NBSIR 74-595

A Procedure for Estimating

Automobile Fuel Consumption on

Congested Urban Roads

David M. Levinsohn, James T. McQueen

National Bureau of Standards

Technical Analysis Division

Urban Systems Program Area

August 1974

Final Report

Prepared for Urban Mass Transportation Administration

Department of T ransportation

Washington, D. C. 20591
\
NBSIR 74-595

A PROCEDURE FOR ESTIMATING

AUTOMOBILE FUEL CONSUMPTION ON

CONGESTED URBAN ROADS

David M. Levinsohn, James T. McQueen

National Bureau of Standards

Technical Analysis Division

Urban Systems Program Area

August 1974

Final Report

Prepared for Urban Mass Transportation Administration

Department of Transportation

Washington, D. C. 20591

U. S. DEPARTMENT OF COMMERCE, Frederick B. Oent. Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

TABLE OF COOTENTS Page

1 . Introduction 1

1.1 Discussion o£ the Problem 1

1.2 Purpose 1

1.3 Organization 1

2. Review of Literature and Research in Progress 1

2.1 Literature 1

2.2 Research in Progress 3

3. Factors Which Affect Auto Fuel Consumption 4

3.1 Vehicle Attributes 4

3.2 Roadway Operating Conditions 5

4. A Procedure for Estimating Automobile Fuel Consumption 7

4.1 The Procedure 7

4.2 Aggregation of Variables 8

4.3 Summary of Procedure and Example 11

5. Recommended Approach to Data Collection 14

iii
ABSTRACr

Energy consumption is an important measure o£ the performance o£ a transportation

system. To be able to accurately measure associated automobile fuel consumption will improve the evaluation of urban transportation alternatives. An estimation procedure is proposed that is designed to be particularly sensitive to automobile fuel consumption in con- gested, peak hour traffic. This procedure is based upon vehicle attributes and roadway

operating conditions which were determined through an extensive review of the auto fuel

consumption literature. Vehicle attributes include characteristics of the automobile that

affect fuel consumption. Roadway operating conditions comprise the types of driving to

which the automobiles are subjected. Vehicles are classified by weight and model year. The

proposed roadway classifications are expressway, arterial, and local street. For each vehicle type category, base fuel consumption rates are determined. These base consumption rates are then modified by adjustment factors which reflect the roadway operating conditions. The

rates are multiplied by the vehicle miles of each vehicle category and summed over all categories to compute the total fuel consumption on the road under analysis. An example application of the procedure including sensitivity analyses is presented. The base fuel consumption rates can be obtained from EPA emissions test data. Research is required to determine

the adjustment factors, particularly under conditions of extreme roadway congestion.

iv
1. INTRODUCTION

1.1 DISCUSSION OF THE PROBLEM

The conservation of automobile fuel is an important benefit of mass transportation, and

an essential statistic of each transit alternative should be the estimated saving in automobile gasoline. Most transit improvements focus upon peak period traffic conditions; however, present techniques for estimating auto gasoline consumption do not adequately account

for this rush hour operating environment of the vehicle (roadway design characteristics,

volume, capacity, etc.) or the characteristics of the vehicles which make up today's traffic volume flows (engines, weight, age, etc.).

When the volume of vehicles on a roadway approaches capacity, congestion occurs. One

effect of congestion is that vehicles, because of frequent needs for deceleration, are not

able to maintain their attempted speeds. This causes an increase in fuel consumption.

Tlius, if congestion is not considered, auto gasoline consumption is understated.

Estimation problems are also caused by the obsolescence of the available data. Vehicle

characteristics have changed considerably since 1967. Most are heavier, and all are fitted

with emissions control devices. Since each of these changes affects gasoline consumption,

estimation procedures developed using pre-1967 data will produce inaccurate results.

1.2 PURPOSE

The ultimate requirement of this and related research is to provide procedures for estimating automobile fuel consumption that may be used by transportation planners in evaluating alternative transportation strategies in congested urban corridors. The purpose of

this report is to summarize the existing literature and research in progress to develop a statement of current knowledge of automobile fuel consumption, to identify inadequacies in

existing techniques that lead to inaccurate estimations of consumption, and to recommend

approaches for the elimination of the inadequacies.

1.3 ORGANIZATION '

The report is divided into five sections. A discussion of the problem of estimating

fuel consumption is found in this section. Section 2 reviews the existing literature and

research in progress on automobile fuel consumption. The third section presents an analysis of deficiencies of existing estimation procedures, including a discussion of the factors affecting fuel consumption. Section 4 proposes a procedure for estimating automobile

fuel consumption based upon the analysis in Section 3. The fifth and final section i-ecommends a data collection procedure for input to the proposed estimation procedure and recom- mends areas for further research.

2. REVIEW OF LITERATURE AND RESEARCH IN PROGRESS

The estimation of automobile fuel consumption has been the subject of research in various areas of the automobile industry and transportation field for several years. Recently,

added importance has been attached to fuel consumption estimation as a factor in the evaluation of urban transportation alternatives. The purpose of this section is to review both

the related literature and research to determine the state-of-the-art and discover any inadequacies which might exist in present methods for estimating auto fuel consumption.

