 29

Economy During Non-Urban Driving," SAE Paper 740592, August, 1974. 

FTP Preliminary Report: March 27, 1998

51

carried out from July through the beginning of October; mechanics

instrumented a total of 101 vehicles. The successful completion

of the study served to add a third city to the instrumented

vehicle database. 

EPA contracted with Radian Corporation to process the

Atlanta data set using the exact procedures employed in the

processing of Baltimore and Spokane data. Due to contract

delays, only initial processing of the data is complete. 

Preliminary data are available for 68 vehicles from Atlanta. 

This limits the usefulness of the Atlanta data for this

preliminary report; however, it is expected that further analyses

will use data from a finalized Atlanta data set.

4.3. Chase Car Approach

The use of chase cars to collect driving patterns data has a

fairly long history. The EPA Highway fuel economy driving cycle

was based on driving patterns data collected by driving a chase

car over 1000 miles of non-urban roadways. For the FTP study, 29

EPA felt the chase car approach could complement the instrumented

vehicle approach in several areas. First, a chase car allows for

the collection of driving behavior data in a non-intrusive

manner. The chase car method also provides a cost-effective way

to collect data for a large sample of vehicles. Finally,

information on the driving environment--road type, congestion

level--can be obtained using chase cars. 
 Sierra Research Inc., "Design and Operation of an Instrumented "Chase Car" 30

for Characterizing the Driving Patterns of Light-Duty Vehicles in Customer Service,"

Draft Report to U.S. EPA, February 28, 1992.

FTP Preliminary Report: March 27, 1998

52

The traditional chase car approach has a major drawback. It

is assumed that the driver of the chase car can simulate typical

driving patterns, either by following specific "target" vehicles

or by "flowing" with traffic. However, EPA's objective was to

not only capture "typical" driving patterns, but also to study

the entire spectrum of drivers and driving patterns, including

low frequency behavior found in the "tails" of the driving

behavior distribution. Clearly a single chase car driver could

not be expected to adequately achieve such a goal. 

4.3.1. Key Features

Under contract with both CARB and EPA, Sierra Research

developed an enhanced chase car equipped with a grill-mounted,

laser rangefinder which can infer the speed of the "target"

vehicle. The laser rangefinder utilizes the same technology 30

found in hand held laser guns currently used by many police

departments to measure vehicle speed. The laser measures the

distance between the "target" vehicle and the chase car (patrol

car). The patrol car itself is instrumented to collect the

vehicle speed; knowing the patrol car's speed and the change in

the distance between the patrol car and target car, the target's

car's speed can be calculated. The laser measures the distance

20 times a second and calculates a one second average. The

laser-enhanced chase car method makes it possible to get an
FTP Preliminary Report: March 27, 1998

53

accurate representation of the driving behavior of the target

vehicle.

The use of the a laser rangefinder is the most prominent

feature of Sierra Research's chase car approach. Other important

characteristics of this approach are in the following discussion. 

Instrumentation

The patrol car was a 1991 Chevrolet Caprice with sufficient

room behind the grill to install the custom-built laser

rangefinder. The car was equipped with a videotape recorder

mounted between the front bucket seats. In addition, the patrol

car speed and manifold pressure were collected once per second. 

A road grade measurement system was developed using two

unidirectional accelerometers mounted on the vehicle floor pan to

collect longitudinal and lateral acceleration. All of the data

were directed to an on-board laptop computer operated by the 

"co-pilot." The co-pilot was responsible for working a handheld

switch box for recording road type, traffic level, and target

vehicle information.

Route Selection

It was imperative that the routes over which the patrol car

would travel and collect information reflect road conditions,

trip conditions, and vehicle/driver variation which are

characteristic of the overall driving patterns for the particular

city being studied. The chase car study employed transportation
 Ibid. 31

FTP Preliminary Report: March 27, 1998

54

planning models for the generation of representative routes. 

These computer-based transportation planning models are used by

urban areas to track travel activity over the road network. 

Information from periodic travel surveys are a principal input

into these models, providing trip information such as trip

origin, destination, purpose, and length. Additional information

on land-use patterns are also incorporated into the model. 

Representative routes are generated using a four-stage process of

trip generation, trip distribution, model choice, and route

choice. The contractor worked with local transportation

authorities to generate 300 routes for each city. 31

Target Selection

A detailed road map was generated for each route. At a predetermined time and start location, the patrol car began the

route and sought its first target vehicle. Target vehicles were

selected at random from candidate vehicles (cars and light

trucks) traveling on the same route as the chase car. If the

target vehicle left the pre-defined route, the chase acquired a

new target vehicle according to prescribed protocol.

Trip ends

Inherent to the chase car approach is an inability to

measure vehicle activity at very beginning and very end of a

trip. Recognizing this, Sierra Research conducted trip ends
 Sierra Research Inc., "Characterization of Driving Patterns and Emissions 32

from Light-Duty Vehicles in California," Draft Final Report to California Air

Resources Board, March 5, 1993.

FTP Preliminary Report: March 27, 1998

55

surveys in Sacramento and Los Angeles. Information on idle 32

time and driving time and behavior prior to accessing the road

network were recorded by observers in the field. The

information can be used in conjunction with the chase car data to

give a more complete profile of driving behavior.

4.3.2. Chase Car Field Results

Sierra Research conducted the first chase car study in

Baltimore during November and December of 1991. After completion

of the data collection effort in Baltimore, the contractor

processed and quality checked the data. Chase car survey work

continued in the spring of 1992 with the Los Angeles study

sponsored by CARB. The Spokane chase car study was carried out

in July of 1992.

Table 4-3 presents a summary of the field results for

Spokane and Baltimore. The chase car study collected data on

over two hundred routes in each city. A large number of targets

were acquired in each city, although the fraction of total route

time with a target was disappointingly low. The bias analysis,

in Section 4.4, discusses the significance of these results.
FTP Preliminary Report: March 27, 1998

56

Table 4-3.

Summary of Chase Car Field Results

Characteristic Both Cities Baltimore Spokane

Number of Routes 467 218 249

Total driving 366,256 191,119 175,137

time (seconds)

Number of targets 1,641 770 871

Total target time 143,668 69,528 74,140

(seconds)

Target time as 39 36 42

percent of total

4.3.3. Los Angeles Chase Car Study

CARB's Los Angeles chase car study ran concurrent with EPA's

studies in Baltimore and Spokane. The first chase car work in

Los Angeles was in conducted in the fall of 1991. Due to data

acquisition problems and revisions to the route selection

methodology, these data were not acceptable to CARB. The

contractor returned to Los Angeles to conduct additional chase

car runs in April and May of 1992. The chase car collected data

for 102 routes, encompassing 28 hours of vehicle operation. The

data collection methods and data processing used in Los Angeles

were entirely consistent with those employed in Baltimore and

Spokane. Section 6.1 presents a limited analysis of the Los

Angeles data.

4.4. Analyses of Potential Bias
 Radian Corporation, "Private Vehicle Instrumentation Survey: Data Bias 33

Analysis Technical Note," Draft Report to U.S. EPA, September 30, 1992.

FTP Preliminary Report: March 27, 1998

57

The objective of both driving survey approaches was to

obtain the most representative characterization of in-use driving

behavior. While the design features of the two survey methods

attempted to explicitly account for all areas of foreseeable

biases, it is the surveys' results which ultimately determine the

success of the two approaches. This section provides a largely

qualitative assessment of the "representativeness" of the chase

car and instrumented vehicle data for Baltimore and Spokane.

4.4.1. Analysis of Instrumented Vehicle Method

EPA's contractor, Radian Corporation, conducted a postsurvey study of several issues pertaining to possible survey

bias. Sampling bias sources included improper representation 33

of vehicle age, make and manufacturer, and performance type. 

Nonsampling sources included accuracy of speed measurement and

the effect on driving behavior of the presence of the datalogger.

Vehicle Selection

The sampling frame in the 3-parameter instrumented vehicle

surveys was the set of vehicles scheduled for inspection during

the month in which the survey was conducted. Radian obtained

lists from Spokane and Baltimore of all vehicles tested during

the survey periods. For both cities these vehicles were

classified by model year (age) and make and compared to the

corresponding distributions found in the sample. Goodness-of-fit
 The selection method was not recorded for 58 vehicle solicitations in 34

Baltimore.

FTP Preliminary Report: March 27, 1998

58

statistics showed no significant difference except in the case of

the Spokane vehicle make variable, in which Mazdas were somewhat

overrepresented.

Vehicle performance type is a rather subjective measure, and

EPA was unable to obtain sufficient information on performance

criteria for all vehicles tested in the two cities during the

survey periods. Therefore, the representativeness of the sample

was judged by comparing the final sample vehicles to the set of

all vehicles that were randomly solicited, which are assumed to

be representative of the target population.

Using power-to-weight values and some subjective judgment

about the "image" of the vehicle, vehicles were classified as

"High-Performance" or otherwise. In Spokane, 3 of 99 (3.0%)

randomly solicited vehicles were classified as High-Performance;

in the final sample, 1 of 75 (1.3%) were High-Performance. For

Baltimore, there were 6 of 86-144 (4.2-7.0%) High-Performance 34

random solicitations, and 4 of 93 (4.3%) High-Performance

vehicles in the final sample. These numbers suggest the

possibility that High-Performance vehicles were underrepresented

in the final sample; EPA has not yet developed a reasonable

adjustment method for this factor. This may be an area for

further study prior to completion of a NPRM. 

Speed Measurement
 Radian Corporation, "Light-Duty Vehicle Driving Behavior: Private Vehicle 35

Instrumentation, Volume 1: Technical Report," Draft Report to U.S. EPA, August 24,

1992. 

 Ibid. 36

FTP Preliminary Report: March 27, 1998

59

Accurate vehicle speed data were critical to the success of

the instrumented vehicle study. The 3-parameter dataloggers used

three different methods for obtaining vehicle speed: magnets on

a drive shaft, OEM speed sensor, or an aftermarket cruise control

attached to the speedometer cable. The acceptable accuracy for

the vehicle speed parameter was +/- .4 mph. 35

A fifth wheel test track study was conducted by the

contractor to test the accuracy of the three speed measurement

methods. The OEM sensor and driveshaft magnet methods showed

very good correlation with the fifth wheel at all tested speeds

(1 to 50 mph). The speedometer cable approach performed well at

speeds greater than 10 mph; however, at low speeds the

speedometer cable approach proved less accurate due to jerky

rotation of the cable. In the field, every attempt was made 36

to keep the speedometer cable straight to minimize the potential

for such erratic low-speed data. Upon quality checking of the

data, EPA does not consider this to be a significant problem.

The vehicle speed resolution for 6-parameter vehicles varied

among manufacturers. The speed data were obtained directly from

the engine's computer (ECU), and were therefore limited by the

resolution required by the vehicle's production specifications. 

Typically, a fine speed resolution is not necessary for a

vehicle's dashboard speedometer. Thus, many of the 6-parameter
 Systems Applications International, "Stratified Comparison and Rounding 37

Effect Analysis of Driving Operation Patterns and Event Characteristics Between

Three-Parameter and Six-Parameter Instrumented Vehicle Data," Draft Report to

American Automobile Manufacturers Association, January 7, 1993.

FTP Preliminary Report: March 27, 1998

60

vehicles only recorded speeds to the nearest 1 mph or less. As a

consequence, the lower precision of the 6-parameter vehicles'

speed is a potential problem, particularly at low speeds. More

importantly, this problem results in low resolution (that is, 1

mph/sec) and an upward bias in the calculated acceleration

rates.37

Datalogger Presence

A driver may alter his or her vehicle operation in response

to the datalogger presence. Reasons for this change in behavior

might include concern that the data could be used as evidence of

illegal or unsafe driving. It is most likely that bias

introduced in this way would tend in the direction of more

conservative driving, that is lower speeds or lower rates of

acceleration.

EPA and Radian conjectured that, if it exists, this type of

bias might be most evident during the initial phase of

instrumentation and would decline as the driver adjusted to the

presence of the datalogger. This motivated an examination of

possible bias based on the "Observation Phase" in which speed and

related values were observed.
FTP Preliminary Report: March 27, 1998

61

For each vehicle in the Baltimore 3-parameter sample, the

speed observations were classified as belonging to the first day

of instrumentation (Observation Phase 1) or later (Observation

Phase 2). Various driving behavior measures were computed for

the two phases. (Section 5.1 contains a discussion of speedrelated driving variables analyzed in this study.) It was found

that during Observation Phase 1, drivers operated at lower

average speeds than in Phase 2, which appears to support the

claim that the datalogger influenced driving behavior in the

initial phase of instrumentation. However, further examination

showed that the time of week also affects average speed:

substantially higher speeds are observed in weekend driving than

on weekdays. (See Section 6.2 for details.) Moreover, due to most

vehicles being instrumented on weekdays, the proportion of

Observation Phase 1 driving on weekends (17.9%) was lower than

for Phase 2 (25.6%). If the average speed during Phase 1 is

adjusted for this difference, it actually is higher than the same

value in Phase 2. Thus, it was decided that driving behavior

probably did not change substantially over the instrumentation

period and that data for both observation phases should be

included in subsequent analyses. Of course, this analysis leaves

unanswered the question of whether instrumented vehicle driving

was more conservative than normal over the full survey period.

4.4.2. Analysis of Chase Car Method

In terms of potential bias, the three areas of concern for

the chase car study are the representativeness of the routes

selected, vehicles, speed measurement, and driving behavior. 
FTP Preliminary Report: March 27, 1998

62

Each of these issues were reviewed following the completion of

data collection.

Route Selection

The method for selecting routes was designed to match the

overall distribution of trips by travel period or trip type, zone

type, and trip length. This method assumes that the

transportation model produces an accurate portrayal of in-use

trip behavior. Accepting this, the subset of the selected routes

which were actually run, closely matches the desired distribution

(For a qualitative comparison see Appendix A, tables A-1 and A2). However, the assumption that the transportation model

produces an accurate portrayal of in-use trip behavior may not be

appropriate, as discussed in Section 4.5.1.

Vehicle Selection

As discussed in Section 4.3.1, the protocol for selecting

target vehicles was designed to obtain a random sampling of

vehicles. In the field this method proved practical; however,

obtaining and maintaining a lock on the target vehicles was only

a limited success. The overall proportion of total route time

with a target locked on was 36 percent in Baltimore and 42

percent in Spokane. The general availability of candidate

vehicles was a significant factor in the low percentage. In

fact, 10 percent of the routes in Baltimore had no target vehicle

data. The corresponding percentage for Spokane was 13.
 Sierra Research Inc., "Chase Car Video Tape Review," Draft technical memos 38

to U.S. EPA, October 9, 1992 and October 29, 1992.

FTP Preliminary Report: March 27, 1998

63

Maintaining a constant laser lock-on for a given target

vehicle also contributed to the lack of target data. A break in

the laser-lock often occurred due to change in grade, on turns,

or lane changes. In order to investigate the lock-on problem,

EPA contracted Sierra Research to analyze the video tapes for

Baltimore and Spokane to identify reasons for losing targets. 38

Table 4-4 presents the results of this analysis.

A second issue related to vehicle selection is the

representativeness of the target vehicle sample relative to the

vehicle population. The video tape review also helps address

this topic. The contractor tabulated the distribution of target

vehicles by vehicle type, model year, and manufacturer, for

Baltimore and Spokane (For a summary of the results see Appendix

A, Table A-3).
FTP Preliminary Report: March 27, 1998

64

Table 4-4.

