EPA-450/3-74-003-f
November 1973
VEHICLE BEHAVIOR
IN AND AROUND
COMPLEX SOURCES
AND RELATED COMPLEX
SOURCE CHARACTERISTICS
VOLUME VI - MAJOR HIGHWAYS
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Water Programs
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
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EPA450/3-74-003-f
IN AND AROUND
COMPLEX SOURCES
AND RELATED COMPLEX
VOLUME VI - MAJOR HIGHWAYS
by
Scott D . Thayer and Jonathan D . Cook
Geomet, Inc.
50 Monroe Street
Rockville, Maryland 20850
Contract No. 68-02-1094
Task Order No. 3
EPA Project Officer: Edwin Meyer
Prepared for
ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Water Programs
Office of Air Quality Planning and Standards
Research Triangle Park, N. C. 27711
November 1973
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This report is issued by the Environmental Protection Agency to report
technical data of interest to a limited number of readers. Copies are
available free of charge to Federal employees, current contractors and
grantees, and nonprofit organizations - as supplies permit - from the
Air Pollution Technical Information Center, Environmental Protection
Agency, Research Triangle Park, North Carolina 27711, or from the
National Technical Information Service, 5285 Port Royal Road, Springfield,
Virginia 22151.
This report was furnished to the Environmental Protection Agency by
Geomet, Inc. , 50 Monroe Street, Rockville, Maryland, in fulfillment
of Contract No. 68-02-1094. The contents of this report are reproduced
herein as received from Geomet, Inc. The opinions, findings, and con-
clusions expressed are those of the author and not necessarily those
of the Environmental Protection Agency. Mention of company or product
names is not to be considered as an endorsement by the Environmental
Protection Agency.
Publication No. EPA 450/3-74-003-f
11
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CONTENTS
Page
List of Figures iv
List of Tables vi
Sections
I Conclusions 1
II Recommendations 2
III Introduction 3
IV Highway Characteristics and Parameters 7
V Traffic Parameters 10
VI Analysis 24
VII Results 34
VIII References 47
IX Selected Definitions 49
m
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FIGURES
No. Page
1 Various Representations of CO Emission Rates as
Functions of Vehicle Speed . 12
2 Various Representations of HC Emission Rates as
Functions of Vehicle Speed 13
3 Various Representations of NO Emission Rates as
Functions of Vehicle Speed 14
4 Typical California Motor Vehicle Travel Distribution
by Annual Average Hour of the Day 17
5 Weekday vs. Sunday Traffic 18
6 Daily Changes in Traffic 18
7 Examples of Monthly Traffic Volume Variations 19
8 Percentage of ADT Recorded During all Hours of the
Year on 113 Selected Urban and Rural Roads, 1959-1960 20
9 Total Motor Vehicle Travel and Forecase for Selected
States . 21
10 Speed-Flow Relationships for Three Different Highways 29
11 Example of Flow-Density Relationship in Limited-Access
Traffic Flow 29
12 Relationships Among V/C Ratio and Operating Speed, in
One Direction of Travel, on Freeways and Expressways,
Under Uninterrupted Flow Conditions 31
13 Expected Growth on New Facility Showing Impact of
Mass Transit Program 33
(Continued)
iv
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FIGURES (Concluded)
No.
14 Isopleths (m sec" ) of Mean Winter Wind Speed Averaged
Through the Afternoon Mixing Layer . 42
p
15 Isopleths (m x 10 ) of Mean Winter Afternoon Mixing Heights 43
16 Isopleths (m sec" ) of Mean Summer Wind Speed AVeraged
Through Afternoon Mixing Layer 44
o
17 Isopleths (mxlO ) of Mean Summer Afternoon Mixing
Heights 45
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TABLES
No. Page
1 Typical Values for Some Highway Design Parameters 7
2 Effect.of Lane Width on Capacity for Uninterrupted
Flow Conditions 8
3 Effective Roadway Width Due to Restricted Lateral
Clearances Under Uninterrupted Flow Conditions 8
4 Alignment Standards in Relation to Design Speed 9
5 Variations in Traffic Flow of Urban Freeways 11
6 Combined Effect of Lane Width and Restricted Lateral
Clearance on Capacity and Service Volumes of Divided
Freeways and Expressways with Uninterrupted Flow 36
7 Average Generalized Adjustment Factors for Trucks on
Freeways and Expressways, Over Extended Section Lengths 36
8 Key to Stability Categories 40
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SECTION I
CONCLUSIONS
1. Since highways are distinctly different from other complex sources,
it has been necessary to develop a methodology for highways which is
different from the general methodology developed in the first report on
shopping centers. This methodology relates the parameters descriptive of
highway traffic to highway parameters. These relationships are subsequently
to be used by the sponsor to develop guidance for relating the highway's
characteristics to air quality.