2.1 LITERATURE

The rate at which an automobile consumes gasoline is influenced by the environmental

conditions in which it operates as well as by the design of the automobile. Thus, a procedure for estimating fuel consumption should consider the characteristics of both the automobiles and the environmental conditions of the roadway on which they operate. This sec- tion reviews the related literature and research to determine the degree to which auto fuel

1
estimation procedures account for diversity o£ both the automobiles and the operating con- ditions which are likely to be encountered.

The great majority of the published research concerning automobile fuel consumption has

been produced or sponsored by either: 1) the automobile industry, 2) the Environmental

Protection Agency, or 3) the transportation planning profession. While each is responsible

for a substantial amount of research in the field of automobile fuel consumption, each

source offers a different perspective on automobile fuel consumption and the analysis of

factors affecting the rate at which fuel is consumed.

1. Automobile Industry

The automotive industry, in papers published by the Society of Automotive Engineers

(SAE) , generally considers the problem of fuel consumption from the point of view of

the individual vehicle. (2,4) These papers are similar in that they contain discussions of the effects of changes in vehicle design characteristics upon fuel consumption. Such characteristics as size, weight, engine design and axle ratio were examined, and the relationship between each characteristic and fuel consumption was isolated and quantified. The majority of the tests in these studies used constant

driving cycles. 1 Tlierefore, data on vehicle fuel consumption for different traffic

volume and type of roadway combinations have not been produced by these industry

studies.

A typical paper from the automotive industry was written by G. J. Huebner, Jr., of

the Chrysler Corporation entitled "Energy and the Automobile -- General Factors Affecting Vehicle Fuel Consumption." (2) A 3600 lb vehicle was used, and the effects

of engine efficiency and displacement, compression ratio, torque converter, transmission type, axle ratio, aerodynamic drag, tires, accessories, vehicle weight, and

emissions control system upon fuel2Consumption were investigated for both a 70 mph

cruise and an urban driving cycle. Huebner concluded that since 1968, vehicle

weight increases and emissions controls had reduced fuel economy substantially,

while the other factors had had little effect on fuel economy. The effects of road

conditions were not investigated for either case.

2. Environmental Protection Agency

The Environmental Protection Agency's (EPA) investigations of automobile fuel con- sumption have been conducted along with the agency's tests of automobile emissions

controls. Federal certification is required for all new cars sold in the United

States, and fuel consumption data are a tangential output of these tests. The data

are generated on a chassis dynamometer which simulates ,3 in a controlled environment, an urban driving cycle of 7.5 miles in length with attendant changes in speed

and rates of acceleration and deceleration. Fuel consumption is not measured directly, but is computed using the carbon balance method. This method computes con- sumption by measuring the amount of carbon in the engine exhaust and comparing it

with the amount in the gasoline burned. The carbon balance method has been found to be within an average of 4.5 percent of fuel consumption when measured directly. (3)

A constant driving cycle is one where the same pattern of accelerations, deceleration, and

constant speeds are repeated for each vehicle.

»'An urban driving cycle simulates the accelerations, decelerations, stops, and speed

changes most frequently encountered when driving in urban areas. This typically includes

many starts and stops, and maximum speeds under 40 mph.

'a chassis dynamometer is a machine which allows a vehicle to be operated in a laboratory

environment. It allows the wheels to turn and simulates the inertia of the vehicle, but

since the vehicle remains stationary, rolling resistance and air resistance are not simulated.

2
Since the consumption data has been collected by model year and vehicle model type,

the data include considerable information about the change in consumption by model

year and vehicle weight. However, as with research by the automotive industry, the

effects of traffic volume and roadway conditions have not been investigated. Thus,

data on vehicle performance under variable roadway conditions have not been produced.

3. Transportation Planning

In the discussions on automobile fuel consumption, the transportation planning literature considers the effect of roadway operating conditions (as opposed to vehicle

characteristics) upon fuel consumption. The primary focus is the effect of such

characteristics of the operating environment as grade, curvature, volume, speed, and

traffic signals. Paul Claffey, in NCHRP 111, has produced one of the most comprehensive studies to date from the transportation planning profession. (5) Auto gasoline consumption was measured for varying operating conditions, including road type,

grade, curvature, speed, surface condition, speed changes, and traffic volume.

NCHRP 111 provides tables of fuel consumption rates as functions of the roadway operating environment. These rates are based upon a standard size car, as represented

by a 4400 lb. 1964 Chevrolet sedan. The report also includes measurements of con- sumption for smaller automobiles in the appendices, but the measurements do not re- flect all of the roadway operating conditions. Claffey 's work is typical of the

other literature in transportation planning in that it does not account for the

variation in fuel consumption as a function of vehicle characteristics, the analysis

being performed on the "typical", or most-used vehicle.