Reasons for Chase Car Losing Target 

Reason for losing target

Baltimore Spokane

Number Percent Number Percent

Target turned off route 250 12.9 331 18.4

Patrol changed roadways to continue 249 12.8 211 11.7

route

End of route 14 0.7 24 1.3

Lost target over hill or around 684 35.2 653 36.3

corner

Target and patrol car out of 216 11.1 131 7.3

alignment while in same lane

Changed lanes (patrol car) 27 1.4 98 5.5

Lost target while merging with heavy 15 0.8 5 0.3

traffic

Patrol stuck at traffic light 19 1.0 23 1.3

Another vehicle came between 36 1.9 23 1.3

 target and patrol car

Lost target because of aggressive 14 0.7 0 0.0

driving

Suspect target driver realized 7 0.4 4 0.2

they were being followed

Target changed lanes 186 9.6 235 13.1

Turned off switch to read map 21 1.1 22 1.2

Navigator bumped switch 28 1.4 19 1.1

Laser error, sunlight interference 95 4.9 0 0.0

Other reason 81 4.2 19 1.1
FTP Preliminary Report: March 27, 1998

65

Aggressive driver

From the start, EPA was aware of, and concerned about, the

ability of the chase car approach to capture aggressive driving

behavior. There was some small fraction of vehicles which were

operated in such a fashion that it was either impossible or

unsafe for the patrol car to "chase" them. It is not possible to

quantify the incidence of these occurrences. A partial answer

can be found, however, from the review of the target vehicle

videotape. As shown in Table 4-4, less than 1 percent of the

cases in which the target was lost was attributable to aggressive

driving behavior of the target (the classification was based on

the videotape reviewer's best judgment). In addition, some 

fraction of the reason "lost due to lane change" (10 percent in

Baltimore and 13 percent in Spokane), can be attributed to

aggressive driving on the part of the target vehicle.

Speed Measurement

The use of the laser was designed to produce an accurate

measurement of the target vehicle's speed on a second-by-second

basis. The preliminary testing of the laser suggested that the

laser provided an accurate measurement of target speed. However,

in the process of comparing the chase car data and instrumented

vehicle data, EPA identified a problem with the target vehicle

speed resolution which resulted in skewed acceleration data under

certain situations. Post-processing of the target speed data
 Sierra Research Inc. "Characterization of Driving Patterns and Emissions 39

from Light-Duty Vehicles in California," Draft Final Report for California Air

Resources Board, March 5, 1993.

 Specific power is a single measure for the combined effect of speed and 40

acceleration. See Section 5.1.1 for more detailed discussion of this measure.

FTP Preliminary Report: March 27, 1998

66

using a smoothing algorithm greatly improved the "fit" of the

laser-based target data.39

4.5. Selection of Principal Data Set

4.5.1. Comparison of the Two Surveys Results

EPA's contractors conducted the chase car and instrumented

vehicles studies in Baltimore and Spokane within a nine month

period in 1991 and 1992. A variety of constraints prevented

simultaneous data collection in each city. Except for potential

seasonal differences, it is valid to compare the results for the

chase car and instrumented vehicle approach. Such a comparison

cannot validate or invalidate an approach, since the absolute

truth is not known. Rather, the comparison serves to identify

substantial differences and allows for an evaluation of these

differences.

In looking at the results from the two fundamentally

different survey approaches, it was reassuring to see that they

paint like portraits of driving behavior, especially in terms of

differences between Baltimore and Spokane. Both methods indicate

higher average speed in Baltimore than Spokane (see Table 4-5). 

This is also true for average specific power. The surveys also 40
FTP Preliminary Report: March 27, 1998

67

agree in the direction of the differences of trip time and

distances; Baltimore trips averaged about 0.5 miles longer than

Spokane trips and the duration of trips in Baltimore were about

three minutes longer than in Spokane.

While the two surveys agree in terms of differences between

the cities, the actual estimates of various driving behavior

measures are clearly different. The chase car method estimates

much higher average speeds than the instrumented vehicle

approach. This difference is, however, consistent with

expectations, given the methodological differences. The chase

car's lack of trip end information (see Section 4.3.1) is likely

to be the primary reason for the higher average speed. 
FTP Preliminary Report: March 27, 1998

68

Table 4-5.

Comparison of Chase Car and Instrumented Vehicle Driving Behavior

Driving Baltimore Spokane

Behavior

Measures Chase Car Instrumented Chase Car Instrumented

Vehicle Vehicle

Speed (mph)

Average 30.70 24.50 29.80 23.24

Maximum 79.50 94.46 83.20 77.55

Standard 21.00 20.52 19.50 17.71

deviation

Number of 191,119 3,365,504 175,137 2,081,199

observations

Acceleration

(mph/sec.)

Minimum -20.80 -19.49 -11.30 -15.46

Maximum 8.13 15.19 7.79 15.95

Standard 1.39 1.50 1.42 1.46

deviation

Power

(mph /sec) 2

Average 22.2 19.28 20.0 17.01 *

Maximum 577.30 557.69 403.00 672.28

Average trip 7.48 4.89 5.83 3.56

length (miles)

Average trip 14.61 12.03 11.72 9.18

time (minutes)

* The average power values presented in this table use total number of seconds of vehicle

operation as the denominator. This differs from average power values cited in Chapter 6,

which use total seconds of positive acceleration as the denominator. The alternative

definition used here was necessary in order to have consistent average power values for

the chase car and instrumented vehicle data.

The differences in the surveys' estimates of trip time and

distance suggest two distinct perspectives on trip-making
FTP Preliminary Report: March 27, 1998

69

activity. Using engine on/off to define a trip (see Section

5.1.2), the instrumented vehicle approach identified a large

number of short trips. In contrast, the chase car routes are

developed using the transportation network model's concept of a

trip which is based on trip purpose. A given chase car route may

be a result of the implicit connection of two or more engine

on/off events. As an example, a single home-to-work chase car

trip could be viewed as 2 separate trips in the instrumented

vehicle approach--home-to-day care and day care-to-work. The

instrumented vehicle's trip patterns perspective, which captures

the full-range of trip behavior, is most consistent with the

objectives of the FTP study. 

4.5.2. Rationale for Choosing 3-parameter Instrumented

Vehicle data

For this preliminary technical report, EPA feels the 3-

parameter instrumented vehicle data are the best representation

of in-use driving behavior currently available. This data set

provides detailed driving behavior for a large cross-section of

vehicles and drivers. All substantive issues on bias or data

integrity have been satisfactorily addressed for the 3-parameter

data set.

The 6-parameter instrumented vehicle data are a valuable

addition to the overall data set; however, two factors limit

their usefulness for this preliminary report. As discussed in

Section 4.2, speed measurement problems impact the accuracy of

the acceleration-based measures for the 6-parameter data set. In

addition, the vehicle selection process for the 6-parameter
FTP Preliminary Report: March 27, 1998

70

vehicles did not attempt to obtain a representative sample;

weighting factors will need to be developed in working with the

data in the future.

In choosing the 3-parameter instrumented vehicle data over

the chase car data, one factor was just the sheer volume of data

obtained by the instrumented vehicle approach. In Baltimore and

Spokane, 3-parameter instrumented vehicles collected over 1500

hours of driving behavior data compared to 100 hours of chase car

data. EPA also considered the surveys' ability to provide data

on a representative cross-section of vehicles and driving. While

we are largely satisfied with the results of the instrumented

vehicle bias analysis, the results of the chase car surveys are

less reassuring. The majority of data from the chase car is for

only one vehicle--the patrol vehicle. The target vehicle data

does capture a large number of vehicles, but typically, the data

are obtained for a very small amount of time. Finally, the

routes (or trips) driven in the chase car approach are

considerably longer than typical trips from the instrumented

vehicle data, casting doubt on whether the chase car routes

properly reflect the range of in-use driving conditions.
FTP Preliminary Report: March 27, 1998

71

Chapter 5. Analytical Methods and Considerations

This chapter describes driving behavior variables and

methods for their analysis based on the survey data discussed in

Chapter 4. Analytical methods consist primarily of traditional

descriptive techniques: arithmetic summary measures, variable

frequency distributions, and supporting graphs. Formal

statistical inference is omitted, in part due to large sample

features of the survey. Nevertheless, choices and emphases in

describing data can easily color the reader's impression of what

those data say about the larger population of drivers and their

vehicle operation. The final section reviews a study of engine

and catalyst temperature cooldown undertaken as part of the

project.

5.1. Driving Behavior Variables

Despite longstanding recognition of the importance of mobile

sources to air quality, surprisingly little research has been

done on driving behavior. This report gives a preliminary, broad

description of the current survey results while concentrating on

those issues that pertain most to vehicle emissions and test

drive cycle policy. The data collected for this report offer a

rich source for future study of driving behavior. EPA

anticipates that researchers in different fields, with other

perspectives, skills, and computing resources, will contribute

new understanding of the information contained in these data.

5.1.1. Speed-Based Measures of Driving Behavior
FTP Preliminary Report: March 27, 1998

72

In planning the survey, EPA attempted to collect basic

measures known or believed to explain vehicle emissions. As

noted earlier, the most fundamental variable, vehicle speed, was

recorded for each second that a vehicle's engine was running. 

These "time series" data have been used to derive two other key

measures: acceleration and specific power.

Acceleration

Acceleration, the change in speed per unit time, is computed

as the simple difference in two successive speed values. This

calculation results in a missing value for acceleration at the

start of each (engine on) trip. By its very nature, the

distribution of acceleration is centered at zero and tends to be

symmetric and unimodal. Thus, the mean and median are not useful

for comparing two or more distributions. Average measures of

dispersion, such as the standard deviation, help describe how a

set of accelerations vary. Percentiles are useful for

summarizing extreme levels of acceleration and deceleration.

The problem with analyzing the acceleration rate is that, by

itself, it has dubious significance. For a given acceleration

rate, the load placed on the engine increases with the vehicle

speed. Thus, the most appropriate measure is the joint

distribution of speed and acceleration. However, this requires

three dimensions to display, making it harder to visualize. As a

useful compromise, the joint distribution of speed and

acceleration can be approximated with a measure of "power" to

allow two-dimensional analysis. This is described below.
 Watson, H.C., E.E.Milkins, M.O. Preston, C. Chittleborough and B. 41

Alimoradian, "Predicting Fuel Consumption and Emissions-Transferring Chassis

Dynamometer Results to Real Driving Conditions," SAE Technical Paper Series, 830435,

1983.

FTP Preliminary Report: March 27, 1998

73

Specific Power

Specific power is defined as the per second change in the

square of vehicle velocity during positive accelerations:

specific power = V - V , V > V f i f i

2 2

where V and V are the initial and final velocity, respectively, i f

in a one-second interval. Interest in this variable is motivated

by the work of Watson, et. al., who introduce a measure called 41

"positive acceleration kinetic energy change per unit distance,"

or PKE. In their work, it is presumed that fuel economy and

emissions are proportional to PKE when measured jointly over a

given trip. Thus, PKE is a cumulative measure of increases in

kinetic energy over a fixed travel distance.

As used in this study, specific power is determined for each

second of driving, unadjusted for distance traveled. It is

closely related to per second speed and acceleration by simple

multiplication:

specific power . 2 x speed x acceleration 

(for acceleration greater than zero). Preliminary work by EPA

suggests that, for explaining second-by-second emissions,
FTP Preliminary Report: March 27, 1998

74

specific power performs better than either speed or acceleration

alone (although the joint distribution of speed and acceleration

is best). In conclusion, specific power appears to give a useful

univariate composite measure of the more intuitive speed and

acceleration variables.

5.1.2. Trip-based Measures

Trip-based analyses provide an alternative, summarized

perspective of driving behavior. The concept of what constitutes

a trip appears intuitively simple; however, a number of practical

issues require some discussion. This section reviews features of

the instrumented vehicle trip definition and describes the main

concepts used in the trip-based analyses.

Trip Definition

The instrumented vehicle survey used engine on-engine off to

define the beginning and end of a trip. In reviewing the initial

trip data it became evident that this definition resulted in

including vehicle stalls as separate trips. To exclude stalls,

EPA directed the contractor to make a slight modification to the

original definition. Using the time between trips, or soak time,

apparent stalls were connected to the appropriate contiguous

trip. As an example, a vehicle sits in the driveway all night

and in the morning the car is started, but stalls after 5 seconds

because it is cold. It then takes 15 seconds to start the car

again and the driver leaves on a 10 minute trip. Using just

engine on-engine off as the trip definition criteria would result
FTP Preliminary Report: March 27, 1998

75

in having the stall treated as a separate, 5-second trip. 

However, if one uses the criteria that if the time between trips

is less than a certain number of seconds, then the "stall trip"

would be combined with the following "real" trip. Using 18

seconds as the soak time threshold, the 5 second "stall trip"

would be connected to the following 10 minute trip and the 15

second soak in-between would be added to make the total trip time

equal to 10 minutes and 20 seconds.

The formal trip definition is:

if Soak < t 18 then,

New trip = Trip + Soak + Trip t-1 t t

The selection of 18 seconds as a soak time threshold is based on

the distribution of in-use soak times and it is consistent with

similar analyses contracted by AAMA and AIAM.

An additional modification to the trip data was made to

insure a representative sample of trips. As discussed in Section

4.2, the vehicles in the survey were recruited and instrumented

at centralized I/M stations. After installation and testing of

the datalogger, the datalogger was placed in a "run" to begin

collecting in-use driving data. However, on many occasions the

vehicle was driven by the mechanic to the front of the I/M

station and parked. At such time the vehicle was shut off and

the keys were turned over to the customer. This trip was

recorded as the first trip for the vehicle. A similar scenario
FTP Preliminary Report: March 27, 1998

76

could occur when the vehicle was returned. EPA felt such trips

were not representative of in-use operation and should be

deleted. To accomplish this task, I/M station trips at the

beginning and end of a vehicle's trip file were identified and

deleted using a distance threshold of 0.2 mile (1,056 feet). 

This resulted in an elimination of 55 trips in Spokane and 67

trips in Baltimore.

Vehicle Operating Mode Definitions

A common measure in trip-based analyses is the proportion of

time spent in the four vehicle operating modes: cruise,

acceleration, deceleration, and idle. In defining the modes, EPA

started with a definition for cruise. Driving at a constant

speed is what is typically thought of as a cruise. However, a

vehicle's speed is rarely constant. For this report, the cruise

mode is defined as a driving period at least three seconds long

during which all acceleration and deceleration are of an average

magnitude of 0.5 mph per second or less over two second

intervals, with speeds measured at one second intervals. The

definitions for acceleration, deceleration, and idle modes fall

out of this cruise definition. Computationally, the four modes

are defined as follows:

1. Find the average 2-second acceleration:

A = (S - S )/2 t t+1 t-1

where, S = vehicle speed at time t t

2. Cruise: If A , A , and A are less than or equal to t-1 t t+1

0.5 mph/sec. in absolute value, and S is t
FTP Preliminary Report: March 27, 1998

77

greater than 1, then values for time t-1, t,

and t+1 are classified as Cruise. 

Acceleration: If A > 0.5 and S > 1, then values for time t t

t is an acceleration.

Deceleration: If A < -0.5 and S > 1, then values for time t t

t is an deceleration.

Idle: If the value for time t does not meet the

conditions for cruise, acceleration, or

deceleration it is classified as idle. This

definition of idle includes some non-zero

speed driving and, thus, differs slightly

from the idle definition used in the

speed/acceleration sections of this report.

Stop Mode Definition

The number of stops per trip or the distance between stops

is another commonly used trip measure. A stop at a traffic light

or stop sign typically involves the vehicle' speed going to zero,

but this is not always true, as in the case of "rolling stops." 

For this report, the stop mode is defined as beginning when the

vehicle slows from above 10 mph to below 4 mph. The stop

concludes when the vehicle's speed exceeds 4 mph, conditional on

the speed exceeding 10 mph without falling below 4 mph. This

definition allows for a "creep" mode within the stop mode. 

5.1.3. Vehicle-Based Measures

An alternative measurement unit for describing driving

behavior is the vehicle that generates the speed and trip

patterns discussed previously. Vehicle-based measures are needed
FTP Preliminary Report: March 27, 1998

78

to judge the representativeness of the vehicle sample and for

analyzing various vehicle factors that may influence driving

behavior. Because this view of the data is less important to

emission test cycle development than the speed- and trip-based

measures, its discussion in this report is comparatively limited.