2. The methodology has been successfully applied to highways with
quantitative results presented in this task report.
3. It is now appropriate to proceed to the next and last type of complex
source (recreational areas), and apply the general methodology appropriately.
-1-
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SECTION II
RECOMMENDATIONS
It is recommended that, as planned, the project officer employ this
methodology to develop guidance for relating the traffic characteristics
of highways to typical and peak air pollution concentrations.
-2-
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SECTION III
INTRODUCTION
INTRODUCTION
OBJECTIVE AND SCOPE
The ability to estimate traffic characteristics for proposed developments
and the resulting effects on air quality is an important prerequisite for
promulgating State Implementation Plans which adequately address themselves
to -the maintenance of NAAQS. Prior to estimating the impact of a develop-
ment (complex source) on air quality, it is necessary that traffic charac-
teristics associated with the source be identified and related to parameters
of the development which can be readily identified by the developer a priori.
The purpose of this study is to jdentify traffic characteristics associated
with specified varieties of complex sources and to relate these character-
istics to readily identifiable parameters of the complexes. The end product
of this task will then be used to develop an Air Pollution Technical Document
which will provide guidance to enable control agencies to relate readily
identifiable characteristics of complex sources to air quality.
The work is being performed in seven sub-tasks. Each sub-task is devoted
to examining vehicle behavior and its relationship to readily obtainable
parameters associated with a different variety of complex source. The
seven categories of complex sources are:
1. Shopping centers (Report EF-263)
2. Sports complexes (stadiums) (Report EF-265)
3. Amusement parks (Report EF-268)
4. Major highways (Report EF-267 - the present report)
5. Recreational areas (e.g., State and National Parks)
6. Parking lots (e.g., Municipal) (Report EF-266)
7. Airports (Report EF-264)
-3-
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This, the sixth task report, describes a special methodology developed for
highways (different from that for the other six complex sources), and the
analysis and results of its application.
APPROACH
Due to internal constraints, the sponsor has been forced to impose a tight
schedule on this project, permitting only two to three weeks for the analysis
and reporting of each sub-task. Accordingly, the employment of readily
available traffic design information for each type of complex has been
suggested as the general approach.
The approach was designed to permit the development of answers to the
following questions posed by the sponsor, where the questions were felt . .
relevant, using available traffic design and behavior data, and available
data on parameters of the complex:
1. How much area is allotted to or occupied by a single motor vehicle?
2. How much or what percentage of the land occupied by the complex source
(and the source's parking facilities) can potentially be occupied by
vehicles? What is the usual percentage?
3. What portion of the vehicles within the complex are likely to be running
at any given time during a 1-hour period? During an 8-hour period? We
are interested in both peak and typical circumstance here.
4. What is the typical and worst case (slowest) vehicle speed over 1-hour
and 8-hour periods?
5. How are moving and parked vehicles distributed within the complex
property?
6. What are the design parameters for each type of complex which are
likely to be known by the prospective developer beforehand?
7. Which ones of the design parameters in number 6 can be most successfully
related to traffic and emissions generated by the complex? What is the best
estimate for relationships between readily obtainable parameters and emissions?
8. What are the relationships of parking "lot" design to parking densities
and vehicle circulation? What represents a typical design and/or a design
which has highest parking densities, lowest vehicle speeds, longest vehicle
operating times?
9. What meteorological conditions (i.e., atmospheric dilutive capacity) are
likely to occur during periods of peak use? What use level is likely to occur
during periods of worst meteorology (i.e., atmospheric dilutive capacity)
-4-
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The technical approach developed and implemented in this report centers
about the highway traffic parameters of volume and speed (or, equivalently,
traffic density and speed, since density is derivable from speed and volume).
It is these parameters which permit estimation of automobile emissions from
highways and therefore allow a determination of the future highway's impact
on NAAQS.