2.2 RESEARCH IN PROGRESS

Research on gasoline consumption for use in transportation planning is currently concentrated in the Transportation Systems Center (TSC) of the Department of Transportation. A

recent effort for the Federal Energy Administration (FEA) involves the collecting of data

on automobile fuel consumption for use in several computer models which test the energy impacts of alternative national policies affecting transportation. These data are being

gathered by a firm that specializes in developing vehicle operating cost data for large

private fleet owners. Fuel consumption data are being collected for model years 1965-1974

for 14 U. S. automobiles and 4 foreign models. The autos are divided into four size

classes: subcompact, compact, standard, and full size. General operating conditions are divided into three classes: mostly urban driving, mostly suburban driving, and mostly

interstate driving. The data are also stratified by climatic conditions into three ambient

temperature ranges. Although stratified explicitly by vehicle type, the data do not directly reflect specific urban operating conditions of road type and traffic volume, as these are imbedded in the three general operating categories.

Another study currently underway at TSC is a computer simulation of an automobile to investigate the effects of various parameters upon fuel consumption. Vehicle characteristics such as weight or operating conditions such as speed may be varied to determine the

effect upon the fuel consumption rate of the simulated vehicle. This simulation model is

also being used to investigate the trade-offs in fuel consumption among vehicle design options, e.g., adding weight to the vehicle without changing engine size or adding weight but

maintaining performance by increasing engine displacement.

Also underway at TSC is a study to investigate alternative procedures for measuring fuel

consumption. In the event that the Government requires auto manufacturers to label their

cars with respect to fuel economy, a uniform measurement procedure will be required, and

answers are being sought for such questions as how to instrument the test vehicle and how

to test it for fuel consumption.

In summary, the existing work on estimating automobile fuel consumption has concentrated

either on the individual vehicle or the roadway environment. Some have analyzed the attributes of the vehicle to determine their effects upon consumption. Other studies have examined the operating environment of the vehicle to determine its effect upon consumption.

However, accurate estimations of the fuel consumption of auto volumes require a knowledge

3
of the joint effect of vehicular attributes and roadway operating conditions, and, to date,

there has not been a synthesis of those two areas of research. Section 4 will introduce an estimation procedure that incorporates both vehicle characteristics and operating conditions in order to more accurately estimate fuel consumption under a number of cases which

are likely to be encountered during peak periods in urban areas.

FACTORS WHICH AFFECT AUTO FUEL CONSUMPTION

The variables or factors which affect fuel consumption will be divided into two categories: vehicle attributes and characteristics of the roadway operating environment. Vehicle attributes are those characteristics of the automobile that directly affect the amount

of gasoline consumed by that vehicle and include weight, engine size and design, shape,

rolling resistance, and accessories. Characteristics of the roadway operating environment

are other factors which affect consumption and include temperature, road type, traffic

I'olume, speed, number of traffic signals, grade, curvature, and the condition of the pavement. While all attributes and roadway operating conditions affect fuel consumption to some degree, the analysis reported in this document will be limited to those attributes and

conditions which have the largest effect on automobile fuel consumption in the urban area. These are automobile weight and design, traffic volume, and road type.

3.1 VEHICLE ATTRIBUTES

Weight has been found to be the single most important vehicle attribute affecting fuel

consumption (1,2). In general, a car with inertia weight of 5000 lbs will consume twice as much gasoline as a car weighing 2500 lbs (under the same operating conditions) because the

engine must perform more work to propel a heavy vehicle than a light vehicle. Another

study found that vehicle weight was correlated tvith fuel consumption with a simple corre- lation coefficient of 0.93. (3)

Data published by the Environmental Protection Agency (EPA) indicates that fuel consumption varies with changes in vehicle model year. (1,3) Model year is actually a surrogate

which reflects automotive design and performance changes which affect fuel consumption.

Major changes have recently been caused by the need to meet Federal emissions standards and

include spark retardation and exhaust gas recirculation. The effects of these changes on

fuel consumption are not readily apparent. There has been a trend of increasing fuel con- sumption from 1967 to the present for vehicles weighing more than 3500 lbs and of slightly

decreasing fuel consumption for vehicles under 3500 lbs. A 4500 lb 1973 model automobile

had nearly a 13 percent greater fuel consumption rate than a similar 1967 model of the same

weight. (3)

Automobile accessories, such as power steering, power brakes, automatic transmissions,

and air conditioning affect fuel consumption. Of these, air conditioning and automatic

transmission have been found to affect fuel consumption most. (1,2) Also, both are increasingly prevalent accessories with 72.6 percent of 1973 American-made cars being sold

with air conditioning and 93.4 percent with automatic transmissions. (9)

Air conditioning has a two- fold effect upon consumption: first, the added weight of

about 100 lbs for the unit results in a 1 to 2 percent fuel consumption penalty. (Smaller

cars are affected more by the added weight than are larger cars.) Second, a much larger

penalty is associated with the operation of air conditioning, as the compressor consumes

energy. The effect upon consumption depends upon the environment of the roadway, driving

speed, and the ambient temperature. The average increase in consumption has been found to be approximately nine percent with a maximum up to 20 percent. (1) Another study measured

a 14 percent increase in fuel consumption with the operation of the air conditioning of a 3600 lb car during an urban driving cycle. (2) Automatic transmissions do not have as great an effect upon fuel consumption as air conditioning, with an average penalty of only

about two percent. (1) Lighter weight cars with smaller engines, however, show up to a six

percent penalty. (4)

4
5.2 ROADWAY OPERATING CONDITIONS

The two most important characteristics of the roadway operating environment, road type

and traffic volume, are interrelated in their effect upon automobile fuel consumption.