The final Baltimore and Spokane 3-parameter samples

contained a total of 168 vehicles, by no means an extremely large

sample. In the analyses presented in Chapter 6. The following

variables measured for each vehicle are reviewed:

* Miles driven per day

* Number of trips per day

* Number of stops per hour of operation

* Number of minutes of operation per day

These and other vehicle-based variables are derived from the

speed-time data or from trip-based variables constructed from

those data.

5.2. Descriptive Methods

Summary statistics developed from the raw data are intended

to paint a basic picture of in-use driving and to distill the

large mass of detail into forms that are readily managed in

searching for important and/or unexpected patterns. Standard

arithmetic measures include a variable's mean, standard

deviation, minimum, maximum, and count. Distributions of

frequency, both percentage and cumulative percentage, furnish

additional detail, along with the joint distribution of speed and

acceleration. These also provide the basis for approximating
FTP Preliminary Report: March 27, 1998

79

distribution percentiles, such as the median. Two and three

dimensional graphs of the distributions give a visual image of

variation patterns.

For reasons discussed in Section 5.4, exact percentile

calculations of speed-related data are not part of the standard

measures generated for this report. These values can be

approximated by interpolation from the frequency distribution of

a given variable. The median, a widely used measure of central

tendency, equals the 50th percentile of a distribution. The

median usually is preferred to the mean (average) when describing

data that are distributed highly asymmetrically, or skewed. 

Unlike the mean, it is unaffected by the tail values and outliers

of the distribution and, thus, is more representative of the

central mass of data. Of the three speed-related variables,

specific power exhibits large positive skewness; speed data

typically display more modest positive skewness; and acceleration

data show little or no skewness. Therefore, in the analyses of

Chapter 6, both mean and median often are reported for the

specific power and speed variables. The distributions of several

trip variables, such as trip time and distance, also tend to be

skewed.

One of the principal goals of this study is to identify the

presence of "aggressive" driving behavior patterns that may

account for disproportionately high levels of vehicle emissions. 

While there is no commonly accepted measure of driving

aggressiveness, it is generally accepted that this condition

corresponds to high speed and/or acceleration levels. It follows
FTP Preliminary Report: March 27, 1998

80

that high levels of specific power also characterize aggressive

driving. For all three variables, then, the upper percentiles,

or "tails," of the frequency distribution provide a useful

measure of this important driving feature. 

"Tail" percentages corresponding to selected values of the

measured variable are used to describe and compare driving

aggressiveness patterns. For example, it was found that in the

3-parameter Baltimore sample, 2.61 percent of driving time is

spent at speeds greater than 65 miles per hour. This is

equivalent to saying that the 97.39th percentile of the speed

distribution is 65 mph.

Note that percentiles associated with the distribution of

specific power are based only on those data for which that

variable is defined: during positive acceleration. This

constitutes less than half of all driving time. In discussions

of this variable, percentiles sometimes are also re-expressed in

terms of the total of all driving time.

5.3. Statistical Accuracy

In describing and comparing results, it is important to

recognize the basic unit that generates a particular measure. 

The "finest" unit of interest is the one-second period of time

during which a vehicle's speed and other characteristics were

recorded. These data form the basis for other variables

aggregated by individual trip (engine-on to engine-off) and by

vehicle.
FTP Preliminary Report: March 27, 1998

81

Of course, a large volume of second-by-second data were

obtained from the Spokane and Baltimore instrumented vehicle

study: over 5.4 million observations for the 3-parameter vehicles

alone. Such a large sample virtually guarantees that observed

values give very accurate estimates of the true population

measures that they represent. Questions about sampling error

arise from the vehicle and trip sample sizes. For the Baltimore

and Spokane 3-parameter vehicles, some 8,459 trip observations

were made on 168 vehicles. Other potential problems involve

nonsampling sources: the representativeness of the vehicle sample

and the manner in which the vehicles were driven during

instrumentation. These issues were addressed earlier in the

discussion of possible bias sources.

In this report, primary attention is given to per-second and

per-trip driving behavior measures because these bear directly on

emission test cycle development. Driving measures based on the

vehicle unit are reviewed cursorily.

5.4. Computer Resource Issues

Analysis of the instrumented vehicle speed data presents a

substantial challenge to the management of computer resources. 

Routine statistical computations on the large data set collected

in the study call for efficient, high-speed processing. At the

same time, the unique nature of the problems and their analyses

requires considerable application of non-routine exploratory

techniques, which tend to be relatively inefficient.
FTP Preliminary Report: March 27, 1998

82

EPA approached these issues by combining private contractor

and Agency resources in order to draw on the strengths of both. 

At the contractor level, processing of the raw data on mainframe

computers reduced the data to the level of "sufficient"

statistics: means, standard deviations, frequency distributions,

and other potentially useful descriptive measures. These outputs

were then studied in more ad hoc fashion by Agency staff working

on personal computers.

Resolving these resource concerns necessarily involved

certain compromises in terms of analytical methodology. For

example, exact calculation of some common descriptive statistics,

such as the median and other quantiles (percentiles), is very

computer-intensive. In most cases, these are excluded from the

current analysis or are approximated from other summary results. 

In general, stratification of speed, acceleration, and specific

power across two or more factors also are excluded.

5.5. Driving Conditions

The speed, trip, and vehicle-based measures discussed

previously describe driving behavior over which the vehicle

operator exercises more or less conscious control. The typical

driver is less conscious of two other factors known to influence

emissions: engine and catalyst temperature and road grade. As

part of this study, EPA investigated the temperature issue in

some depth. Road grade requires additional effort.

5.5.1. Engine and Catalyst Cooldown
FTP Preliminary Report: March 27, 1998

83

In order to predict the effect of engine and catalyst

temperature on emissions, it is necessary to estimate the

proportion of time these vehicle systems spend at different

temperature levels. Those levels are essentially constant after

the vehicle is driven for a certain period. However, at engine

start-up, engine and catalyst temperatures vary as a function of

several factors, including the length of the preceding soak.

Starting temperatures decrease with soak time. Knowledge of

the rate of these temperature drops, combined with the

distribution of soak times, enables estimation of the

distribution of start-up temperatures in-use. When this is

further combined with information on emission-temperature

relations, a picture of overall emissions during start-up

emerges.

New Research

This section describes research into cooldown rates

described above. In August, 1991, EPA entered into a cooperative

agreement with the New York State Bureau of Air Research to

obtain test data and analysis of the engine and catalyst cooldown

processes and factors affecting the cooldown rates. Researchers

at the Bureau's Albany Emissions Laboratory (AEL) performed a

series of forty tests on six different vehicles. In each test,

the vehicle was driven under actual road conditions until the
FTP Preliminary Report: March 27, 1998

84

engine and catalyst temperature were stabilized, typically for a

period of about thirty minutes. The engine was then turned off

to begin a soak, which varied from 2 to 10 hours. Throughout

each test, component temperatures and other variables were

recorded at intervals of fifteen seconds.

Figures 5-1 and 5-2 display measured temperature for engine

coolant and catalyst, respectively, for one such test. After the

engine has been turned off the engine coolant continues to rise

for several minutes before beginning its decline toward ambient;

catalyst temperatures begin the descent immediately.

The goal of this study was to construct functions of engine

coolant and catalyst temperature versus soak time. AEL

researchers attempted to account for several vehicle and

environmental factors expected to influence these functions,

including engine size, ambient air temperature, wind speed, and

pavement condition (wet or dry). In a quasi-experimental design,

these factors were varied in an effort to fit a response surface
FTP Preliminary Report: March 27, 1998

85

Figure 5-1

Plymouth Acclaim Engine Cooldown Curve

over a reasonable range of operating conditions.
FTP Preliminary Report: March 27, 1998

86

Figure 5-2

Plymouth Acclaim Catalyst Cooldown Curve
FTP Preliminary Report: March 27, 1998

87

Results

In a series of progress reports, AEL researchers adopted a

two-stage approach to the problem of temperature prediction. The

first step was to fit component temperature as a nonlinear

function of soak time for each of the tests. This produced a set

of equation coefficients that varied across tests. In the second

step, these coefficients were regressed against the vehicle and

environmental variables to enable prediction of the coefficients

for any prespecified values of those variables.

Proposed forms for the engine coolant and catalyst

temperature-time functions were based on known physical

properties. Both are exponential functions of the form:

Temperature = K*exp{ß*soaktime} + C

where K, ß, and C are constants for a given set of vehicle and

environmental factors. In general, ß must be negative in order

for temperature to decrease as soaktime increases. Using linear

regression, the relations between the fitted coefficients and the

vehicle and environmental variables were estimated in equations

of the form:

Coefficient = b + b *(engine size) + b *(ambient temp) 0 1 2

 + b *(wind speed) + b *(pavement condition) 3 4
FTP Preliminary Report: March 27, 1998

88

In validating the AEL coolant analysis, EPA applied this

method to the 6-parameter data from the Baltimore and Spokane

studies. The 6-parameter vehicles were instrumented to record

engine coolant temperature (but not catalyst temperature). A

data file was created with values for soak time and for coolant

and ambient temperature at start and end of trip. While these

data were generated in uncontrolled experimental conditions, they

produced temperature-time equation fits similar to those of the

AEL experiments.

In adapting the AEL results for application to the survey

driving data, EPA considered several modifications of the AEL

equations to better conform with the requirements of the current

study. For the engine coolant formulas, this review failed to

produce an appreciable improvement and the AEL equations were

left intact for subsequent analyses. For several reasons, the

catalyst equations were altered to some degree.

It was learned that one of the test vehicles, an Oldsmobile

Cutlass, was equipped with a pelletized catalyst, an obsolete

technology. It was therefore decided to delete from

consideration the 10 tests performed on this vehicle. In

addition, catalyst cooldown for the single truck, a Ford Ranger

used for 11 tests, behaved very differently than in the cars. 

These tests were analyzed separately.

The final equations are used in Chapter 6 to help assess the

impact on vehicle emissions of cold start driving behavior. They

have the following form (t equals soak time):
FTP Preliminary Report: March 27, 1998

89

 Engine

 Coolant = k*exp{ß*|t-J|} + C

Temperature

where

k = - 0.6221*(Average Ambient Temp)

- 0.0237*(Pavement)

- 0.0058*(Number of Cylinders) + 0.9695

ß = - 0.0179*(Average Wind Speed)

+ 0.0394*(Number of Cylinders) - 0.7058

J = 0.1097

C = 0.9660*(Average Ambient Temp) + 0.0582
 U.S. Environmental Protection Agency. "Passenger Car Fuel Economy: EPA and 42

Road." Report No. EPA 460/3-80-010, September 1980, p.119.

FTP Preliminary Report: March 27, 1998

90

 Catalyst

Temperature = k *exp{-4*t} + k *exp{ß*t} + C 1 2

 where

ß = 1.6269*(Average Ambient Temp)

- 0.1588*(Average Wind Speed)

+ 0.2486*(Number of Cylinders) - 3.0453

k = 0.5224 1

k = 1.2482 2

5.5.2. Road Grade

As mentioned previously, the road gradient data collected

through the chase car study may be of questionable value due to

potential inaccuracies introduced by a variety of sources. 

Consequently, the Agency is currently not in a position to

present data with respect to the distribution of road gradients

in Baltimore. A 1980 EPA report to Congress summarized some U.S.

Department of Transportation (DOT) data on the nationwide

distribution of road gradient by the percent of vehicle-miles

traveled (VMT) (See Table 5-1). From these data it is also 42

possible to calculate a nationwide VMT-weighted average for road

gradient of 1.6 percent. Even though over time the mileage up

will equal the mileage down, the response of vehicle emissions to

road gradient is nonlinear and the increased emissions due to an

increased load on an upgrade will not be compensated for by the

decreased load experienced on an equal downgrade.
FTP Preliminary Report: March 27, 1998

91

Table 5-1

Road Gradient Distribution

by Percentage of Vehicle-Miles Traveled

(Miles Traveled Up = Miles Traveled Down)

Road Gradient Percent of

(%) VMT

< 0.5 35

0.5 - 1 20

1 - 2 15

2 - 3 10

3 - 4 8

4 - 5 6

5 - 6 4

> 6 2
FTP Preliminary Report: March 27, 1998

92

Chapter 6. Discussion and Analysis of Driving Survey

Results

This chapter provides a brief summary of the results

obtained from driving surveys in all four cities: Baltimore,

Spokane, Atlanta, and Los Angeles. A detailed discussion follows

of the Baltimore survey results; EPA feels that Baltimore results

are the most representative at this point. The chapter concludes

with comparisons between the FTP and the Baltimore results.

 

6.1. Summary of In-Use Driving Results for Four Cities 

The four cities for which driving patterns data were

collected serve as the basis for EPA's study of urban, in-use

driving behavior. In looking at the results, it is important to

keep in mind that differences among cities are attributable to

not only driver behavior differences, but also to transportation

network differences, in terms of both the road network

configuration and usage. The road network can be thought of as

the driving environment faced by each driver and, as such, it can

have a significant impact on driving behavior.

6.1.1. Driving Behavior - Second-by-Second Analysis

Table 6-1 provides summary measures of speed, acceleration,

power, and several trip measures for each city. As discussed in

Section 4.3.3, the chase car data for Los Angeles are not

directly comparable with the data for the other cities; however,

certain qualitative evaluations are still valid across cities.
FTP Preliminary Report: March 27, 1998

93
FTP Preliminary Report: March 27, 1998

94

Table 6-1

Comparison of Driving Behavior for Four Cities

Driving Baltimore Spokane Atlanta Los

Behavior Angeles

Measure Both Exeter Rossville

sites

Speed (mph)

Average 24.50 20.90 28.11 23.24 28.80 28.35

Maximum 94.46 83.72 94.46 77.55 96.48 80.30

Standard 20.52 18.47 21.80 17.71 22.60 20.15

deviation

Number of 3,365,504 1,686,890 1,678,614 2,081,199 3,010,672 99,729 

seconds

Acceleration

(mph/sec.)

Minimum -19.49 -13.31 -19.49 -15.46 -18.62 -15.00

Maximum 15.19 15.19 14.65 15.95 16.69 10.41

Standard 1.50 1.54 1.46 1.46 1.53 1.74

deviation

Number of 3,360,550 1,684,228 1,676,322 2,077,008 3,006,675 99,625 

seconds

Power

(mph /sec.) 2

Average 46.02 45.38 46.62 40.14 51.72 58.97

Maximum 557.69 557.69 479.82 672.28 723.12 769.10

Standard 42.96 41.51 44.29 40.82 48.11 49.11

deviation

Number of 1,407,908 686,347 721,561 880,258 1,318,072 45,251

seconds

Average trip 4.89 3.99 5.89 3.56 6.04 7.78

length

(miles)

Average trip 12.03 11.55 12.56 9.18 12.59 16.45

time

(minutes)
FTP Preliminary Report: March 27, 1998

95

Average 0.87 0.64 1.13 0.81 1.09 1.26

distance b/w

stops

Percent idle 21.12 23.91 18.32 17.91 17.38 11.78

operation

 Speed

Atlanta had the highest average speed among the four cities,

followed by Los Angeles, Baltimore, and Spokane. A look at the

frequency distribution of speed affords a more interesting

picture of driving behavior. Figure 6-1 compares the

distribution of speeds for the four cities. The cities share a

characteristic tri-modal distribution. The first mode, at idle, 

is similar for three of the cities: 17 percent for Atlanta, 18

percent for Spokane, and 21 percent for Baltimore. The low

percentage of idle in Los Angeles, at 12 percent, is likely due

to the chase car survey methodology which excludes trip ends

information (see Section 4.3.1).

In Spokane, a second mode is very obvious for speeds between

30 and 35 mph. For Los Angeles and Baltimore this mid-speed mode

is less significant; it is actually shifted out to 35 to 45 mph

speed range for Atlanta. A third mode at 55 to 60 mph is

distinct for Baltimore and Spokane; Los Angeles and Atlanta also

have a slight mode at 60 to 65 mph. In both cases, this third

mode is indicative of freeway operation.