The effort was planned to focus on major highways; further discussions with
the government project officer indicated that it would be best to concentrate
on arterial roads or greater. As will be seen, it was subsequently determined
that a quantitative treatment could be developed, within the time available,
for highways with full control of access; the treatment is also considered
applicable, with proper exercise of judgment, to some cases of partial
control of access. The cases treated are those of "uninterrupted (or con-
tinuous) flow", where speed-volume-density relationships have been reasonably
well quantified. Interrupted flow, whose most common example is the city
street with signalized intersections, is considered difficult to describe
in terms of speed-flow relationships, even by specialists in the field,2
and is not treated here.
For the case of non-congested flow, traffic volume* (vehicles per hour) is
determined by the demands (vehicles per hour) at the various entrances to
and exits from the highway. Average highway speed is found to vary in an
inverse fashion with the ratio of volume of traffic to capacity. Capacity
is defined to be the maximum volume to be accommodated by the highway under
ideal circumstances and is determined primarily by highway design characteristics.
Hence both volume and speed are highly dependent on highway design characteristics,
The interrelationships of volume, speed and density which have been measured
on many occasions, and have undergone significant analytical interpretation,2
form the basis for our methodology for highways.
* While the terms volume and flow are used to denote different parameters
in traffic and highway engineering (volume being used to represent periods
of an hour or more, and flow periods of less than an hour), we have found
it convenient to use them interchangeable in this study.
-5-
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An additional point which renders this study different from those for the
other six complex sources: each of the others required-examination of the
emission characteristics of automobiles only to the extent necessary to
insure that the traffic behavior methodology could be used by the sponsor
in conjunction with such emission data, It has been necessary, in this
study of highways, to go more deeply into automotive emission data because
of the different nature of this problem (especially the variety of speeds
encountered).
The sponsor has expressed interest in information on the expected frequency
of occurrence of new highways of various sizes; in our contacts with staff
of the Federal Highway Administration1 we have been told that their data are
presently being processed to develop this type of information; the sponsor
may contact them in the near future to obtain it.
As a final note, this study must be recognized as a condensed analysis of
a massive amount of information in a field significant in its own right -
that of traffic and highway engineering. As a result, shortcuts have been
taken to focus on only those facets of the problem considered most important,
and the elements considered most relevant to the problem at hand; there is
thus the likelihood that points considered significant in traffic and high-
way engineering problems per se may have been minimized as a result. The
analysis should be read with this point in mind.
-6-
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SECTION IV
HIGHWAY CHARACTERISTICS AND PARAMETERS
The principal item which characterizes a highway is its spatial design.
A map showing the proposed layout of the highway will show such features
as right-of-way, lateral clearance, number of lanes, lane widths, highway
curvature, location of traffic blockages such as signals or toll booths,
and manner of access. Typical values for some of these parameters are
shown in Table 1. Some of these variables characterize the level of service
of the highway, which represents the capacity of the highway to tolerate
specified traffic volumes at specified maximum safe operating speeds.
Other such variables, such as the right-of-way, are not reflective of the
level of service but are important in other ways in air quality impact
analysis. Right-of-way defines the area surrounding the highway which is
excluded from use except for the highway. Hence right-of-way can be used
to determine where receptors might be stationed to measure pollutant con-
centrations, or simply distances at which air quality might be of concern.
Table 1. TYPICAL VALUES FOR SOME HIGHWAY DESIGN PARAMETERS
No. of lanes 2 to 8
Lane Width - 9 to 12 feet
Lateral Clearance 0 to six feet
Right-of-Way 50 to 400 feet
Speed Limit 40 to 80 MPH
-7-
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The spatial distribution of the highway in conjunction with land use maps
and trip generation studies should be used to determine anticipated demands
on the highway at particular segments, depending on the amount of residential,
commercial and industrial development present or anticipated.
The effect of two of these parameters - lane width and lateral clearance -
is seen in Tables 2 and 3 in terms of reduced highway capacity.
Table 2. EFFECT OF LANE WIDTH ON CAPACITY
FOR UNINTERRUPTED FLOW CONDITIONS2
Capacity
(% of 12-ft Lane Cap.)