Roads have been categorized into three types for fuel consumpt-ion analysis: expressways,

arterials, and local streets in downtown areas (referred to hereafter as Central Business

District (CBD) streets). In free flowing traffic conditions, the road type does not have

an effect upon fuel consumption at a given speed; however, if there is congestion, vehicle

fuel consumption will vary with road type.

Traffic volume is related to road type in as much as the volume of traffic that constitutes congestion varies according to the type of road and the number of lanes on that road.

The Claffey study determined that increases in volume can increase fuel consumption by as much as 9 percent on an expressway and by as much as 21 percent on a CBD street. Since

these statistics do not include fuel consumption rates under extreme traffic congestion,

service levels E and F (Table 1) , they understate the potential effect of congestion on

fuel consumption.

Speed is another factor that has a direct effect upon automobile fuel consumption. Much

has been written on the effect of speed upon fuel consumption on the open road; many also

present fuel consumption rates as functions of speeds which can be maintained on the open

road. However, an average speed that could be maintained in free flowing traffic on the

open road might differ substantially from a similar one attained during congested peak period traffic. This is due to the interaction of an individual vehicle with other vehicles

and traffic control devices. This interaction requires that speed be considered in con- junction with traffic density and road type. The Claffey study showed that the dimension

of speed that is important when related to traffic volume is the attempted speed of the

automobile. 4 The study showed that fuel consumption at attempted speeds of 45 mph and 60

mph on an expressway (under the same traffic conditions - Level of Service D) can differ by

as much as 8.5 percent. On an urban arterial the difference between 30 mph and 40 mph was

12 percent (Level of Service D) . Again, these values do not include extreme traffic con- gestion.

The prediction or estimation of fuel consumption on a road is dependent upon the attributes of the vehicles in the traffic stream and the roadway operating conditions. The most

important vehicle attributes affecting fuel consumption are vehicle weight and vehicle

model year. The most important roadway operating conditions are road type, traffic volume,

and attempted speed. The problem is that, to date, no method nor data are available so that all of these factors can be simultaneously taken into account.

Attempted speed is the speed at which the driver of a vehicle would like to travel but

might not be able to due to slower vehicles in a heavy traffic stream which block his path.

It can be approximated by the posted speed limit of the road.

5
Table 1

LEVEL OF SERVICE

LEVEL OF SERVICE DESCRIPTION

A Free flow with low volumes and high speeds.

B Stable flow with operating speeds beginning

, ,. to be restricted somewhat by traffic condi-

, .

> tions.

C , r Stable flow but speeds and maneuverability

are more closely controlled by the higher

volumes.

D , • Approaching unstable flow with tolerable

. . operating speeds being maintained though

; considerably affected by changes in opera-

. ting conditions.

E • * Flow is unstable with volumes at or near

• '

: capacity of the road.

F , , . : Forced flow operation at low speeds where

volumes are below capacity. Speeds are reduced substantially and stoppages may

occur for short or long periods of time

because of doivnstream congestion.

Source: "Highway Capacity Manual", Special Report 87, Highway Research Board, Washington,

D. C. , 1965

6
4. A PROCEDURE FOR ESTIMATING AUTOIOBILE FUEL CONSUMPTION

This section discusses a proposed procedure for estimating automobile fuel consumption.

This procedure will require weight, model year and accessory (primarily air conditioning)

information about vehicles in the flow of traffic; and road type, traffic volume, and at- tempted speed information about the roadway operating environment.

Ideally, total automobile fuel consumption can be computed by multiplying vehicle miles

traveled by appropriate consumption rates (expressed in gallons per mile) which are functions of the five previously discussed variables:

C = ZVM CR.

I where: I = indices for vehicle type, traffic

" volume, road type, speed, and accessory

usage

VM = vehicle miles

CR = consumption rate

The critical question, of course, is how to specify the function CR.

4.1 THE PROCEDURE

Present procedures are inadequate for estimating the fuel consumption of autos traveling

over either of the three road types under high traffic volumes. Data and procedures for

correcting the deficiency are discussed in the remainder of this document.

The preceding discussions have presented factors to be considered in the estimation of

automobile fuel consumption with a specific flow of traffic. 5 Paragraph 4.1 presents a com- putational procedure v\liich follows the approach presented in NCHRP 111. The idea is to develop base auto consumption rates and to modify these base rates with adjustment factors

which reflect the operating environment of the roadway. The product of the base rates and

the adjustment factors will constitute a gasoline consumption rate function which incorporates both vehicle and roadway operating conditions. Estimated gasoline consumption will

be given by:

(2) C = Z (BC • A ) (VM ) w,my w,my,r,t,s,a w,my,r,t,s,a

= Z (CR ) (VM ) w,my,r,t,s,a w,my,r,t,s,a

^This estimation procedure was developed for automobiles but could be generalized to include

trucks as well.
where:

C = total fuel consumption (gallons)

CR = consumption rate for each combination of weight and

model year category, road type, traffic volume,

attempted speed, and accessory usage

BC = base fuel consumption rate in gallons per mile

for each weight and model year category

VM,A = vehicle miles and adjustment factors for each

combination of weight and model year category,

road type, traffic volume, attempted speed,

and accessory usage

a = accessory usage

w = vehicle weight category

r = road type ^

.'

my = vehicle model year category

t = traffic volume

s = attempted speed

4.2 AGGREGATION OF VARIABLES

A preliminary investigation was undertaken to determine a potential stratification of

the variables and the sensitivity of the estmation procedure to different stratifications.