Acceleration
FTP Preliminary Report: March 27, 1998

96

As presented in Table 6-1, there are not large differences

in the summary acceleration measures for the four cities. In

addition, the analytical importance of acceleration rates is

limited (see discussion in Section 5.1.1.).

Specific Power

The power term serves to combine speed and acceleration into

a single variable, as discussed in Section 5.1.1. The average

specific power was highest for Los Angeles (58.97 mph /sec) and 2


FTP Preliminary Report: March 27, 1998

99

Atlanta (51.72 mph /sec), and lowest in Spokane (40.14 mph /sec). 2 2

Baltimore's average power, 46.02 mph /sec, placed it in the 2

middle.

Figure 6-2 presents the frequency distribution for specific

power for the four cities. Since power is calculated only for

positive values of acceleration (see Section 5.1.1), the power

distributions are based on total number of seconds of positive

acceleration (1,407,908 seconds out of 3,365,504 seconds of total

vehicle operating time, or 42 percent). Differences in the

frequency distribution of the power value across cities gives an

indication of the varying "intensity" of driving in the four

cities. For example, 63 percent of driving time in Spokane

occurred with power values less than or equal to 40--such power

values correspond to low speed and near-cruise driving. In

contrast, only 44 percent of power values in Los Angeles were 40

or less. The corresponding percentages for Atlanta and Baltimore

were 52 and 57 percent, respectively.

The upper tail of the power distribution is of particular

interest, as it may be the best single measure of aggressive

driving. The percentage of specific power values greater than

200 is highest for Los Angeles (1.5 %); Atlanta, with 1.25

percent, was next, followed by Baltimore (0.77%) and Spokane

(0.57%).

6.1.2. Driving Behavior - Trip Analysis
FTP Preliminary Report: March 27, 1998

100

Speed, acceleration, and power measure driving behavior on a

second-by-second basis. Alternatively, driving behavior can be

examined over a more extended period of time. Trip-based

measures summarize driving behavior from the time the vehicle is

started until it is turned off-again.

Overall, trip measures for the four cities show an ordering

of the cities similar to that found for the speed and

acceleration measures (these are also in Table 6-1). The average

trip length in Spokane was 3.56 miles while Baltimore's average

trip distance was 4.89 miles and Atlanta's was 6.04 miles. Not

surprisingly, trip durations reiterate this pattern with trips

averaging about 9 minutes, 12 minutes and 13 minutes in Spokane,

Baltimore, and Atlanta, respectively. The Los Angeles trip

measures were considerably longer than the other cities. The

average L.A. trip covered 7.78 miles in about 16 minutes. It is

likely that these long trips can be attributed in large measure

to methodological differences between the chase car and

instrumented vehicles approaches (see Section 4.5.1).

6.1.3. Choosing Representative Survey Data

The analysis of in-use driving behavior requires a more

detailed examination than provided by the summary statistics

described previously. For this preliminary report, the 3-

parameter instrumented vehicle data from Baltimore and Spokane

are the only practical candidates for such analysis, as discussed

in Section 4.5.2. However, the summary statistics presented in

the previous section suggest a certain uniqueness to Spokane
FTP Preliminary Report: March 27, 1998

101

driving patterns relative to the other three cities. This calls

into question Spokane's appropriateness for representing other

ozone nonattainment areas. The discussion following outlines

features of Baltimore's instrumented vehicle data which make it

the best candidate for representing ozone nonattainment areas in

this preliminary technical report on in-use driving behavior.

As discussed in Section 4.2.1, the Baltimore data can be

categorized according to the I/M stations from which the vehicles

were recruited. The Exeter station is located in the central

part of Baltimore. In contrast, the Rossville station is in a

suburban location. A comparison of the driving behavior from the

two sites gives a view of urban/suburban driving differences. 

The assumption is made that vehicles recruited in a suburban site

will have a tendency to be driven primarily in an suburban

environment, likewise for the urban-station vehicles.

The average speed for Exeter (urban) was much lower than

Rossville (suburban), 20.90 mph and 28.11 mph, respectively. The

average trip length was 3.99 miles for Exeter compared to 5.89

for Rossville. These differences are consistent with

expectations of suburban and urban driving patterns; higher

speeds and longer trips are presumed for a suburban road network

with a large percentage of freeways and extensive land-use

patterns.

After taking this disaggregated view of Baltimore, driving

patterns for the Exeter station turn out to be similar to those

found in Spokane, and the Rossville station's driving behavior is
FTP Preliminary Report: March 27, 1998

102

very consistent with that found in Atlanta. Figure 6-3 compares

the speed distribution of vehicles from the Baltimore-Exeter

station to Spokane and Figure 6-4 compares Baltimore-Rossville

with Atlanta. Furthermore, the average trip length for

Baltimore-Exeter is similar to that in Spokane, 3.99 miles vs.

3.56 miles. Baltimore-Rossville and Atlanta also have very close

mean trip lengths of 5.89 miles and 6.04 miles, respectively. 

Thus, it appears that Spokane driving most closely represents

driving patterns found in a urban, central city location, while

failing to capture the suburban driving patterns common in many

of the major metropolitan areas.

FTP Preliminary Report: March 27, 1998

104

The driving behavior differences described for Spokane and

the other cities have many potential explanations. One

compelling explanation is the differences in the configuration

and utilization of the road network. For example, the Spokane

road network is much smaller than Baltimore's. Spokane has only

one east-west freeway and a handful of principal arterials. In

contrast, Baltimore has an extensive and diverse road-network

system. Data from the chase car study can be used to gather some

insight into the differences in the road-network systems of

Spokane and Baltimore. Table 6-2 shows the fraction of chase car

operation time by road type. The Baltimore chase car data

contain a much larger fraction of freeway operation than the

Spokane data, 18.9 percent vs. 11.2 percent. Almost 80 percent

of Spokane driving took place on arterials and collectors

compared to 64 percent of total driving time in Baltimore.
FTP Preliminary Report: March 27, 1998

105

Table 6-2

Comparison of Road Type Distribution for Baltimore and Spokane

Road Type Baltimore Spokane

Number of Percent of Number of Percent of

 seconds total driving seconds total driving

Private 2,407 1.26 1,571 0.90

Local 22,932 12.00 12,678 7.24

Arterial/ 122,156 63.92 138,249 78.94

Collector

Freeway 36,113 18.90 19,592 11.19

Ramp 7,382 3.86 3,047 1.74

Carpool lane 3 0.00 0 0

Other 126 0.07 0 0

Total 191,119 100.00 175,137 100.00

Based on the these factors, EPA feels that it is

inappropriate to use a combined Spokane and Baltimore data set to

represent urban in-use driving behavior. A more accurate

representation can be made analyzing Baltimore separately. 

(Adding Atlanta to the Spokane and Baltimore data set might be

even better, but as the Atlanta data are not yet available for

detailed analyses, this is currently not a viable option.) The

similarity between Spokane and the Baltimore-Exeter station

ensure that Spokane-type driving is included, and the BaltimoreRossville data helps to capture the type of driving which is more

consistent with that seen in Atlanta and Los Angeles. Thus, the

remainder of the driving behavior analysis will focus on just

Baltimore driving patterns. (One exception is the trip start

activity section; all instrumented vehicle data are used.) We
FTP Preliminary Report: March 27, 1998

106

expect future analyses of the instrumented vehicle data will

include Spokane and Atlanta. 

6.2. Analysis of Baltimore Driving Conditions

The comparison of driving across four cities gave several

descriptive measures of Baltimore driving behavior. This section

provides additional detail on overall patterns found in the 3-

parameter instrumented vehicles sampled in that city, along with

comparisons of driving over the levels of several factors likely

to influence behavior. The 93 vehicles and some of their

characteristics are listed in Appendix B.

6.2.1. Speed-based Measures of Driving Behavior

The speed, acceleration, and specific power variables form

the basis for analyzing second-to-second driving behavior.

Methods of analyzing data for these variables were discussed in

Chapter Five. It is useful to consider these variables in

absolute terms and in comparisons across the levels of driving

determinants that influence driving behavior.

Overall Driving Behavior

In discussing overall speed-based descriptive measures for

Baltimore, two data sets are useful: the complete three-parameter

second-by-second sample, and the subset of these data for nonzero speeds corresponding to that portion of operation when the
FTP Preliminary Report: March 27, 1998

107

vehicle is moving. In Appendix C, Tables C-1 to C-4 and Figures

C-1 to C-3 provide summary statistics, distributions, and graphs

for speed, acceleration and specific power of the Baltimore data

for the full and vehicle-moving sets. After removing idle-mode

data, average speed rises from 24.50 mph to 31.06 mph. The

median speed is less affected by extreme values, and thus is

smaller in the positively skewed speed distribution. For

Baltimore, the approximate median is 23.70 mph for all driving,

and 30.25 mph for non-idle driving.

Speeds ranged to nearly 95 miles per hour; 2.61% of total

driving time, and 3.30% of time while moving, were spent over 65

mph. Accelerations varied from about -20 to +15 mph per second;

4.25% of driving time is spent in decelerations below -3 mph/sec

(5.18% of non-idle); 3.17% of the time is positive accelerations

above +3 mph/sec (4.03% of non-idle); the remaining 92.58%

(90.79% non-idle) of driving time lies between -3 and +3 mph/sec.

The distribution graphs reflect driving time-in-mode

patterns quite typical of vehicle second-by-second data. The

tri-modal speed distribution was commented on earlier. 

Accelerations exhibit a standard unimodal pattern around zero. 

Cumulative graphs of acceleration are used in this report because

they tend to draw a clearer distinction between two or more

distributions.

The less intuitive specific power is a multiplicative

composite of speed and acceleration. Mean and median specific

power are 46.02 mph /sec and 34.69 mph /sec. Appendix Figure C-3 2 2
FTP Preliminary Report: March 27, 1998

108

shows the cumulative distribution of specific power for

Baltimore. (Because it is defined only for positive

acceleration, there is no comparable distribution that includes

the idle mode.) The distribution of specific power is highly

skewed, reflecting the joint tailing off of speed and

acceleration. As with acceleration, the cumulative graph

presents a more useful picture of the specific power

distribution; this form is used throughout this section. Of all

driving time, 0.22% is spent at specific power above 200 (0.77%

of driving in the positive acceleration mode). 

The joint distribution of speed and acceleration describes

the percentage of driving time spent in different modes. It's

importance to emissions is discussed in Chapter 5. Appendix

Table C-5 gives this distribution for the entire Baltimore 3-

parameter data set. Figure 6-5 presents a three dimensional view

of this distribution. This surface graph is characteristic of

such data, with a large spike at zero speed and acceleration

(idle mode) and a ridge of varying height corresponding to the

speed pattern observed earlier. In Figure 6-6, the same data are

portrayed with the idle portion of driving removed. This better

contrasts the speed-acceleration pattern of distribution during

non-idle operation.

A second way of visualizing the interaction between speed

and acceleration is to compare the "conditional" distributions of

acceleration at different speed levels. Figure 6-7 presents this

comparison for several speed categories. Each distribution

corresponds to a specific row in the speed-acceleration matrix of
FTP Preliminary Report: March 27, 1998

109

Appendix Table C-5. These are essentially cross-sectional views

of acceleration-frequency plane from the speed-acceleration

surface distribution. They reveal the pattern of relatively low 
Figure 6-5
FTP Preliminary Report: March 27, 1998

111

Distribution of Speed and Acceleration - Baltimore
Figure 6-6
FTP Preliminary Report: March 27, 1998

113

Distribution of Speed and Acceleration - Baltimore

Idle Excluded
Figure 6-7
0%

60%50%40%30%20%10%

ACCELERATION (MPH/SEC)

0 -5 mph 15-20 mph 30-35 mph45-50 mph 60-65 mph 75-80 mph

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

FTP Preliminary Report: March 27, 1998

115

Conditional Distributions of Acceleration

Baltimore 3-Parameter Vehicles
FTP Preliminary Report: March 27, 1998

116

variation in acceleration at both low and high speeds, where

speed changes are physically or legally constrained.

Driving Behavior Determinants/Factors

While a number of factors are known to influence individual

driver behavior, the survey conducted for the current study

obtained only limited information on such factors. In

particular, no data were collected on driver demographics. Some

vehicle characteristic information was identified, and other

variables relating to time of observation were generated from the

datalogger output. This section examines the speed,

acceleration, and specific power results across several available

factors in terms of speed-based and vehicle-based measures.

Stratification of speed-based measures includes vehicle type

and age, time of day and time of week.

Vehicle Type. One possible factor in explaining how

vehicles are driven is the vehicle itself. This is a complex

issue, since vehicles can be characterized in many different

ways. One possible set of variables might be described as

engineering-based: engine size, power-to-weight ratio,

transmission type, or body style. A second set relates to the

vehicle's primary functions: work commuting, long-distance

travel, family errands, freight transport. Another set of

vehicle variables might in some way reflect the owner's selfimage: conservative, outdoors-oriented, sporty.
FTP Preliminary Report: March 27, 1998

117

These and other potential factors are correlated and

choosing factor levels is rather subjective. In this study,

virtually no information is available on the second and third

factor types discussed above. Limited data have been obtained

concerning the survey vehicle engineering characteristics. In

the analysis that follows, a "Vehicle Type" factor is defined on

the basis of power/weight, body type, and assumptions concerning

owner/driver "perception" of the vehicle's performance

capabilities.

Using this approach, three vehicle categories were

identified:

! Luxury/Sedan/Station Wagon

! Pickup/Van/Utility

! Sports Car/High Performance

The Baltimore 3-parameter sample consisted of 68 Luxury/Sedan/

Station Wagon vehicles, 21 Pickup/Van/Utility, and 4 Sports

Car/High Performance. (See Appendix B for a list of survey

vehicles.)

In Appendix C, Tables C-6 to C-9 and Figures C-4 to C-6

present summary statistics, distributions, and graphs describing

each of the vehicle categories. Average speed is lowest for the

Sedan class (24.07 mph) and highest for the Pick-up class (25.59

mph). Mean specific power is somewhat higher for the High

Performance category.
FTP Preliminary Report: March 27, 1998

118

Comparing the upper percentiles for the speed distributions

gives essentially the same picture as the mean values. If

aggressive driving is judged by high speed levels, then no clear

picture emerges. The percentage of driving over 55 mph is

greater for the Sport category (12.48%) than for the other

vehicles (Sedan, 10.65%, and Pickup, 10.60%). This order changes

for speeds over 65 mph: the Sport vehicles exceeded that level

only 1.15% of the time; the Sedan class, 2.98%; Pick-up, 1.82%.

Similarly, high acceleration levels do not clearly identify

one vehicle type as being more or less aggressively driven. 

Accelerations over 3 mph/sec occurred with highest frequency

within the Sedan group (3.32%) and lowest for the Pick-up class

(2.85%)

The specific power variable presents a somewhat more

definitive result. Using high specific power as a measure of

aggressive driving, the Sport group displays by far the greatest

proportion of high values. For example, Sport vehicles exceeded

a specific power level of 200 mph /sec during 0.76% of driving 2

(1.89% while in positive acceleration mode) of driving. This

compares with 0.26% and 0.39% (0.63% and 0.90% in positive

accelerations) for the Sedan and Pick-up groups, respectively.

It might also be instructive to look at how low performance

vehicles are driven. However, these data are not currently

available due to the lack of a systematic definition of "low

performance." Developing this definition and analysis of driving

behavior for these vehicles is planned for the future.
FTP Preliminary Report: March 27, 1998

119

Vehicle Age. The age of the sample vehicle was used to

define three categories corresponding approximately to the

evolution of engine and emission control technologies. These

categories include:

 Age Category Model Years

 Old Pre-1975

 Medium 1975-1982

 New 1983-1992

The Baltimore 3-parameter vehicles included only members of

the Medium and New age groups. Of the 93 vehicles, 15 fall in

the Medium class, and 78 are New.