Lane
Width
(Ft)
12
11
10
. 9
2-Lane
Highways
100
88
81
76
Multilane
Highways
100
97
91
81
Table 3. EFFECTIVE ROADWAY WIDTH DUE
TO RESTRICTED LATERAL CLEARANCES UNDER
UNINTERRUPTED FLOW CONDITIONS2
Clearance From
Pavement Edge
to Obstruction,
Both Sides
(Ft)
6
4
2
0
Effective
Width of
Two 12-ft
Lanes
(Ft)
24
22
20
17
Capacity
of Two
12-ft
Lanes
(X of
Ideal)
100
92
83
72
-8-
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Highways are also characterized by curvature - both horizontal and vertical -
and by grade, in addition to other alignment parameters; However the net
significance of such parameters, at least from our standpoint, is to determine
the design speed of the highway. Table 4 illustrates how design speeds
relate to alignment parameters.
Table 4. ALIGNMENT STANDARDS IN RELATION TO DESIGN SPEED2
Design
Speed,
in MPH
20
30
40
50
60
70
Minimum
Radius of
Horizontal
Curves
in Feet
100
250
450
750
1100
1600
Maximum
% of Grade
12
10
8
7
5
4
Min. Forward
Sight Distance,
in Feet
150
200
275
350
475
600
Min Length
of Vertical
Curve for Each
1% Change of
Grade, in Feet
10
20
35
70
150
200
The design parameters of a highway are useful in our analysis only to
the extent that they determine the capacity and design speed of the highway.
Hence if either of these measures is available directly from the highway
engineer, then it can be used directly. Generally design speed will be
available, since it will normally be the same as the highway's speed limit.
Section VII, Results, gives the procedure for determining highway capacity
from design characteristics.
-9-
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SECTION V
TRAFFIC PARAMETERS
Traffic flowing through a non-congested highway segment can be character-
ized sufficiently for our purposes by its volume and average speed.
Volume is the number of vehicles passing over a given road segment during
a given time, normally 1-hour or more. A complete set of definitions is.
given in Section IX. Table 5 gives some typical traffic volumes for
urban freeways.
SPEED
Average speed is the arithmetic mean of speeds of vehicles passing over
a segment of highway during a given time-frame. If vehicle emissions
were independent of speed - that is, could be expressed as a constant
number of grams of emission per mile per vehicle, we would not be con-
cerned with vehicle speed.
The dependence of vehicle emissions on speed is seen in Figures 1, 2,
and 3. Note that for each pollutant there are four lines: one corres-
ponds to the familiar average trip speed and one to steady state speed.
The other two reflect data from reference 7 on emissions during periods
of acceleration and deceleration of speed in 15 mph increments (0-15,
15-30, 30-45, 45-60, 60-45, 45-30, 30-15, and 15-0). The points are
plotted at the average speed of each increment. Average trip speed is
defined as the total distance covered from start (i.e., starting of
engine) to stop (i.e., turning off of engine) divided by the time required
to make this trip. Typically average trip speed reflects periods of
idling, such as encountered at signalized intersections. Steady state
speed is the speed of a vehicle during periods of approximately constant
-10-
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Table 5. VARIATIONS IN TRAFFIC FLOW OF URBAN FREEWAYS
B
No.
Location of Count Station of
Lanes
New England
Maine
0.7 mi W of US 1A, Bangor 4
1.7 mi from Augusta 4
Massachusetts
2.3 mi W of Mattapoisett 4
New Hampshire
0.5 mi from Concord 4
0.6 mi from Concord 4
0.5 mi S. Jet. US 3 4
2.0 mi from Manchester 4
Rhode Island
Pawtucket River Bridge 6
Middle Atlantic
New York
Long Island Expwy. at 82nd . 6
Street
15 mi from N.Y. City, 6
Nassau Co.
Cross Island Pkwy. at 114th 6
Ave., New York City
New England Thrwy., 6
N.Y. City
Route
Number
1-395
1-95
US 6
1-93
1-93
1-93
1-193
1-95
1-495
NYS 495
Direc.
of
Travel
EB
WB
Both
N8
SB
Both
Both
Both
Both
Both
Both
EB
WB
Both
EB
WB
Both
Both
NB
SB
Both
NB
SB
Both
AADTa
7,120
2,167
15,988
11,804
11,554
4,035
6,352
19,216
127,910h
119,300
66,610
47,420
Peak Day
10,534
6,013°
25,509C
20,737r
21,167°
15,925C
9,622
24,293
157,940
-
92,000C
f
65,970°
Volume in Selected Highest Hours As a Percentage
of Annual Average 24-Hour Volume (AADT)
Max.