1. Vehicle Attributes

The stra1;;ification of consumption rates by vehicle weight and model year was based

upon consumption data presented by Hellman and Austin of EPA. (3) The data were

stratified by model year from 1957 to 1974, and by weight from 1750 to 5550 lbs.

The consumption data were presented by individual model year and by weight intervals

of 250 lbs from 1750 to to 3000 lbs and by 500 lb intervals from 3000 to 5500 lbs, a total of 18 model year categories and 11 weight class categories. (The entire

data set is presented in Table 2)

Further examination of the EPA data revealed that the greatest changes in fuel con- sumption occurred between the 2750 and 3000 lbs categories and the 4000 and 4500 lbs

categories. Therefore the weight classes have been aggregated into three new categories: less than 3000 lbs, between 3000 and 4000 lbs, and greater than 4000 lbs.

Analyzing the model year stratification in the same manner, the largest changes oc- curred between 1967 and 1968, 1970 and 1971, and between 1973 and 1974. Austin and

Hellman aggregated the 1957-1967 data in their analysis because these were the model

years that had no Federal emissions requirements to be met.

Retaining this aggregation as one category, the new categories for model year are: up to and including 1967, 1968 through 1970, 1971 through 1973, and finally 1974.

Therefore, the number of vehicle weight model year categories has been reduced from

198 to 12. The sensitivity of the estimation procedure to a further aggregation of the vehicle

attribute data was also briefly investigated. The methodology employed in this sen- sitivity analysis involved using the estimation procedure with the aggregate data

set (the 12 data points) and aggregating the weight categories while maintaining the

weight stratification. In each case, the estimates of total fuel consumption were

compared with thie estimate produced during the full aggregate data set. It was

8
found that aggregating the weight categories into a single 4500 lb class (standard

size automobile) while maintaining model year stratification resulted in a 20.3

percent higher estimate of total fuel consumption than when all 12 data points were

used. By contrast, aggregating model year into a single year, 1969, resulted in

only a 2.0 percent lower estimate of total consumption than was obtained using the

full aggregate data set.

2. Roadway Operating Conditions

The only study to date that has collected and analyzed fuel consumption data for a broad range of roadway operating conditions is presented in Na^RP 111. (5) The

analysis of the sensitivity of fuel consumption estimation to roadway operating con- ditions is based upon this work.

The fuel consumption data in NCHRP 111 are stratified by three road types: six lane

expressway, six lane urban arterial with no curb parking, and a six lane CBD street

with parking in both curb lanes. The data are further stratified by traffic volume,

attempted speed, and number of traffic control signals per mile.

a. The volumes on each road type produce conditions that range from Level of Service A through Level of Service D. Data are not available for Service Levels

E and F. (Refer to Table 1, page 6.)

b. The attempted speeds are given on each road type in the ranges appropriate for

that particular type of facility, i.e., 45-60 mph on expressways; 30-40 mph on

the urban arterial; and 25 mph on the CBD street.

c. The number of traffic control signals per mile are grouped according to the road

type, with none on the expressway; between 0 and 2 on the urban arterial; and

between 0 and 10 on the CBD street.

The sensitivity of the estimation procedure to stratification of road type may be

investigated by examining the data for a given Level of Service, speed, and number

of signals per mile to ascertain the differences in consumption. The reported data

showed a potential difference of 53 percent between the urban arterial and the CBD

street for equal level of service and speed. The effect of traffic volume was examined for each road type by comparing per mile consumption rate at service levels A

and D and at the same attempted speed. For the expressway a 7 percent increase was

observed as increased volumes pushed the service level down from A to D; no differ- ence was observed on the arterials; and a 21 percent increase was observed on the

CBD street.

An increase in the attempted speed on an expressway from 45 mph to 60 mph produced

9 percent higher fuel consumption while an increase from 30 mph to 40 mph on an urban arterial resulted in a 39 percent increase in fuel consumption (both at Level

of Service D)

Finally, an increase in the number of traffic control signals per mile from 0 to 2 produced a 58 percent increase in consumption on an urban arterial, and an 180

percent increase in consumption for an increase of from 0 to 10 signals per mile on

a CBD street.

The tentative conclusions that may be reached about the necessary stratification of

roadway operating conditions suggest that road type should be differentiated, and

the three types presented in NCHRP 111 (expressway, urban arterial, and CBD street) will be retained in this study. The effect of traffic volume is not as clear as the

effect of road types; however, Claffey did not gather data for Levels of Service E and F which intuitively would seem to have a more pronounced effect upon consumption

than Levels of Service A through D. Further investigation of traffic volume effects

seems necessary and will be discussed in Section 5.