It might be assumed that vehicle age is a proxy for various

factors influencing driving behavior. For example, older

vehicles tend to be used for purposes resulting in lower mileage

accumulation and less highway driving. Comparing driving

behavior across age categories should illuminate these factor

effects.

Appendix C Tables C-10 to C-13 and Figures C-7 to C-9 show

summary statistics, distributions, and graphs for the speedrelated variables in these categories. On average, newer

vehicles were driven some 4 mph faster than those in the medium

age category (25.10 mph vs. 21.15 mph). Accelerations among the

medium age vehicles actually vary somewhat more than for the new

category (standard deviation 1.53 mph/sec vs. 1.49 mph/sec). 
FTP Preliminary Report: March 27, 1998

120

Average specific power is slightly higher in the new vehicle

category (46.15 vs. 45.22 mph/sec). Median specific powers

compare similarly: New, 34.77 mph /sec and Medium, 34.18. 2

High speed driving is more common in newer vehicles, with

11.33% of driving time spent over 55 mph compared to 7.32% for

the Medium category. As with the summary statistics, the

acceleration and specific power distributions differ less

dramatically.

Time of Day. Because the dataloggers recorded the exact

time of day for each speed observation, it is a simple matter to

categorize speed-related measures by that factor. As a first cut

in relating driving behavior to time of day, the twenty-four hour

period was subdivided into the following classes:

1:00 am to 6:00 am

6:00 am to 9:00 am Morning Rush

9:00 am to 4:00 pm

4:00 pm to 7:00 pm Evening Rush

7:00 pm to 1:00 am

An observation was included in a given time interval if the

trip containing the value began during that interval. Thus,

summary statistics of actual driving for a given class may

include data values for driving that occurred in a later

interval. Presumably, vehicle operation depends on trip purpose

and traffic conditions, both of which in turn are related to trip

start time.
FTP Preliminary Report: March 27, 1998

121

In Appendix C, Tables C-14 to C-17 and Figures C-10 to C-12

summarize the speed, acceleration and specific power levels for

the five time categories. Average speed was lowest during the

evening rush hour of 4:00 to 7:00 pm (22.98 mph) and highest

during the morning rush period (25.42 mph) and night driving

period of 9:00 pm to 1:00 am (25.43 mph).

Daytime driving, between 6:00 am and 7:00 pm exhibits the

largest percentage of high speed driving (over 55 mph). Extremes

of acceleration and specific power do not appear to be highly

influenced by time of day.

Time of Week. A second time classification of interest is

weekday versus weekend. As with time of day, there is reason to

suspect that different trip and traffic congestion patterns for

the two periods may influence driving behavior.

Summary statistics, distributions, and graphs comparing

these categories appear in Appendix C Tables C-18 to C-21 and

Figures C-13 to C-15. The hypothesis that differences exist for

the two parts of the weeks appears to be supported in terms of

the Baltimore 3-parameter speed variable. Average weekday speed

is substantially lower than on weekends: 23.92 mph and 26.35 mph,

respectively. Differences between acceleration and specific

power statistics are less pronounced.

The distributions of speed-related variables give further

insight into their relation to time of week. For example, 9.81%

of weekday driving exceeded the speed 55 mph; this increased to
FTP Preliminary Report: March 27, 1998

122

13.56% on weekends. This reinforces the impression of higher

speed weekend driving. However, using acceleration and specific

power to indicate driving aggressiveness, no evidence indicates

that weekend driving is more extreme than on weekdays.

6.2.2. Trip Measures

The nature and patterns of vehicle trips are another

important area in the study of driving behavior. This section

examines such trip characteristics as trip duration, trip

distance, time between trips, and vehicle operating modes

summarized over the trip. In addition, extremes of speed-based

measures are analyzed from a trip perspective. 

For this study, a trip basically begins when the engine is

turned on and ends when the engine is shut off. (See Section

5.1.1 for a more detailed discussion of trip definition.) This

definition allows for very short trips, such as moving a vehicle

in the driveway. Even zero distance trips are possible with this

definition--a car is started, probably with the intent of driving

it somewhere, but never moved. In fact, 64 zero mile trips were

recorded in Baltimore.

The 3-parameter instrumented vehicles in Baltimore generated

a total of 4,681 trips. The average trip covered 4.89 miles and

lasted an average of 12 minutes. The median values for trip

distance and duration indicate a much shorter "typical" trip of

8.8 minutes in duration and 2.53 miles in distance.
FTP Preliminary Report: March 27, 1998

123

Figures 6-8 and 6-9 present the frequency distribution for

trip time and distance. A little more than a quarter of the 

FTP Preliminary Report: March 27, 1998

125

trips are a mile or less. Only about 12 percent of the trips

were longer than 10 miles. In terms of trip duration, about 5

percent of the trips were very short, lasting a minute or less.

Over the course of a trip, a vehicle will spend a certain

fraction of time in each of four operating modes: idle, cruise,

acceleration, and deceleration. In Baltimore, the average trip

involved 23 percent idle operation, 34 percent cruise, 22 percent

acceleration and 20 percent deceleration. (See Section 5.1.2 for

mode definitions.)

Trip Factors/Determinants

The characteristic features of a trip are dependent upon

numerous factors including trip purpose, driver attitude, trip

location, and the driving environment encountered during the

course of the trip. None of these factors can be determined from

the survey data. However, certain known variables can be used as

a proxy for one or more of these unknown factors. Knowing the

time of day when a trip took place, it may be possible to make

some crude aggregation of such factors as trip purpose, driving

environment, and possibly driver attitude. For example, a trip

occurring between 6am and 9am could be considered a morning

commute trip driven under congested conditions by a driver who

may react to a very familiar driving environment in an aggressive

manner.

This section considers several factors for which trip data

were collected. Time of day and time of week (weekday vs.
FTP Preliminary Report: March 27, 1998

126

weekend) are examined along with vehicle type and vehicle age. 

These preliminary analyses serve as a starting point for further

work in understanding the complex nature of trip patterns.

Time of Day. As in Section 6.2.1, time of day is separated

into 5 categories, selected to roughly represent 2 periods of

peak vehicle operation (6am-9am, 4pm-7pm) and 3 periods of offpeak operation (1am-6am, 9am-4pm, 7pm-1am). Table 6-3 presents

summary statistics for trip time, distance, and idle time

stratified by time of day. A large fraction of the trips

occurred during the off-peak times, with nearly half of all

trips taken during the mid-day period. Almost twice as may trips

were taken in the afternoon peak period than in the morning peak. 

The morning peak period had the longest average trip distance

(6.65 miles) while mid-day trips averaged only 4.45 miles. 

Average trip time was greatest during the early morning off-peak

period. From these summary measures it appears that the

afternoon peak-period looks more like the mid-day off peak than

the morning peak.

Time of Week. Trips were also stratified by weekend and

weekday to examine the need for isolating weekday operation for

future analyses. Trip time and distance are typically longer on

weekends than weekdays. These differences are not particularly

large; the median trip length of 2.56 miles for weekends compares

to a median of 2.35 miles for weekdays. The variation in

distance and duration weekend trips is considerably greater than

found for trips taken during the week. The standard deviations

shown in Table 6-3 illustrate these differences.
FTP Preliminary Report: March 27, 1998

127

Vehicle Type. It is reasonable to expect that the type of

vehicle could have an impact on trip behavior. For example, a

commuting car may only be used to make long trips to and from the

workplace and a sports utility vehicle may be used for long trips

on weekends. The vehicle type classification for this report is

very broad, categorizing vehicles as either: luxury, sedan,

station wagon; pick-up, van, utility; and sport/high performance

(see Section 6.2.1). 

The average trip time of sport/high performance vehicles

(12.71 minutes) was longer than for either pick-ups (12.56

minutes) or sedans (11.8 minutes). Trip length differences among

the vehicle types were less distinct. The average trip length

for pick-ups was 5.32 miles, slightly higher than mean trip

length of 5.22 for sports/high performance (see Table 6-3). The

sedans average trip length was just 4.72 miles. Interestingly,

the median trip length was longer for sport/high performance

vehicles than pick-ups. The contrasting results for the mean and

median suggest that the pick-up class had some extremely long

trips which only impacted the mean. In fact the longest trip for

pick-ups was 113 miles compared to only 37 miles for the

sport/high performance vehicles.

Vehicle Age. The age of a vehicle can affect the way the

vehicle is driven in terms of the type and frequency of trips. 

Concerns regarding a vehicle's reliability may lead people to

limit their driving of older vehicles to short trips. To take a

very cursory look at the influence of vehicle age on trip

patterns, EPA categorized vehicles into three broad age groups:
FTP Preliminary Report: March 27, 1998

128

pre-1975, 1975-82, 1983-1992. For Baltimore, the I/M program

restricted recruitment to 1977 and newer model year vehicles;

thus, only the two newer categories are considered here.

The newer vehicles' (1983-92) average trip time was 12.36

minutes and covered an average of 5.14 miles. Both measures were

considerably shorter for older vehicles(1975-82), with a mean

trip time of 10.48 minutes and a trip length average 3.7 miles

(see Table 6-3).
Table 6-3.

Baltimore Stratified Trip Measures 

Characteristic between

Trips Trip time (minutes) Trip Distance (miles) Distance

stops(miles) Number Percent Mean Median Std. Mean Median Std.

of total

Time of Day1am to 6am 100 2.14 14.37 15.06 9.23 5.85 3.77 5.87 1.336am to 9am 572 12.22 15.78 13.28 12.46 6.65 3.91 8.95 1.089am to 4pm 2,284 48.79 10.89 7.74 11.39 4.45 2.18 8.29 0.804pm to 7pm 1,047 22.37 11.97 8.80 10.85 4.55 2.45 6.07 0.787pm to 1am 678 14.48 12.44 9.05 12.80 5.27 2.80 8.74 1.01Time of WeekWeekday 3,578 76.44 11.94 8.87 10.85 4.75 2.56 7.04 0.84Weekend 1,103 23.56 12.32 8.53 14.06 5.35 2.35 10.49 0.97Vehicle TypeLuxury, sedan, 3,294 70.37 11.80 8.43 11.77 4.72 2.37 8.15 0.81station wagonPick-up trucks, 1,207 25.79 12.56 9.28 11.70 5.32 2.80 7.80 1.04van, utilitySports, high 180 3.85 12.71 10.88 9.72 5.22 3.89 5.82 0.96performanceVehicle Age1975 to 1982 816 17.43 10.48 7.43 10.82 3.70 1.81 6.62 0.64
FTP Preliminary Report: March 27, 1998

130

1983 to 1992 3,865 82.57 12.36 9.05 11.83 5.14 2.66 8.23 0.92

Trip-based Analysis of Speed, Acceleration, and PowerThe driving behavior exhibited over the course of a given trip varies greatly and is

affected by a host of factors. Thus, even on a trip basis, one would expect to seeconsiderable variation in driving behavior variables such as speed, acceleration, andpower. A trip-based analysis of the extremes of these driving behavior variables providesan alternative perspective of the incidence of aggressive driving.

High speed operation(> 60 mph) accounts for 7 percent of total driving time. On a

trip basis, it is clear that the majority of trips in Baltimore do not involve driving athigh speeds. Only about 14 percent of all trips had a maximum speed greater than 60 mph. Figure 6-10 shows the cumulative distribution of maximum speed per trip.

In marked contrast to high speeds, extreme values of power are found on

disproportionately large fraction of trips relative to their overall frequency. About 30percent of all trips had a maximum power value greater than 200, while as a percentage oftotal instances of positive power values, a power value of 200 or greater accounts for
FTP Preliminary Report: March 27, 1998

131

only 0.77 percent (0.22 percent of total vehicle operation). 

(See Figure 6-11)

High acceleration rates illustrate this point as well. Nine

out of ten trips contain accelerations greater than 3 mph/second,

yet these accelerations only account for about 3 percent of total

driving time. (See Figure 6-12)

These differences illustrate how certain driving parameters

are closely related to the type or purpose of a trip (high speed

operation), while others (speed and acceleration) are a more 



 Radian Corporation, "Light Duty Vehicle Driving Behavior: Private Vehicle 43

Instrumentation, Volume 1," Draft Report to U.S. EPA, August 24, 1992.

FTP Preliminary Report: March 27, 1998

135

universal trip characteristic. From this perspective, the

"tails" of the acceleration and power distribution take on

greater significance.

6.2.3. Vehicle-based Measures

Several summary statistics were compiled from the survey to

describe driving behavior variation among vehicles. These are

reviewed briefly here, for all Baltimore 3-parameter driving, and

for several factor comparisons. Refer to the Radian Corporation

report for additional detail on these and related variables.43

The Baltimore 3-parameter sample contained 93 vehicles. 

Variables described in this section were listed earlier: miles

driven per day, number of trips per day, number of minutes of

operation per day, and number of stops per hour of operation.

In summarizing the data for these variables, it was

appropriate to weight their values to account for differing

amounts of instrumentation time and operation time. Therefore,

in the outcomes reported below, means and standard deviations for

the first three variables are weighted by the number of days the

vehicle was instrumented; for the last variable, those statistics

are weighted by the number of hours the vehicle was operated.

In Appendix C, Table C-22 gives summary results for the 93

Baltimore vehicles. Mean daily mileage is 30.56, while one
 "Soak period" is the time after the vehicle has been turned off until it is 44

restarted.

FTP Preliminary Report: March 27, 1998

136

vehicle averaged over 105 miles per day. Daily time of operation

averaged over 75 minutes; number of trips, 6.25 per day; and

number of stops, over 35 per hour of operation.

The vehicle-based measures used in the discussion of overall

Baltimore driving behavior were summarized by the levels of

several factors, including vehicle type and age, and time of

week. As before, means and standard deviations are weighted

appropriately.

Comparisons of these statistics for several stratification

factors are found in Appendix C Tables C-23 to C-25. The factor

levels are defined in Section 6.2. Some general observations

follow. Newer vehicles were driven somewhat longer and farther

per day, and averaged fewer trips and fewer stops per hour. 

Weekday driving produced substantially higher average time,

higher distance and number of trips than weekdays. For the

vehicle type factor, average daily usage is considerably higher

for the Pickup/Van/Utility class than for the Sedan and Sport

classes; the per hour stop rate of Sedans was somewhat higher. 

Presumably, these outcomes relate to differences in trip purpose

associated with these vehicle types.

6.2.4. Vehicle Soak

The soak period is a critical factor in the level of 44

emissions generated after a vehicle start. For illustration,
 The data is from a test program run at EPA's National Vehicle and Fuel 45

Emissions Laboratory in Ann Arbor, MI. All the soaks were conducted with an ambient

temperature of 75"F and second-by-second emission data were gathered on a modal test

bench. The vehicles tested were a 1992 Ford Crown Victoria with a 4.6L engine, a

1992 Dodge Dakota pickup with a 5.2L engine, a 1991 Honda Accord with a 2.2L engine,

a 1991 Dodge Caravan with a 3.0L engine, and a 1990 Pontiac 6000 with a 3.1L engine.

FTP Preliminary Report: March 27, 1998

137

Figure 6-13 shows the average emissions after soak periods

ranging from 10 minutes to overnight for 5 vehicles run over the

first micro-trip of the FTP (that is, the first 125 seconds of

the cycle). The data indicate that HC and CO emissions during 45

FTP Preliminary Report: March 27, 1998

139

the first 2 minutes of operation can increase by a factor of 10

after long soak periods, relative to 10 minute soak periods, and

NOx by a factor of 5. Even relatively short soaks of 45 minutes

increased all emissions by a factor of 4-5. 