19.6
21.4
32.4
32.1
16.8
20.4
21.4
24.7
15.3
11.2
11.1
10.1
9.4
9.4
8.4
11.9
14.4
12.4
12.6
10th
13.3
11.5
22.5
23.2
11.4
15.9
16.6
20.2
14.3
8.8
10.3
9.7
8.4
8.8
8.3
10.6
13.4
10.7
11.4
20th
12.9
10.1
20.8
20.5
11.0
15.3
15.6
19.1
13.7
-
9,8
9.2
8.2
8.6
•8.3
10.3
12.8
10.0
10.9
30th
12.1
9.8
18.9
19.2
10.9
14.8
14.9
18.2
13.4
-
8.6
8.7
8.1
8.3
8.3
10.1
12.6
9.6
10.8
40th
11.7
9.6
17.3
17.7
10.7
14.3
14.5
17.4
13.1
-
8.2
8.3
8.0
8.2
8.2
10.0
12.1
9.5
10.6
50th
11.4
9.5
16.5
17.3
10.6
13.9
14.1
16.5
12.8
-
-
8.2
7.9
8.1
8.2
10.0
11.8
9.3
10.5
100th
-
_
.
_
10.2
_
12.5
14.2
12.1
-
-
-
7.6
7.6
8.1
9.6
10.8
9.0
10.0
200th
_
_
_
_
9.7
_
_
-
11.2
-
-
-
7.1
7.1
7.9
8.8
9.9
8.4
9.2
a. Average Annual Daily Total for calendar year 1962 except where noted otherwise.
b. For calendar year 1961.
c. Peak day occurred on a Saturday or Sunday.
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CO
Emission
Rate,
gin/mi.
IUUU'«
8
6
I
"t
100 1
•8
' 7
6
5
on 4
3
2
10 l
8
7
6
5
V
\
\
\
\
\
.
\
\
-s
"X
s\
\v
* it
"\.
^ ^
V
Ns
s
<>•
^^5*
,^
"<^-
[Steady state
v.
**'
...—
~~ • —
Speed
^.Acc
"^
1 f"
-.J -1 -
"Decel
eration
elerat
ion
age Trip Spe<
:d
10
20 30 40
Vehicle Speed, mph
50
60
Figure 1. Various representations of CO emission
rates as functions of vehicle speed.
-12-
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emission
Rftte,
qm/mi.
8
7
6
S
4
3
2
100;
8
7
6
S
4
n
3
2
10 I
8
7
6
S
4
3
2
1 ,
\
\
\
S
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v
X.
VVN.
"*>>
\
\
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V^.
^
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^-^,
"^^
X
"*
c^-^
..^
-^~
zcelen
'• —
Steady State Speedy
- ^— -
-^ — '
^Dec
-I
.elerat
ion
^.A.V<
• — '— '
:rage "
"rip Sp
eed
10
50
60
20 30 40
Vehicle Speed, mph
Figure 2. Various representations of HC emission
rates as functions of vehicle speed.
-13-
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100'°
9
NOX
Emission 4
Rate, 3
gm/mi.
1.QJ
O.li
0
10
20 30 40
Vehicle Speed, mph
50
60
Figure 3. Various representations of NO emission
rates as functions of vehicle speed. x emission
-14-
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speed. While average trip speed is more appropriate for in-town driving
conditions, it is apparent th-at steady-state speeds are more applicable
to limited access highways, where the design features demand that vehicles
enter and exit at high rates of speed, and only stop due to congestion or
emergency situations. The average trip speed curves are included for
comparison purposes.
It is also apparent that normally speed will be approximately constant
between interchanges, since the cross-sectional design of a highway is
generally constant between interchanges. However, it is to be expected
that speed will vary from one segment between interchanges to another,
since cross-sectional design and speed limits vary in this fashion. There-
fore, it is necessary to evaluate speeds for each segment between inter-
changes where highway design is known to change.