9
TABLE 2

FUEL ECONOMY IN MILES PER GALLON FOR VARIOUS

MODEL YEARS AND INERTIA WEIGHT CATEGORIES

(--INDICATES NO DATA)

INERTIA WEIGHT

MODEL

YEAR 1750 2000 2250 2500 2750 3000 3500 4000 4500 5000 5500

57 -- 26.,4 -- -- 14,,7 13.0 12.5

58 -- 25,,3 18.2 -- 13.,2 13,.6 15.2 12,. 5 8,.6 --

59 -- 28.,6 -- -- 15..2 15,,0 13.2 12,.7 13,.8 --

60 -- 20..4 -- 22.3 24.,5 15,.7 12.4 10,.8 10,,9 . _-

61 -- 29.,4 -- 20.9 16.,3 17,,2 11,,4 14.0 10,. 5 10,.6 --

62 -- 25..8 -- -- 18.,0 16.3 13.0 13.8 12,,6 10,,8 --

63 -- 23.,2 19.5 -- 16,,1 14.7 12,,6 12.0 11,,1 10,,6 --

64 -- 22.,8 -- -- 17.,3 16,,2 13,.7 12.9 11,,4 11,,0 --

65 -- 23.,8 -- -- 18.,3 15,,2 13,,7 12.3 11,.7 10,.3 --

66 -- 20.,9 -- 12.7 14.,9 14,.6 13,,9 12.3 12,.1 11,.3 9.3

67 -- 22.,6 25.7 -- 18.,7 15,,9 13,.1 12.1 11,.6 11,,2 10.3

68 -- 19.,3 20.5 18.5 19.,7 15,.6 13,.3 12.0 11,,3 9,,5 --

69 -- 22.,2 20.3 18.8 15,,4 13,.3 11.9 11,,3 9,,1 10.8

70 -- 23.,4 19.3 17.5 18., 5 15,.9 13,.3 12.0 10,,9 10,.1 9.9

71 27.2 22..6 21.4 19.3 18,,3 14,.8 12,,2 11.7 10,,7 9.,6 10.9

72 23.,0 21.9 19.6 20.,0 14,,4 13.,3 11.1 10.,7 9,,6 9.3

73 24.8 23.,8 21.9 19.7 17.,5 15,,6 13.,9 10.8 10.,1 9.,3 8.6

74 24..1 21.4 18.7 17. 7 14.,8 13.,7 10.8 9.,6 9.,1 8.2

75 20.1 17.4 16. 6 14..3 10. 1 9. 6 8.4

57-67

Aver. 23.,2 21.7 19.1 17. 1 15.,4 13. 5 12.6 11. 7 10.9 10.5

Source: "A Report on Automobile Fuel Economy", United States Environmental Protection

Agency, Office of Air and Water Programs, Office of Mobile Source Air Pollution

Control, October 1973.

10
4.3 SUMMARY OF PROCEDURE AND EXAMPLE

A prototype of the auto fuel consumption estimation procedure advocated in paragraph 4.2 was developed; it is based upon two vehicle attributes and three roadway operating conditions. The prototype estimation procedure uses base consumption rates for various combinations of vehicle weight and model year and adjustment factors for combinations of road type,

traffic volume, and attempted speed. However, available data did not allow for the detailed

disaggregate computations described previously: Vehicle Miles (VM^

my r t s a'' approximated by the product of P , a distribution for weight and model year categories; M ^ , ' ^ w,my' * J b > x,t,s

the length of a roadway of type r in miles with a volume per hour of t autos and a speed

limit of s mph; and T^, the total number of automobiles on the roadway during the time

period under study. The influence of accessory usage was not included. The formulation of

the procedure is:

(3) C=TZM^Z*P BC -A r r r,t,s ^^^^ w,my w,my w,my,r,t,s

T^ = total number of vehicles on road type r

P = percentage of vehicles in class w,my

w,my

M^

^

^ = number of miles of travel on road type

' ' r with t vehicles per hour and speed limit

of s mph

BC^ = base consumption rate in free -flowing

' ^ traffic for vehicle in class w and my

(gallons per mile)

^w my r t s ~ adjustment factor for vehicle class w,my

' ^' ' ' on road type r with traffic volume t and

attempted speed s

11
A simple example may be used to illustrate the estimation procedure." Assume that an estimate of total automobile fuel consumption on a six lane expressway during the AM peak

period is desired. The hypothetical situation is as follows:

speed limit: 55 mph

length of expressway: '

^

' 7.4 miles

inbound peak hour traffic volume: 4900 vehicles per hour (VPH)

(Level of Service C)

total AM peak period (2.5

' hours) traffic: 12,000 autos'

Vehicle Model Year and Weight Distribution

in Freeway Traffic Stream^

1955- 1968- 1971-

1967 1970 1973 1974

<3000 lbs .068 .088 .065 .030

3000-4000 lbs .124 .161 : , .120 .055

>4000 lbs .078 .101 .075 . .035

Base Consumption Rate Matrix [gallons per vehicle-mile (gpvm)]

1955- 1968- 1971-

1967 1970 1973 1974

<3000 lbs .050 .050 .051 .051

3000-4000 lbs .074 .077 .079 ^ .068

>4000 lbs .087 .093 .103 .106

The example is for illustrative purposes only. Results are based upon data from NCHRP 111

and therefore should only be used to estimate auto fuel consumption under conditions corre- sponding to service levels A through D

r While the estimation procedure can theoretically be applied to any time period, it is envisioned that the adjustment factors will be based on hourly flows. Therefore, the peak

period may be divided into smaller time periods, where different volumes prevail; and

hence, different adjustment factors are required. This example assumes that the same set

of adjustment factors apply to the entire peak period.