This emission increase occurs due to engine and catalyst

cooldown. Typical engine and catalyst cooldown curves can be

generated using the equations developed from the Albany Emission

Laboratory work, as discussed in Section 5.5.1. Figure 6-14

shows the predicted cooldown curves for a 6-cylinder vehicle at

75 F with a 4 MPH wind and dry pavement. The curves demonstrate o

that the catalyst cools off much faster than the engine. Thus,

most of the emission increase found when increasing soak time

from 10 minutes to about an hour is probably due to catalyst

cooldown. After an hour, engine cooldown becomes increasingly

important as the soak time continues to increase and the catalyst

cooldown becomes less important (as the catalyst light-off

temperature is around 650 F, the catalyst is close to being o

completely cold after 45-60 minutes).

Because of the potential emission impacts, one of the

primary objectives of the driving surveys was to examine the

distribution of actual soak times. Figure 6-15 summarizes the

distribution of soak times found for the Baltimore 3-parameter

data set. A more detailed distribution is presented in tabular

form in Appendix D.

The soak distributions include a large number of relatively

short soak periods. This indicates that drivers frequently
FTP Preliminary Report: March 27, 1998

140

string trips together. In fact, only about 18% of the trips

occurred after a soak period greater than 8 hours, indicating

that single trips directly to work (or some other destination)

and then home again for the night are more the exception than the

rule.


FTP Preliminary Report: March 27, 1998

143

To help put the soak time information into context, the

cooldown formulas developed in Section 5.5.1. were applied to the

Baltimore soak distributions to develop a profile of engine and

catalyst temperatures at vehicle start (assuming an ambient

temperature of 75 F, wind of 4 MPH, and dry pavement). These o

temperature distributions are presented in Table 6-4. It should

be noted that these temperature distributions overstate the

initial temperatures to some degree, as it was assumed that the

engine and catalyst had reached normal operating temperatures

prior to the soak period, which is not true for every soak.
FTP Preliminary Report: March 27, 1998

144

Table 6-4

Temperature Distribution at Trip Start

Catalyst Engine

Temperature Frequency Temperature Frequency

( F) (Percent) ( F) (Percent) o o

800+ 22 180+ 51

700-800 9 160-180 7

140-160 6

600-700* 7 120-140 6

100-120 7

500-600 6 <100 23

400-500 5

300-400 5

200-300 6

<200 40

*Catalyst light-off range

The data indicate substantial differences between catalyst

and engine conditions upon vehicle start-up. A high proportion

of vehicle starts (at least 62%) occur with catalyst temperatures

below the light-off range. However, the engine is quite warm

for most vehicle starts. Only about 23% of starts occur with

engine temperatures below 100 F and about 64% of all starts had o

engine temperatures above 140 F. o

6.2.5. Trip Start Driving Activity
FTP Preliminary Report: March 27, 1998

145

Vehicle soak periods impact emissions by affecting engine

and catalyst temperatures at the start of the next trip; the

longer the soak period, the more the engine and catalyst cool

off. How the vehicle is operated after start-up may also impact

emissions, as it affects the rate at which the engine and

catalyst warm up and the level of emissions from the vehicle

during the warmup period.

Most of the exhaust emissions from modern, properly

operating vehicles occur during the first 1-3 minutes of

operation after the vehicle has been started. However, this was

not the case when the current FTP driving cycle was developed. 

Vehicles in the late '60's always ran with rich air/fuel ratios

and did not have catalysts. Start emissions from these vehicles

were not very different from hot stabilized emissions and how the

vehicles were driven during the engine warmup period had little

impact on overall emission levels. Thus, there was little need

to evaluate whether vehicles are driven differently after they

are started.

The introduction of the catalyst and more sophisticated fuel

control systems dramatically altered the relationship between

emissions during the warmup period and hot stabilized emissions. 

Hot stabilized emissions have been reduced to relatively low

levels with modern catalysts, plus improvements in fuel systems

ensure efficient catalyst operation even during moderate

transient operation. However, emissions shortly after cold (and

warm) starts have not been reduced proportionally due to the need

to warm up the catalyst and the inability to immediately enter
FTP Preliminary Report: March 27, 1998

146

closed-loop operation. Thus, how vehicles are driven shortly

after they have been started is a much more important

consideration today than when the FTP was developed.

One concern that EPA investigated was whether or not the

length of the soak period influenced driving behavior after

start-up (for example, are vehicles driven differently after

overnight soaks at home than they are after short soaks at

stores). Therefore, the first step of the analysis was to divide

the start data into cold, warm and hot starts. A cold start is

where both the engine and catalyst are cold, a warm start is

where the engine is warm and the catalyst is cold, and a hot

start is where both engine and catalyst are warm (as catalysts

cool off much faster than engines, starts with warm catalysts and

cold engines are not a concern). Based on previous test

programs, it is known that catalysts warm up after about two

minutes of driving. The work by New York's Albany Emission

Laboratory indicated that catalysts cool off to fairly cold

levels in about 45 minutes. Thus, a start was classified as

having a warm catalyst if two conditions were met: (1) the

previous trip prior to the soak was less than two minutes in

duration, and (2) the soak period was less than 45 minutes. If

either the previous trip was less than 2 minutes or the soak

period was longer than 45 minutes, the start was classified as

having a cold catalyst.

For engine classification, Radian developed engine warmup

and cooldown algorithms from the 6-parameter data vehicles (which

monitored coolant temperature). The warm-up algorithm related
 Time was used instead of distance because the data was gathered in second- 46

by-second increments. A distance criteria would also create problems when

generating new testing cycles, which are time based.

 Reference: Technical Note Under Contract No. 68-C1-0079 In Response to Work 47

Assignment 1-05, "Evaluation of Trip Start Activity, Subtask 1b, Definition of Trip

Start Condition", by Sierra Research, dated April 12, 1993.

FTP Preliminary Report: March 27, 1998

147

engine temperature rise to idle time and vehicle speed while

moving. The cooldown algorithm related engine temperature fall

to ambient air temperature, engine displacement, and soak time. 

These algorithms were then used by Radian to predict engine

temperature at the beginning of every trip in the 3-parameter

data set (both Spokane and Baltimore). If the predicted engine

temperature at the beginning of the trip was within 10 C of the o

ambient temperature, the start was classified as having a cold

engine. If the difference was greater than 10 C, the start was o

classified as a warm engine.

The next step in assessing start driving behavior was to

define the length of time after a vehicle start that driving 46

should be classified as "start", as opposed to hot stabilized. 

To facilitate this analyses, EPA decided to strip off the initial

idle period and analyze it separately from the data after the

vehicle began to move. This was done because of the substantial

impacts of ambient temperature on the initial idle time and to

equitably compare driving after cold, warm, and hot starts.

The criteria established for the start period was to

encompass the portion of the warm-up period that relates to

elevated emission levels. Based upon an analysis of trip start

emission activity by Sierra Research, a coolant temperature 47
FTP Preliminary Report: March 27, 1998

148

threshold of 140 F was selected as the point at which elevated o

emission levels largely disappear during vehicle warmup.

Using the 140 F coolant temperature threshold, Radian o

analyzed the 6-parameter coolant temperature data for the amount

of time it took to reach 140 F on each trip, excluding the o

initial idle. The distribution of these times are listed in

Appendix E, both for all trips (Table E-1) and broken down by

cold, warm, and hot starts (Tables E-2 to E-4, respectively). 

The data indicate that over 50% of all cold starts reach 140EF

coolant temperature after 240 seconds of operation. Also, almost

95% of all warm starts reach 140EF coolant after 240 seconds and

over 90% of all starts, combined, reach 140EF after 240 seconds. 

Thus, 240 seconds, excluding the initial idle period, was

selected as the "start" portion of overall driving. To further

distinguish the sequence of driving behavior after starts, this

240 second period was broken up into three equal 80 second

"phases" for analysis purposes.

Radian analyzed speed, acceleration, and power distributions

for each combination of the 80 second phases and the different

trip start definitions. The averages and standard deviations for

each group, as well as for overall driving, are presented in

Table 6-5. 

The averages and standard deviations indicate that driving

behavior is not significantly affected by the length of the soak

time. For any given time phase, the averages of speed,

acceleration, and power are very similar for each start category
FTP Preliminary Report: March 27, 1998

149

(i.e. cold, warm, and hot). Graphs comparing the actual

distribution of speed and power are presented in Appendix E

(Figures E-1 to E-3 show the speed distribution by start category

for phases 1-3, respectively; Figures E-4 to E-6 show the

cumulative power distributions). The distributions for each

individual phase show remarkable similarity across all three

start categories. Thus, the only driving behavior that needs to

be considered as a function of soak time is the initial idle

period.

The data in Table 6-5 also indicate that driving behavior

immediately after a start (be it cold, warm, or hot) is different 
FTP Preliminary Report: March 27, 1998

150

Table 6-5

Trip Start Driving Behavior by 80-Second Phases

Start Type Number of Speed (mph) Acceleration Power

Phase of Seconds (mph/sec) (mph /sec)

Start

2

Mean Std. Mean Std. Mean Std.

1 Cold 154,397 14.4 11.9 .25 1.87 41.4 42.4

Warm 171,764 14.2 12.0 .23 1.84 40.5 43.4

Hot 313,433 14.5 12.6 .23 1.79 40.5 43.3

Wgt. 14.4 12.3 .24 1.82 40.7 43.1

avg.

2 Cold 145,684 22.5 15.1 .02 1.67 48.6 44.6

Warm 156,818 21.1 15.0 .01 1.66 47.1 44.5

Hot 273,546 21.4 15.5 -.01 1.63 45.9 42.4

Wgt. 21.6 15.3 .00 1.65 46.9 43.5

avg.

3 Cold 134,348 25.4 16.8 .01 1.53 47.0 44.1

Warm 141,539 24.0 16.2 .00 1.57 45.6 43.2

Hot 233,245 23.5 16.6 -.02 1.54 44.9 42.1

Wgt. 24.1 16.6 -.01 1.55 45.7 43.0

avg.

Hot 3,365,299 28.1 20.3 -.04 1.42 43.5 41.6 *

*Hot stabilized operation (that is, driving after the first 240 seconds of each trip).

than after stabilized operation has been reached. The average 

speeds for all the start phases are lower than for hot stabilized

driving (that is, driving after the 240-second start period),

especially phase 1. Phase 1 also has lower average power values
FTP Preliminary Report: March 27, 1998

151

than all the other phases, although phase 2 and phase 3 actually

have higher average power values than hot stabilized driving.

Figure 6-16 displays the speed distribution for Baltimore

and Spokane by phase (i.e. each of the first three 80 second

phases of driving, plus driving after the first 240 seconds, with

the different start types combined). This figure shows that the

speed distribution during the initial 80 second phase is

completely different than for subsequent driving, with the most

frequent driving speed range being 0-5 mph and the frequency

dropping steadily with increasing speed. The speed distributions

for all subsequent driving phases have more similar shapes, with

the most frequent speed range being 30-35 mph. Phase 2 and Phase

3 of start driving behavior are especially similar, although

Phase 3 has a little less driving in the 5-25 mph range and a

little more in the 45-65 mph range. The speed distribution for

hot stabilized driving (that is, the "rest of trip") differs in

having substantially more driving in the 50-75 mph range. 

Cumulative power distributions for each phase of driving are

presented in Figure 6-17. These distributions demonstrate that

the aggressiveness of the driving for each phase is more similar

than the speed distributions. Phase 1 spends substantially more

time at very low power levels, but the distribution of power

levels above 80 is very similar to hot stabilized driving. Phase

2 and Phase 3 are virtually identical to each other, although

both are a little more aggressive than either Phase 1 or hot

stabilized driving.


FTP Preliminary Report: March 27, 1998

154

Initial Idle

The distribution of the initial idle time for all trips (3-

parameter, Spokane and Baltimore) has been analyzed and is

presented in Appendix E for each start category (Tables E-5 to E7 for cold, warm, and hot starts, respectively). Table 6-6

summarizes the average and median initial idle times for Spokane

and Baltimore.

Table 6-6

Initial Idle Time, Baltimore and Spokane

Start Type Mean Median

(seconds) (seconds)

Cold start 59 13

Warm start 28 8

Hot start 15 5

Overall 29

The large discrepancy between the mean and the median

initial idle time is due to a significant number of extended

idles. The data indicate that 10% of all cold starts had initial

idles greater than 2 minutes, and 5% exceeded 5 minutes. This

discrepancy needs to be addressed in the course of generating

representative test cycles.
FTP Preliminary Report: March 27, 1998

155

The data also indicates that cold starts have much longer

initial idle times than hot starts. This also needs to be

addressed during cycle development.

One potential problem with the Baltimore and, especially,

Spokane data is that the studies were conducted during colder

weather when people frequently let their cars warm up before

driving off. The average daily temperature during the survey

period was roughly 40 F and 50 F for Spokane and Baltimore, o o

respectively. The initial idle periods would probably be shorter

during the summer months more representative of ozone

nonattainment, especially after a cold start.

One way to mitigate this concern would be to use the Atlanta

data to calculate initial idle time, as the Atlanta survey was

conducted during August and early September. The average daily

temperature during the survey period in Atlanta was between 70

and 75"F. A comparison of the Atlanta to the combined Spokane

and Baltimore initial idle times is presented in Table 6-7 (the

distribution of initial idle times for Atlanta are presented in

Appendix E, Tables E-8 to E-10 for cold starts, warm starts, and

hot starts, respectively).
FTP Preliminary Report: March 27, 1998

156

Table 6-7

Initial Idle Time for Baltimore/Spokane vs. Atlanta

Start Type

Baltimore and Spokane Atlanta

Mean Median Mean Median

(seconds) (seconds) (seconds) (seconds)

Cold start 59 13 28 9

Warm start 28 8 18 7

Hot start 15 5 12 5

Proportion of Driving Occurring After Starts

The final factor that will be needed to assess the emission

impact of start driving behavior is the proportion of overall

driving represented by the start driving behavior. A breakdown

of the survey data into miles driven from the start of each trip

is presented in Figure 6-18 (the cumulative distribution can be

read as "the proportion of all driving that was driven less than

or equal to the number of miles on the X-axis after the start of

a trip"). The reduction in the miles spent in each speed range

as the miles since the start of the trip increases reflects the

high percentage of short trips in the data base.

FTP Preliminary Report: March 27, 1998

158

6.3. Comparisons to the Federal Test Procedure

The Federal Test Procedure is intended to represent in-use

driving behavior. The Baltimore 3-parameter data offer an

empirical source for judging the success of the FTP in capturing

real driving characteristics. This section compares the FTP and

Baltimore data using the various descriptive tools employed

earlier.

6.3.1. Speed-based Measures

Using the speed-related data, driving on the FTP is more

conservative than that observed in Baltimore. Table 6-8 compares

summary statistics on speed, acceleration and specific power for

the two sources. Typical FTP speeds are lower than for the

Baltimore data: the mean FTP speed is 19.59 mph (24.14 mph when

the vehicle is moving), compared to 24.50 mph (31.06 mph when

moving) for Baltimore. Accelerations in Baltimore range from -15

to +20 mph/sec, with standard deviation 1.50; the FTP range is

much narrower, -3.3 mph/sec to +3.3 mph/sec, with standard

deviation 1.40. Specific power (during positive acceleration)

for the Baltimore sample averaged 46.02 mph /sec; the median is 2

34.7. For the FTP, the mean and median are much lower, 38.60 and

21.6, respectively.

Tables 6-9 to 6-11 and Figures 6-19 to 6-21 contrast the

distributions of speed-related variables for Baltimore with those

of the FTP. As is well documented, the FTP contains no speeds
FTP Preliminary Report: March 27, 1998

159

above 56.7 mph, and no positive or negative accelerations in

excess of 3.3 mph/sec. In Baltimore, roughly 8.5% of speeds

(between 6.35% and 10.73%) fell above 56.7 mph. For

accelerations, about 2.5% of driving exceeded 3.3 mph/sec, while

around 3.5% of decelerations were under -3.3 mph/sec.
FTP Preliminary Report: March 27, 1998

160

The largest FTP specific power is 192 mph /sec. In the 2

Baltimore survey, about 0.3% of all driving time exceeded that

level (about 1% of time in positive acceleration mode).