When capacity is exceeded, and traffic begins to slow down as density
and congestion increase, it is probable that there will be significant
amounts of acceleration and deceleration about the nominal speed for the
case in point. Since Figures 1-3 show significant differences of these
curves from the steady-state ones (especially in the case of acceleration),
it is suggested that, where the traffic density represents a demand which
significantly exceeds capacity, the mean of the two curves (acceleration
and deceleration) may be used to more properly reflect the emission vs.
speed relationship.
TRAFFIC DEMAND
If a highway's design is adequate to meet traffic demands, then the volume
of traffic on a given highway segment is determined by these demands.
Consider the diagram below, which shows the relationship between traffic
demands and actual traffic volume for one direction of a limited access
highway.
-15-
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Segment i
\
VH
V. and V... represent the demands placed on the segment i, with
16 it
V. = V. +V. representing the actual volume of traffic which the segment
1 . I C It
experiences. The traffic demand on the next segment (i+T) down stream is
V1.+l = Vl,e+Vi+l,t where Vi+l,t = VVl.x' Hence a." knowledge of
traffic demands at the various entrances to and exits from a highway is
necessary to determine expected traffic volumes.
Traffic demands are subject to two basically different types of time
variations, one being periodic, the other non-periodic. Principal peri-
odic variations include diurnal, weekly and seasonal variations. Non-
periodic variations include normal growth patterns, generated traffic,
and development traffic, in addition to the implementation of public
transit programs.
Periodic Variations in Traffic Demand
Typical diurnal variations are shown in Figure 4 for highways in urban
and rural environments. Both curves show an expected increase in traffic
volume during the daylight hours, while urban traffic shows morning and
evening peaks corresponding to commuting traffic. Since these peaks are
related to the work cycle, it is to be expected that diurnal variations
during the weekend are substantially different. The contrast is shown
in Figure 5 which gives diurnal variation for weekdays on one curve and
for Sunday on the other. Note that the morning peak for weekdays is
entirely absent for Sundays, and the evening peak is shifted to later
hours for Sundays, probably reflecting returning weekend traffic. Day
of week variations are also significant and vary substantially for urban
and rural traffic, as can be seen in Figure 6. While there is likely to
be consistency in these variations from one urban region to another,
rural variations are probably less constant. It is apparent from these
-16-
-------
figures that the worst traffic volumes will exist on Friday evenings, a
time when return-from-work and weekend traffic overlap.
8
7
6
H 5
"8 4
3
2
1
0
x
\
\
^
.-
y
-'i
i
i
f
P
X
-\
X
\
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l—"*^
^- —
/
— —*
/
/
y
/ N
Ur
V
\
"bai
V
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\
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1 —
<
\
K
Rural
^
"\
\
\
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\
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12M 2 4 68 10 12N 2 46 8 10 12M
Hour of Day
Figure 4. TYPICAL CALIFORNIA MOTOR VEHICLE TRAVEL DISTRIBUTION BY
ANNUAL AVERAGE HOUR OF THE DAY2
-17-
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10 r
o
14-1
>
16
12
10J
4 8 12N 4
Hours of Day
Figure 5. WEEKDAY VS SUNDAY TRAFFIC2
iural
^Urban
._ 1—
S M T W T F S
Day of the Week
Figure 6. DAILY CHANGES IN TRAFFIC2
12M
-18-
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Seasonal variations are shown in Figure 7, for rural and urban Washington
state and for Tucson. Of particular interest here is the dramatic differ-
ence in these seasonal variations. While both rural and urban Washington
state reach peaks during the summer months, Tucson experiences a low in
traffic volume during these months. These differences are due largely to
climatological differences between Washington and Tucson. Due to the
widely varying climatological conditions from one part of the country to
another, it is apparent that these variations in traffic demand for a
particular area must be based on volume data for the region in which that
area is located.
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\ V
\
\
\
>
/
ti
/
\
,\
- — t _
Tucson, 1958-60
Urban
1961
s''
\^
\
^
_^
\
\
(Sources: Washington State Dept.
- of Highways and Tucson, Ariz. , —
Area Transp. Dept. Study)
i i i i i i i i
J FM AM JJ AS OND
Month of Year
Figure 7. EXAMPLES OF MONTHLY TRAFFIC VOLUME
VARIATIONS2
-19-
-------
Typically traffic demands are considered in terms of average annual daily
traffic and peak hour traffic. Table 5 gives some observed values for
these two volume measures for several highways. Normally traffic design
2
is based on the thirtieth highest hourly traffic volume anticipated.