'if research determines that model year and weight are independent variables, this joint

distribution can be replaced by two separate distributions, one each for vehicle weight

and model year.

,12
Values of Adjustment Factors for six lane expressway with attempted speed 55 mph and 4900

VPH (Level of Service C) (ignoring accessory usage.)

1955- 1968- 1971-

1967 1970 1975 1974

<3000 lbs 1.04 1.05 1.05 1.06

3000-4000 lbs 1.05 1.05 1.06 1.06

>4000 lbs 1.05 1.05 1.06 1.06

The estimation procedure is divided into three steps

Step 1: Determine Vehicle Miles Traveled for Each Vehicle Type

vehicle miles traveled = (total vehicles) (I in w,my class) (road length)

for my = 1967 and w<3000 lbs

vehicle miles = (12,000) (.068) (7.4) = 5994

Similar calculations are performed for each w and my pair.

Step 2: Compute Base Fuel Consumption

base fuel consumption

for my

from the Base Consumption

Rate Matrix, BC

from vehicle miles computed

in step one, V

= (base consumption rate) (vehicle miles)

= 1967 and w < 3000 lbs

= .050

= 5994

fuel consumption (.050) (5994) = 299.7 - 300 gallons

Base Fuel Consumption Matrix (gallons)

1955- 1968- 1971-

1967 1971 1973 1974

<3000 lbs 300 389 294 136

3000-4000 lbs 816 iini 839 333

>4000 lbs 605 838 690 328

13
Step 3: Compute Adjusted Fuel Consumption

Each value in the base consumption matrix is multiplied by its corresponding

adjustment factor:

for my = 1967 and w<3000: 300 (1.04) = 312 gallons

Adjusted Fuel Consumption Matrix (gallons)

1955- 1968- 1971-

1967 1971 1973 1974

<3000 lbs 312 408 309 - 144

3000-4000 lbs 857 1156 889 353

>4000 lbs 635 880 731 348

And then summing the values in the Adjusted Fuel Consumption Matrix, the total consumption

for inbound automobiles during the AM peak period is: C = 7022 gallons

5. RECOMMENDED APPROACH TO DATA COLLECTION

The rate at which gasoline is consumed by a particular auto during a peak period (as

shown in Section 4.2) can be approximated by the product of the base (free flow) rate of

consumption (BC) for that auto and an adjustment factor (A) which represents the effect of

roadway operating conditions on gasoline consumption. Thus, R = BC x A.

Currently, only the base consumption rates can be obtained for a wide range of model

year and vehicle weight categories. Adjustment factors to account for varying roadway

operating conditions are only available for a single model year-weight category and three

roadway types. The adjustment factors which are available were developed for a 1964 automobile and more recent data on changes in vehicle weight and operating characteristics are not

available. In addition, adjustment factors (nor the data for their development) are not

available for the estimation of fuel consumption under extreme traffic congestion (Service

Levels E and F) . The effect of these deficiencies is to understate peak period gasoline

consumption and to generally make transportation planning less sensitive to fuel consumption

impacts

Since current base fuel consumption rates can be derived from the EPA data which are provided annually, a desirable approach would be one which combines the EPA rates with adjustment factors to be computed at regular intervals (e.g., three or four year intervals).

Adjustment factors would be developed to link EPA measurements with various roadway operating conditions.

Before the estimation procedure is implemented, a pilot study should be undertaken to

compare the results obtained using the proposed procedure and those obtained from small but

detailed field data collections. If a satisfactory correspondence between predicted and

observed consumption is observed, appropriate field data collections should be scheduled to obtain the gasoline consumption data necessary for implementing the proposed procedure.

To implement the proposed or a comparable procedure for estimating auto fuel consumption, certain data (including the EPA data) must be assembled. These data should allow for
the estimation o£ consumption under: 1) frequently encountered conditions of extreme traffic congestion; 2) high usage of accessories, primarily air conditioners; and 3) various

automobile mixes. Such a data set should allow for stratification by:

1. Roadway type (freeway, arterial, or local street)

2. Attempted speed (or speed limit)

3. Stops per mile (signal lights or stop signs)

4. Roadway volume or Level of Service (especially volumes approaching roadway capacity)

5. Gasoline consumption (gallons per mile)

6. Vehicle weight

7. Vehicle model year characteristics (design changes affecting consumption)

8. Accessory usage

The data in the initial collection should be analyzed to minimize the future collection

of non-essential information and to strengthen the mechanics of the collection procedure.

With respect to the mechanics of the data collections, particular attention should be focused on the choice of time intervals between data collections and sample size. At the

present time, it appears that minimum data requirements for computing the adjustment factors

should include fuel consumption data on at least one vehicle from each vehicle weight and

model year category for operations on each type of roadway. For freeways, the data would

consist of consumption rates for a set of volume and attempted speed combinations. For

arterials and local streets the data would consist of consumption rates for combinations of

volume, attempted speed, and traffic control conditions. Whether or not the data should be

collected for more than one vehicle would be decided only after a detailed study design was

completed.