Table 6-8 

Summary Statistics 

Baltimore and FTP 

Baltimore 3-Parameter Vehicles

Speed (mph)

FTP Baltimore

Mean 19.59 24.50

St Dev 14.69 20.52

Max 56.70 94.46

Min 0.00 0.00

Count 1,369 3,365,504

Acceleration (mph/sec)

FTP Baltimore

Mean 0.00 0.00

St Dev 1.40 1.50

Max 3.31 15.19

Min -3.31 -19.49

Count 1,368 3,360,550

Power (mph /sec) 2

FTP Baltimore

Mean 38.60 46.02

St Dev 31.74 42.96

Max 191.85 557.69

Min 0.01 0.00

Count 544 1,407,908
FTP Preliminary Report: March 27, 1998

161

Table 6-9

DISTRIBUTION OF SPEED

Baltimore and FTP

Baltimore 3-Parameter Vehicles

Baltimore FTP

Speed Cum. Cum.

(mph) Count Percent % Count Percent %

0-0 710,890 21.1 21.1 258 18.8 18.8

0-5 207,698 6.2 27.3 76 5.6 24.4

5-10 181,442 5.4 32.7 74 5.4 29.8

10-15 201,708 6.0 38.7 80 5.8 35.6

15-20 209,420 6.2 44.9 129 9.4 45.1

20-25 231,917 6.9 51.8 265 19.4 64.4

25-30 280,040 8.3 60.1 248 18.1 82.5

30-35 308,521 9.2 69.3 86 6.3 88.8

35-40 253,681 7.5 76.8 42 3.1 91.9

40-45 173,105 5.1 82.0 8 0.6 92.5

45-50 125,294 3.7 85.7 27 2.0 94.4

50-55 121,004 3.6 89.3 52 3.8 98.2

55-60 147,398 4.4 93.7 24 1.8 100.0

60-65 125,917 3.7 97.4 0 0.0 100.0

65-70 66,826 2.0 99.4 0 0.0 100.0

70-75 17,485 0.5 99.9 0 0.0 100.0

75-80 2,803 0.1 100.0 0 0.0 100.0

80-85 253 0.0 100.0 0 0.0 100.0

85-90 91 0.0 100.0 0 0.0 100.0

90-95 11 0.0 100.0 0 0.0 100.0

Total 3,365,504 100.0 1,369 100.0
FTP Preliminary Report: March 27, 1998

162

Table 6-10

DISTRIBUTION OF ACCELERATIONS

Baltimore and FTP

Baltimore 3-Parameter Vehicles

Baltimore FTP

Accel

-20 to -19 1 0.0 0.0 0 0.0 0.0

-19 to -18 0 0.0 0.0 0 0.0 0.0

-18 to -17 0 0.0 0.0 0 0.0 0.0

-17 to -16 0 0.0 0.0 0 0.0 0.0

-16 to -15 0 0.0 0.0 0 0.0 0.0

-15 to -14 3 0.0 0.0 0 0.0 0.0

-14 to -13 2 0.0 0.0 0 0.0 0.0

-13 to -12 11 0.0 0.0 0 0.0 0.0

-12 to -11 34 0.0 0.0 0 0.0 0.0

-11 to -10 44 0.0 0.0 0 0.0 0.0

-10 to -9 149 0.0 0.0 0 0.0 0.0

-9 to -8 545 0.0 0.0 0 0.0 0.0

-8 to -7 1,801 0.1 0.1 0 0.0 0.0

-7 to -6 5,756 0.2 0.2 0 0.0 0.0

-6 to -5 17,199 0.5 0.8 0 0.0 0.0

-5 to -4 41,575 1.2 2.0 0 0.0 0.0

-4 to -3 76,005 2.3 4.3 76 5.6 5.6

-3 to -2 120,151 3.6 7.8 56 4.1 9.6

-2 to -1 219,036 6.5 14.4 85 6.2 15.9

-1 to 0 767,792 22.8 37.2 258 18.9 34.7

0 702,538 20.9 58.1 349 25.5 60.2

0 to 1 899,795 26.8 84.9 314 23.0 83.2

1 to 2 267,355 8.0 92.8 125 9.1 92.3

2 to 3 133,587 4.0 96.8 49 3.6 95.9

3 to 4 62,415 1.9 98.7 56 4.1 100.0

4 to 5 27,375 0.8 99.5 0 0.0 100.0

5 to 6 10,639 0.3 99.8 0 0.0 100.0

6 to 7 3,712 0.1 99.9 0 0.0 100.0

7 to 8 1,649 0.0 100.0 0 0.0 100.0

8 to 9 776 0.0 100.0 0 0.0 100.0

9 to 10 325 0.0 100.0 0 0.0 100.0

10 to 11 187 0.0 100.0 0 0.0 100.0

11 to 12 62 0.0 100.0 0 0.0 100.0

12 to 13 20 0.0 100.0 0 0.0 100.0

13 to 14 8 0.0 100.0 0 0.0 100.0

14 to 15 2 0.0 100.0 0 0.0 100.0

15 to 16 1 0.0 100.0 0 0.0 100.0
Baltimore FTP

Accel

FTP Preliminary Report: March 27, 1998

163

Total 3,360,550 100.0 1,368 100.0
FTP Preliminary Report: March 27, 1998

164

Table 6-11

DISTRIBUTION OF POWER

Baltimore and FTP

Baltimore 3-Parameter Vehicles

Baltimore FTP

Power

(mph /sec) Count Percent Cum. % Count Percent Cum. % 2

0 - 20 477,725 33.9 33.9 182 33.5 33.5

20 - 40 308,000 21.9 55.8 154 28.3 61.8

40 - 60 215,625 15.3 71.1 104 19.1 80.9

60 - 80 149,080 10.6 81.7 49 9.0 89.9

80 - 100 100,442 7.1 88.8 30 5.5 95.4

100 - 120 64,170 4.6 93.4 11 2.0 97.4

120 - 140 38,864 2.8 96.2 5 0.9 98.3

140 - 160 22,726 1.6 97.8 5 0.9 99.3

160 - 180 13,099 0.9 98.7 1 0.2 99.4

180 - 200 7,386 0.5 99.2 3 0.6 100.0

200 - 220 4,306 0.3 99.5 0 0.0 100.0

220 - 240 2,566 0.2 99.7 0 0.0 100.0

240 - 260 1,565 0.1 99.8 0 0.0 100.0

260 - 280 1,023 0.1 99.9 0 0.0 100.0

280 - 300 582 0.0 99.9 0 0.0 100.0

300 - 320 302 0.0 100.0 0 0.0 100.0

320 - 340 187 0.0 100.0 0 0.0 100.0

340 - 360 103 0.0 100.0 0 0.0 100.0

360 - 380 72 0.0 100.0 0 0.0 100.0

380 - 400 42 0.0 100.0 0 0.0 100.0

400 - 420 20 0.0 100.0 0 0.0 100.0

420 - 440 8 0.0 100.0 0 0.0 100.0

440 - 460 7 0.0 100.0 0 0.0 100.0

460 - 480 7 0.0 100.0 0 0.0 100.0

480 - 500 0 0.0 100.0 0 0.0 100.0

500 - 520 0 0.0 100.0 0 0.0 100.0

520 - 540 0 0.0 100.0 0 0.0 100.0

540 - 560 1 0.0 100.0 0 0.0 100.0
FTP Preliminary Report: March 27, 1998

165

Total 1,407,908 100.0 544 100.0
FTP Preliminary Report: March 27, 1998

166



FTP Preliminary Report: March 27, 1998

170

Combinations of speed and acceleration also can be compared

in order to judge how well the FTP "covers" the range of in-use

driving. To make this comparison, it is necessary to define in

the two-dimensional sense what is meant by FTP and non-FTP

driving. Figure 6-22 displays the speed-acceleration plane with

points corresponding to all realized combinations of these two

variables. Both variables are given to an accuracy of 0.1. 

These points can be said to define an "envelope" of driving, but

there currently is no generally accepted method of defining the

boundary of this region.

 Typical two-dimensional displays of speed and acceleration

imply that the maximum absolute value of acceleration tends to

decrease as speed increases. This suggests one possible

definition for the FTP envelope: maximum acceleration (or

deceleration) at a given speed equals the largest acceleration

(deceleration) occurring at or above that speed. The outline in

Figure 6-22 is drawn using this definition. All points on or

inside this line represent FTP conditions; all points outside are

considered non-FTP driving.

Figure 6-23 displays the scatter of speed-acceleration

points occurring in the Baltimore sample outside the FTP envelope

as defined above. These points represent about 18% of total

Baltimore driving time.

Two other views of the FTP/non-FTP contrast are useful. 

Figure 6-24 depicts the difference in the distribution of driving

time found in the FTP and the Baltimore in-use data. This draws
FTP Preliminary Report: March 27, 1998

171

attention to those combinations of speed and acceleration that

are under- or over- represented by the FTP. The distribution of

Baltimore non-FTP driving is shown in the surface graph of Figure

6-25, which helps emphasize regions of relative importance.
Figure 6-22
0 10 20 30 40 50 60

-1-2-3-4

43210

SPEED (MPH)

FTP Preliminary Report: March 27, 1998

173

FTP Speed-Acceleration Envelope
Figure 6-23
0 10 20 30 40 50 60 70 80 90 100

-10-15-20

-5

50

201510

SPEED (MPH)

FTP Preliminary Report: March 27, 1998

175

Baltimore 3-Parameter Speed-Acceleration Range

with FTP Envelope
FTP Preliminary Report: March 27, 1998

176

Figure 6-24

FTP Envelope: Difference between Baltimore and FTP

Balt % is percent of all Baltimore Driving Time
FTP Preliminary Report: March 27, 1998

177

Figure 6-25

Distribution of Speed and Acceleration

Baltimore Driving Outside FTP Envelope
 Kruse, Ronald E., and Thomas A. Huls, "Development of the Federal Urban 48

Driving Schedule," SAE Paper #730553. 

FTP Preliminary Report: March 27, 1998

178

6.3.2. Trip Comparison

The FTP driving cycle was based on a driving route developed

to represent driving patterns found on a morning commute trip in

Los Angeles, circa 1966. The original route was over 12 miles

long, considerably longer than the average trip length of 7.45

miles. In generating the FTP driving cycle, EPA shortened the

actual driving trace to 7.45 miles while maintaining the

characteristics of the longer cycle.48

Table 6-12 compares the FTP "trip" to overall trip data for

Baltimore, as well as to Baltimore's typical morning commute

trips. The FTP is longer in duration and distance than either of

the Baltimore trip measures, although the FTP most closely

resembles the Baltimore morning trips.

Comparisons can also be in terms of the proportion of time

spent in the four operating modes: idle, cruise, acceleration,

and deceleration. The percentage of idle time is slightly higher

for Baltimore compared to the FTP. Cruise operation accounts for

a little more than a third of operating time for the FTP and on

the Baltimore trips. The differences between the in-use data and

the FTP for the other operating modes are also minor.
FTP Preliminary Report: March 27, 1998

179

Table 6-12

Comparison of Trip Measures for FTP and Baltimore

Trip Measure Baltimore

FTP

All Trips Morning

commute trips

Duration (minutes)

Average 22.82 12.03 15.78

Median 22.82 8.80 13.28

Distance (miles)

Average 7.45 4.89 6.65

Median 7.45 2.53 3.91

Percent time in mode*

Idle 19.58 22.81 24.34

Cruise 36.23 34.38 34.18

Acceleration 24.18 22.46 21.89

Deceleration 20.01 20.36 19.59

Average distance 0.41 0.87 1.08

between stops

*See Section 5.1.2. for definition of modes.

As shown in Figure 6-26, the FTP consists of a series of 18

hills, or microtrips, separated by a short periods of idle. If

one thinks of the idle between the trips as a stop, the average

distance between stops on the FTP is 0.41 mile. For all

Baltimore trips, 0.87 mile is the average distance between stops,
FTP Preliminary Report: March 27, 1998

180

more than double the FTP distance. During the morning commute

period, the corresponding distance grows to 1.08 miles.

FTP Preliminary Report: March 27, 1998

182

At 7.5 miles, the FTP is longer than the average Baltimore

trip (4.9 miles); the median Baltimore trip length indicates that

"typical" trips are even shorter, only 2.5 miles. The frequency

of stops on the FTP is also uncharacteristic of in-use trips,

with more frequent stops on the FTP. Despite these differences,

the FTP and Baltimore trips disagree only slightly in the

proportion of time in various operational modes. 

 

6.3.3. Vehicle Soak

The FTP includes two soaks; a 10 minute soak and an

overnight soak. Clearly, when compared to Figure 6-15 of Section

6.2.4, the 10 minute soak in the FTP is appropriate, as 0-10

minute soaks were the largest mode in Baltimore. However, almost

40% of the soaks in Baltimore occurred with soak periods between

10 minutes and 2 hours, the most critical period for catalyst

cooldown. To illustrate this point, Table 6-13 compares the

temperatures at trip start calculated for the Baltimore data

(from Table 6-4) with engine and catalyst temperatures calculated

from the cooldown formulas for the FTP.

The data demonstrate that the FTP substantially overstates

both the proportion of starts with hot catalysts (that is,

catalyst temperatures above the light-off range) and the

proportion of starts with cold engines. The FTP implicitly

assumes that 57% of all starts occur with hot catalysts, while

the actual data indicate that only about 30% of all starts occur

with hot catalysts (as discussed in Section 6.2.4., the Baltimore

figures in Table 6-13 are likely to overstate the actual catalyst
FTP Preliminary Report: March 27, 1998

183

and engine temperatures upon start-up). The FTP also assumes

that 

Table 6-13

FTP vs. Baltimore Temperature Distribution at Trip Start

Catalyst Engine

Temperature Frequency (%) Temperature Frequency (%)

Range ( F) Range ( F) o o

Balt. FTP Balt. FTP

800+ 22 -- 180+ 51 57

700-800 9 57 160-180 7 --

140-160 6 --

600-700* 7 -- 120-140 6 --

100-120 7 --

500-600 6 -- <100 23 43

400-500 5 --

300-400 5 --

200-300 6 --

<200 40 43

*Catalyst light-off range

43% of all starts occur with cold engines, while the data suggest

that less than a quarter of all starts occur with engine

temperatures within 25 F the ambient temperature. o

From an overall emission inventory point of view, the

overstatement of hot catalysts and cold engines in the FTP are

offsetting factors which may largely cancel each other out. 

However, while the 10 minute and overnight soaks in the FTP

represent the most frequent modes of soak periods, they offer no
FTP Preliminary Report: March 27, 1998

184

incentive for manufacturers to delay catalyst cooldown. 

Catalysts remain above light-off temperatures after a 10 minute

soak and it is not practical to delay cooldown enough to obtain a

benefit after an overnight soak. Thus, there may be a "lost

opportunity" for inclusion of an intermediate soak time in the

FTP. Such an intermediate soak time, if it proved to be

feasible, would offer an incentive to improve catalyst light-off

times or delay catalyst cooldown (which could be done with

catalyst insulation). Thus, the feasibility and potential

emission benefits of an intermediate soak period should be

included in future emission assessments.

6.3.4. Trip Start Driving Activity

As indicated earlier, the FTP, overall, has lower speeds and

is less aggressive than actual driving behavior. However, the

reverse occurs during the first few minutes of driving after

vehicle starts. Both the FTP and actual driving begin with an

initial idle period (20 seconds for the FTP and variable for

actual driving, depending on the type of start and ambient air

temperature). The FTP then goes into a 105 second micro-trip

with an average speed of 23.1 MPH. The average speed during the

first 80 seconds of actual driving after the initial idle is only

14.4 mph (as reported in Section 6.2.5). The FTP follows the

first micro-trip with a 39 second idle, then launches into the

most aggressive section of the entire cycle, reaching speeds over

50 mph 200 seconds after the initial idle. The actual driving

activity during the second and third 80-second phases averages
FTP Preliminary Report: March 27, 1998

185

about 23 mph, including 27 seconds of idle, and only has about 6%

of total operation above 45 mph. 