Figure 8 shows the relationship between peak hour traffic and AADT.
025 0 50 . 1.0 SO 100
PERCENTAGE OF HOURS IN YEAR
100 IOOO
Figure 8. PERCENTAGE OF ADT RECORDED DURING ALL HOURS OF
THE YEAR ON 113 SELECTED URBAN AND RURAL ROADS, 1959-1960.l
This figure shows that, for one direction of travel, the highest peak
hour for urban highways is approximately 13% of the AADT. From the dis-
cussion of the periodic variation in demand, it is seen that this peak
demand will usually occur during the late afternoon hours of a Friday,
the time of year depending on climate. Warmer climates will tend to have
a winter peak, cooler climates a summer peak, corresponding to increased
recreational traffic during these times.
-20-
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Non-Periodic Variations in Traffic Demand
Traffic growth factors are discussed in some detail in reference 3. This
publication outlines the three types of traffic growth which have been
mentioned above. Normal traffic growth is that growth which is due to
the general increase in the number and use of motor vehicles. Such traf-
fic growth is clearly indicated by Figure 9, which shows the number of
vehicle miles traveled from 1920 to the early 1960's, with expected
growth trends indicated beyond.. It is a relatively straightforward
matter to apply this normal growth factor to existing traffic demands to
arrive at an expected traffic demand in ten years subsequent to completion
of a highway facility. This normal growth can be calculated based on
2
4.6% increase in travel miles per year.
y
I
UJ
tf)
z
o
CD 0
1920
1930
1940
1950
YEAR
I960
1970
I960
Figure 9. TOTAL MOTOR VEHICLE TRAVEL AND
FORECAST FOR SELECTED STATES3
Generated and development traffic will be less easily evaluated, since
these traffic demands depend principally on the region through which the
highway passes. Generated traffic is defined as "motor vehicle trips
(other than public transit) which normally would not have been made if
the new facility had not been provided." An example of a generated trip
is one which was previously made to a different destination, but for
which the change is attributable to the attractiveness of the improved
highway.
-21-
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Increases due to generated traffic demand are usually experienced within
a year or two after the opening of a new highway facility, since general
public awareness of improved access due to the new facility is achieved
during that time. Increases due to generated traffic are widely varying
and will have to be based on judgment. For rural highways, generated
traffic is likely to run greater than 5% (of existing traffic volume),
but less than 25%.
Development traffic is defined as that traffic "due to improvement on
adjacent land over and above the development which would have taken place
3
had not the new or improved highway been constructed." "[his increase in
traffic is likely to continue for years subsequent to completion of a new
highway. The magnitude of increase due to development traffic is depen-
dent largely on the availability of land for development adjacent to the
new highway. Highways constructed through a highly developed area will
experience little growth in volume due to development traffic, although
they may experience the shorter term generated growth, as described above.
Highways which are constructed through a completely restricted region,
such as a federal reserve, or park, should experience no development
growth. On the other hand, land is typically available for development
along a new highway, and the amount of this land which is readily accessi-
ble and its zoning, along with other factors which make development
attractive, may be studied to estimate the number and types of develop-
ments which would be expected. From this information the number of trips
expected to be generated may be evaluated for each interchange. In terms
of our previous notation, V. and V. , the demand volumes at a particular
interchange i may be predicted, or at least the proportions of these
demands which are due to development.
Future plans for.public transit programs (other than those which already
exist) must be taken into account in the evaluation of traffic parameters
of proposed highways. Vehicle demand (V. and V. ) is reduced by effective
1 A 1 C
public transit programs.
Thus, the importance of trip generation analyses, both for present and
future expected demands, cannot be overemphasized.
-22-
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Percentage Use by Trucks
Since trucks are considerably less maneuverable than passenger cars, a
significant percentage of trucks on a highway will adversely effect
capacity. For example, studies have shown that, even in level terrain,
20% usage level by trucks will reduce highway capacity by 17%; in
2
mountainous terrain, the reduction is 58%. Therefore the evaluation of
highway capacity (as specified in Results) has taken into account per-
centage use by trucks.