In order to minimize the amount of fuel consumption data that needs to be collected, an investigation of the response of the vehicle types to various operating conditions should

be conducted. This would determine if an adjustment factor is required for each combination

of road type, traffic volume, attempted speed, accessory usage, vehicle weight and model

year or if a smaller set may be compiled (e.g., one for each road type). Any reduction in

the number of variables considered would substantially reduce the data requirements of the

procedure.

If implemented, this approach to estimating gasoline consumption will allow planners to more accurately predict the energy impacts of transportation alternatives. The range of

policy alternatives for which fuel consumption can be estimated will be broadened considerably, including 1) alternative auto size ownership policies, 2) alternative auto driver

diversion approaches, and 3) alternative traffic control policies. The costs incurred will

only be those associated with the generation of adjustment factors, and through a systematic

sensitivity analysis, these costs may even be reduced.

15
REFERENCES

"A Report on Automobile Fuel Economy". United States Environmental Proection Agency,

Office of Air and Water Programs, Office of Mobile Source Air Pollution Control,

October 1973.

G. J. Huebner, Jr., "Energy and the Automobile - General Factors Affecting Vehicle Fuel

Consumption". Presented at the 1973 SAE National Automobile Engineering meeting,

Detroit, Michigan, May 1973.

Thomas C. Austin and Karl H. Hellman, "Passenger Car Fuel Economy - Trends and Influencing Factors", paper 730790 presented at SAE National Combined Farm, Construction and

Industrial Machinery and Fuel and Lubricants meetings, Milwaukee, Wisconsin, September

1973.

C. E. Scheffler and G. W. Niepoth, "Customer Fuel Economy Estimated from Engineering

Tests", paper 650861 presented at SAE National Fuel and Lubricants meeting, Tulsa,

Oklahoma, November 1965.

Paul J. Claffey, "Running Costs of Motor Vehicles as Affected by Road Design and

Traffic", National Cooperative Highway Research Program Report 111, 1971.

Jerome W. Hall, Roy B. Sawhill, and James H. Matteson, "Vehicle Characteristics of Fuel

and Travel Time on Urban Arterials and Freeways", Highway Research Record 314, 1970.

Robley Winfrey, Economic Analysis for Highways , International Textbook Company,

Scranton, Pennsylvania, 1969.

David A. Curry and Dudley G. Anderson, "Procedures for Estimating Highway User Costs,

Air Pollution, and Noise Effects", National Cooperative Highway Research Program Report

133, 1972.

"1973/74 Automobile Facts and Figures", Motor Vehicle Manufacturers Association,

Detroit, Michigan, 1974.

USCOMM-NBS-DC

16
MBS-IMA (REV. 7.73)

U.S. DEPT. OF COMM. 1. IHMil.K AT ION OK Ki-I-'ORT NO. 2. Cov't Accession

BIBLIOGRAPHIC DATA No. SHEET NBSIR 74-595

3. Recipient's Acces.sion No.

4, Ti'i i.i- AM) siiin rn.i; A Procedure for Estimating Automobile Fuel

LiOnsuiup tion on Liongcscca. uiucui r\uciuo

5. Publication Date August 1974

6. Performing Organization Code

7. AirrMOR(S)

David M. Levinsohn, James T. McQueen

8. Performing Organ. Report No. NBSIR 74-595

9. PERFORMING ORC.ANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4314371

11. Contract/Grant No. AT 400010 DOT

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Urban Mass Transportation Administration

Department of Transportation

Washington, D.C. 20591

13. Type of Report & Period Covered

Final Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most sigriificant informatiort. If document includes a significant

bibliography or literature survey, mention it here.) Energy consumption is an important measure of the performance of a transportation system..

To be able to accurately measure associated automobile fuel consunption will improve the

evaluation of urban transportation alternatives. An estimated procedure is proposed that

is designed to be particularly sensitive to automobile fuel consumption in congested,

peak hour traffic. This procedure is based upon vehicle attributes and road\sray operating

conditions which were determined through an extensive review of the auto fuel consumptioli

literature. Vehicle attributes include characteristics of the automobile that affect

fuel consunption. Roadway operating conditions comprise the types of driving to which

the automobiles are subjected. Vehicles are classified by weight and model year. The

proposed roadway classifications are expressway, arterial, and local street. For each

vehicle type category, base fuel consumption rates are determined. These base consunption rates are then modified by adjustment factors which reflect the roadway operating

conditions. The rates are multiplied by the vehicle miles of each vehicle category and

summed over all categories to compute the total fueld consumption on the road under analysis. An example application of the procedure including sensitivity analyses is presented. The base fuel consumption rates can be obtained from EPA emissions test data.

Research is required to determine the adjustment factors, particularly under conditions

of extreme roadway congestion.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

automobile fuel consumption; congestion; energy conservation; fuel consumption; impact

assessment; roadway operating environment; urban roads; vehicle characteristics

18. AVAILABILITY [X Unlimited

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