A comparison between the FTP and the actual driving activity

from the Spokane and Baltimore data (combining cold, warm, and

hot starts), both broken down into 80-second phases, is presented

in Table 6-14.

Table 6-14

Comparison of FTP and Actual Start Driving Behavior

(by 80-second phases after initial idle)

Speed Acceleration Power % Time

at

Avg St Dev Avg St Dev Avg St Dev Idle*

Phase 1

Actual 14.4 12.3 .24 1.82 40.7 43.1 13.5

FTP 22.7 5.9 .38 1.15 34.1 0.0

Phase 2

Actual 21.6 15.3 .00 1.65 46.9 43.5 17.3

FTP 11.9 13.2 -.06 1.51 39.9 49.0

Phase 3

Actual 24.1 16.6 -.01 1.55 45.7 43.0 16.1

FTP 46.0 12.1 .35 1.03 68.2 0.0

*Not including the initial idle time.

These 80-second phase comparisons are skewed by the fact

that all of the idle time during the first 240 seconds of the FTP

(after the initial idle) occurs in one long 39-second segment

that falls in phase 2, while idle time is distributed across all

3 phases of actual driving. Thus, a more meaningful comparison
FTP Preliminary Report: March 27, 1998

186

would be to compare the first micro-trip on the FTP (Hill 1) with

phase 1 of the in-use driving and to compare the next section of

the FTP (106 to 240 seconds after the initial idle) with phase 2

and 3 combined. As the first micro-trip on the FTP is already

longer than the first phase of actual driving (105 seconds v.

80), the 39-second idle should be assigned to the second section

of the FTP to keep the initial driving comparisons as compatible

as possible. These comparisons are presented in Table 6-15.

Table 6-15

Revised FTP and Actual Start Comparison

Speed Acceleration Power % Time

 at 

 Avg St Dev Avg St Dev Avg St Dev Idle*

Phase 1 - Actual 14.4 12.3 .24 1.82 40.7 43.1 13.5

Hill 1 of FTP 23.1 7.2 .00 1.45 31.8 1.0

Phase 2+3 Actual 22.8 15.9 .00 1.60 46.3 43.3 16.7

2nd Phase of 29.8 22.6 .40 1.06 64.8 28.1

FTP**

*Not including the initial idle period.

**Starts 106 seconds after the initial idle and ends 240 seconds after the initial idle. 

Includes the 39-second idle between the 1st 2 micro-trips.

The data clearly show that the speeds on the FTP are much

higher than encountered in-use after a vehicle start. Figure 6-

27 presents the speed distributions. The solid lines compare

Hill 1 of the FTP to phase 1 of in-use driving; the FTP has far

less driving in the 0-15 mph range and much more in the 15-35 mph

range. The dashed lines compare the 2nd section of the FTP to

phase 2 and 3 of actual driving combined; the FTP has less
FTP Preliminary Report: March 27, 1998

187

driving in both 0-15 mph and 30-40 mph ranges, plus far more in

the 45-60 mph range. 

The aggressiveness of the driving is also skewed, with the

first micro-trip on the FTP less aggressive than Phase 1 and the

second micro-trip on the FTP clearly overaggressive. Figure 6-28

compares the cumulative power distributions (that is, the amount

of driving occurring at or below a given power level). Hill 1 of

the FTP spends less time at the higher power levels (i.e. the

line rises faster) than Phase 1 of actual driving, while the 2nd

section of the FTP spends far more time at the higher power

levels. The speed and power data clearly show that the FTP 


FTP Preliminary Report: March 27, 1998

190

contains speed/acceleration events that are seldom encountered

in-use during the first 240 seconds of operation.

The impact of driving behavior immediately after a vehicle

start on exhaust emissions is subject to interpretation. Higher

speeds and loads increase the mass flow through the engine, which

would cause more engine out emissions to pass through the

catalyst unconverted during warm or cold starts. On the other

hand, the engine and catalyst heat up faster during higher mass

flows, decreasing the length of time needed for fuel enrichment

and catalyst light-off. Compounding the assessment is the fact

that the first micro-trip of the FTP has much higher speeds than

are encountered in-use during a similar period after a start, but

is also less aggressive. The overall impact of these offsetting

factors should be investigated in future emission assessment and

testing programs. Of course, for compatibility with cycles

developed for hot stabilized operation, any start cycle developed

for use in emission assessments should be based only on the

Baltimore 3-parameter data.

Initial idle time

Table 6-16 presents the initial idle times for

Spokane/Baltimore (combined), Atlanta, and the FTP.
FTP Preliminary Report: March 27, 1998

191

Table 6-16.

Initial Idle Times 

Start City Mean Median

Type (seconds) (seconds) 

Cold Start Spokane/Baltimore 59 13

Atlanta 28 9

FTP 20 20

Warm Start Spokane/Baltimore 28 8

Atlanta 18 7

FTP N/A N/A

Hot Start Spokane/Baltimore 15 5

Atlanta 12 5

FTP 20 20

The data show that the initial idle time after a cold start

on the FTP falls between the mean and median initial idle times

for Atlanta. Thus, the cold start initial idle time appears to

be reasonable. However, the initial idle time after a hot start

on the FTP is substantially longer than the mean initial idle

time in Atlanta, and four times as long as the median time in all

the cities.

Proportion of Driving Occurring After Starts

Figure 6-29 compares the distribution of miles driven since

the start of the trip on the FTP to the actual distribution. As

the FTP is one trip of 7.5 miles, its distribution appears as a

straight line up to 7.5 miles. Due to substantial numbers of

trips 

FTP Preliminary Report: March 27, 1998

193

longer than 7.5 miles in-use, the actual distribution shows a

large discrepancy in the range of 5-10 miles from the start of

the trip. However, this is not likely to be of concern from an

emission point of view, as the engine and catalyst are fully

warmed up and stabilized after 1-3 miles of operation. Whether

the driving after this point occurs 3-7.5 miles from the start of

the trip or extends out to higher mileages should not impact the

level of hot stabilized emissions.

The differences between the FTP and actual driving less than

one mile from the start of the trip are of much more interest, as

this relates to the amount of driving that occurs before

catalysts or engines have warmed up. While these differences are

much smaller in terms of absolute frequency than the differences

at higher miles, the relative differences are substantial. Table

6-18 compares the FTP distribution of the first mile driven from

the start of the trip to the distribution recorded in Baltimore.
FTP Preliminary Report: March 27, 1998

194

Table 6-17

FTP v. Baltimore Comparison of Miles Driven from Start of Trip

Mileage Range from FTP Baltimore In-Use 

Start of Trip Proportion Proportion Increase (%)

0-0.25 3.33 % 4.78 % 43 %

0.25-0.50 3.33 % 4.42 % 33 %

0.50-0.75 3.33 % 4.09 % 23 %

0.75-1.00 3.33 % 3.81 % 14 %

0-0.67 8.93 % 12.01 % 34 %

(Equivalent to Hill 1

of FTP)

It is clear that the FTP substantially underestimates the

amount of time vehicles actually spend in the initial portions of

a trip.

6.3.5. Road Grade

The current FTP speed-time trace was developed without

attention to the effects of road gradient. The original road cycle

in Los Angeles contained at least one freeway on-ramp (Harbor

Freeway, southbound), which was no doubt approached at some rate of

acceleration, yet the transcription of the road cycle to a

dynamometer trace assumed a flat surface. Thus, the additional

engine loads caused by operation on freeway on-ramps and other

gradients are not adequately represented.
FTP Preliminary Report: March 27, 1998

195

Regardless of the current limitations of road gradient data,

a comparison of in-use driving with the FTP is relatively easy: the

FTP is flat, the world is not. The DOT data presented in Table 5-1

illustrates, in nationwide terms, that fully 30% of VMT are

traveled on gradients greater than 2 percent, and almost half occur

on gradients greater than 1 percent. It is quite clear that

emission effects due to road gradient are not represented on the

current FTP and that this should be an area of concern for the

Agency, but it is also clear that more work is needed to better

identify these effects and develop a better understanding of the

amount of driving that occurs in urban areas on various road

gradients.
FTP Preliminary Report: March 27, 1998

196

Chapter 7. Test Cycle Development Methods and Approach

7.1. Test Cycle Methods

A driving cycle is a time series of vehicle speeds occurring

at successive (equally spaced) time points. For emission testing,

a test driving cycle attempts to synthesize real driving conditions

with respect to a number of measures, including speed,

acceleration, specific power, trip patterns, road grade, and

temperature. As a practical matter, the cycle must be of

reasonable length and it must be "driveable," that is, consist of

speeds and accelerations that can be physically realized by

vehicles running on a dynamometer.

Following review of the adequacy of the current Federal Test

Procedure, EPA will determine whether the driving cycle represented

by the FTP should be modified to ensure that future testing better

represents actual in-use driving. The comparison of the FTP with

results of the driving behavior research presented in Chapter 6 of

this report will be a central consideration in developing and

judging new test cycles. A full assessment will require combining

these pure driving measure comparisons with emission test data.

Generation of a specific cycle requires the choice of a method

for selecting the speed sequence. Researchers and practitioners

have proposed and used a number of different methods. These are

briefly reviewed in the current section. In the following

sections, development of a sample cycle is discussed, together with

EPA's plans for new cycle generation.
FTP Preliminary Report: March 27, 1998

197

Drive cycle development aims to satisfy two general goals of

vehicle emission testing: emission control and emission inventory.

Emission control strategies are designed to limit emissions

occurring across the range of actual driving behavior and

conditions. Emission inventory estimates are obtained by combining

vehicle test results with fleet and mileage information. This

distinction has an important implication for test cycle

development. A cycle used to achieve emission control does not

necessarily have to be representative of overall in-use driving.

It may artificially emphasize aspects of driving behavior most

critical to emission control.

7.2. Method Types

The report bibliography cites previous work on drive cycle

method and development. Most of this research is of an applied,

empirical nature, with little substantial theoretical basis.

Method that have been studied fall into two general categories:

segment-splicing and Monte Carlo simulation. A third method might

be called the "engineering" approach, whereby a cycle is developed

using criteria that do not necessarily include the frequency of

occurrence of in-use driving patterns.

7.2.1. Segment-splicing Methods

Probably the most widely accepted approach to cycle generation

is to select and splice together segments of observed speed-time

trace data that satisfy some set of driving behavior criteria. The

current FTP was created by applying this method to speed-time data
 Kruse, R.E. and T.A. Huls, "Development of the Federal Urban Driving 49

Schedule," SAE Technical Paper Series, 730553, 1973.

FTP Preliminary Report: March 27, 1998

198

collected during morning rush-hour driving in Los Angeles,

California. 49

Perhaps the greatest appeal of the segment-splicing approach

lies in its use of real driving sequences that are known to be

driveable (although it is not necessarily true that all on-road

driving can be replicated on laboratory dynamometers). This method

does, however, raise a number of questions and problems. The most

obvious issue is the selection of segments in order to adequately

meet cycle requirements. In practice, this has been performed by

sampling in a variety of ways, at random or otherwise, from an inuse database.

Once a set of segments is chosen, they must be connected to

produce a single cycle. If the segments are full "microtrips"

(extending from one idle to another), this splicing operation is

straightforward and is not likely to jeopardize the driveability of

the full cycle. However, it sometimes is desirable to use only

parts of microtrips (called kinematic sequences or modal segments)

which begin and/or end at non-zero speed. Connecting such segments

must be done more carefully in order to achieve realistic driving

behavior.

7.2.2. Monte Carlo Simulation

Simulation provides an alternative method for generating a

drive cycle. In a Monte Carlo simulation, a model is established
 Smith, M. Heavy-Duty Vehicle Cycle Development, U.S. Environmental Protection Agency (EPA-460/3- 50

78-008), 1978.

FTP Preliminary Report: March 27, 1998

199

for describing how actual driving occurs. The model includes both

deterministic and probability, or "stochastic," elements to account

for known physical properties of driving as well as unexplained

random phenomena. Using random number generation to simulate the

probability component, a time sequence of speeds is produced with

characteristics resembling those of the underlying model.

The principal challenge in applying this technique is to

create a suitable model. The typical model will be estimated by

fitting a set of assumptions to in-use data. For example, in

developing EPA's heavy duty test cycle, second-to-second speed 50

transitions were assumed to obey a "Markov" process, in which the

probability of a given speed at the end of the second depends only

on the vehicle's speed at the end of the previous second. These

conditional probabilities were estimated from in-use driving data

frequency distributions. However, evidence exists that the model

used in that work is not sufficiently complex to capture all

important emission-related elements of real driving, such as the

duration of acceleration events. Even the current large sample of

second-by-second data may not permit accurate estimation of

parameters needed to describe such behavior.

The Monte Carlo simulation approach offers considerable

flexibility in modeling driving behavior and generating cycles. It

is less popular than the segment-splicing method for the reasons

given and because resulting cycles are "artificial" - the secondto-second sequences have not actually been driven in-use. This
FTP Preliminary Report: March 27, 1998

200

raises the fear that a simulated cycle may not be driveable or, in

emission testing, may yield emissions that do not or cannot occur

in real driving.

7.2.3. Engineering Cycles

An engineering cycle is a speed-time trace that satisfies

certain criteria not generally based on the frequency distribution

of in-use driving behavior. For example, it may be the designer's

judgment that a cycle should include certain extreme levels of

speed, acceleration, or specific power. These objectives can be

achieved without reference to in-use data, possibly using simple

"straight-line" construction methods. This approach to cycle

development is useful as a way of forcing certain conditions that

may occur infrequently, or to implement feasibility testing.

7.3. Cycle Criteria

Most applications of cycle methodology have attempted to

represent the full range of second-to-second driving behavior; that

is, all combinations of speed and acceleration likely to occur inuse. In some circumstances, it may be desirable to construct a

cycle that overrepresents various types of driving. In particular,

in considering modifications to the FTP, a cycle with a

disproportionately high amount of non-FTP driving has potential

advantages. This poses special problems, especially for the

microtrip-splicing method, because it may be difficult to find

largely non-FTP microtrips that are sufficiently short in length to
FTP Preliminary Report: March 27, 1998

201

obtain a cycle of realistic duration. In splicing of modal

segments or Monte Carlo methods, this is less of a concern.

7.4. Cycle Validation

Once a cycle is generated by either the segment-splicing or

Monte Carlo method, it is necessary to test how well it satisfies

the objectives of the problem. Usually, this involves matching

various characteristics of the cycle to comparable features of inuse driving. The features may include the summary statistics and

distributions used in Chapter 6 to describe the Baltimore 3-

parameter instrumented vehicle sample. A cycle is considered valid

if its statistical properties are reasonably close to those of the

data supporting its generation. In practice, this involves

selecting a set of arbitrary statistical tests (or "filters"). A

cycle that passes these tests at a desired level of significance

becomes a candidate for testing. This process is normally followed

by a more qualitative evaluation to assure selection of a realistic

cycle.

7.5. Current EPA Test Cycle Development

As part of the current study of non-FTP driving, EPA has

undertaken the development of new test cycles using the survey data

described in this report. This effort includes reviewing cycle

development methods and cycle validation techniques and programming

of cycle generation algorithms. Through two of its contractors,

the Agency currently is engaged in creating cycles for use in
FTP Preliminary Report: March 27, 1998

202

emission testing and as candidates for new regulation, pending

final conclusions from this study.
FTP Preliminary Report: March 27, 1998

203

List of Appendixes

The following appendixes are presented in their order of first

occurrence in the text of the report.

Appendix A. Chase Car Method Bias Analysis: Supplementary Tables

Appendix B. Baltimore 3-Parameter Vehicle Characteristics

Appendix C. Summary Statistics, Distributions, and Graphs

Appendix D. Baltimore Soak Periods

Appendix E. Trip Start Driving Activity