-23-
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SECTION VI
ANALYSIS
There are two analytic approaches to consideration of the impact of new
or improved highways which must be considered. The first of these is the
meso- or macro- approach.* This approach assumes that the air quality
impact of highways can be measured by the number of vehicle miles traveled
during a given time period, and the average speed of vehicles traveling
during the time period. The impact of a new highway in a region will be
measured by the number of additional vehicle miles, and the effect that
the new facility has on average speed.
The second and intuitively simpler approach is the micro approach. This
approach considers the highway as a line source of emissions (or a series
of line sources), the intensity of which fluctuates with time and location.
A diffusion model is used to predict pollutant intensities due to the
highway, and these intensities are compared to background concentrations,
which may also be based on model predictions or on direct measurements.
The micro approach has the advantage that local variations in pollutant
concentrations can be ascertained and the effect of weather conditions can
more accurately be taken into account.
Fortunately the same parameters which enter into the macro analysis can
be readily applied to the micro analysis. While the key variables in
macro analysis are vehfcle miles traveled and average speed, the key
variables in micro analysis are vehicle density and average speed. Let
us consider a typical highway segment of length d with a traffic volume
* DOT customarily uses the term "meso" and EPA."macro" for this concept.
-24-
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of V vehicles per hour traveling at a speed of s mph.
traveled during a time period T (1 or 8 hours) is
VMT = T x V x d.
i
On the other hand, vehicle density is simply
The vehicle miles
For a given segment of highway the analysis variables for macro analysis
are readily transformed to those of micro analysis.
Let us consider a segment of highway in one direction with n exchanges,
one of which is shown below.
At this interchange denoted by i, the exit volume is denoted by V. ,
I /\
the entering volume by V. , and the through volume by V... V. and
Ic 1 t 16
V- are demands on the highway (as discussed in the previous section)
I X
which must be known either in terms of a ratio [V. V(V. +V..)] or absolute
IX I X 1 U
numbers. V. is the sum of previous interchange differences (V. -V. )
I I C I J\
and the initial volume V (that is, the entering volume demand at the upstream
end of the segment being considered). That is,
i
If we denote the distance in miles between interchange i and i+1 as di,
the total vehicle miles travelled along the highway being considered is
n
VMT=£ dixVi
i=o
-25-
-------
where d is the distance from the upstream end of the segment to the
first interchange, and d is the distance from the last or nth interchange
to the downstream end of the segment.
The above notation can be adapted to the situation where an interchange
is "one-sided"; that is, an exit or entrance exists, but not both. For
these the appropriate V. or V. may simply be set equal to zero. This
I /\ i vi
approach could also be used for the situation where the distance between
an exit and an entrance is extraordinarily great, as
•\
«*-
— "2
.d,_
3=0
This situation has been represented by treating the interchange as two
separate interchanges labeled 1 and 2 and by setting V, and V2 both equal
to zero.
As indicated previously speed has been shown to be a critical variable in
the evaluation of impact of highways on air quality. For carbon monoxide
and hydrocarbons the rate of emission in grams per mile decreases as the
steady-state speed of the vehicle increases, to about 30 mph, and is
relatively unchanging with further increases. For NO , emissions increase
/\
with increasing speed above 15 mph.
-26-
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Occasionally situations arise where the speed is not constant between
interchanges. As an example, the cross-sectional design of the highway
might change, as in the case of changing lateral clearance sometimes
necessitated in urban areas. As another example a change in the terrain
might alter the vertical and horizontal al.inement of the highway, hence
altering the operating speed. These situations may be treated consistently
with the notation above by defining the segment between interchanges into
subsegments, each having relatively constant speed. Consider the following
example:
Lateral Clearance
>
3t
'It
A stretch of highway which would normally be treated as one segment has
been divided into two, to reflect differing operating speeds caused by a
change in lateral clearance. The point at which the speed changes is
defined here as a "pseudo" interchange, with both \L and V^g set to zero.
Such a refined segment breakdown will not be necessary unless the speed
difference is significant.
For the macro approach, it is possible to take into account speed varia-
tions by summarizing results in the following fashion: X VMT at 40 mph,
Y at 45 mph, etc. It may be more convenient to use the concept of a speed
correction factor, as suggested in reference 3. However, the speed
correction factor used there is for average trip speed, while we wish to
use a correction factor based on steady state speeds (see Figures 1 to 3).
If, say, 30 mph is selected as the standard speed, the speed correction
factor for NO at 60 mph is approximately 3.5. If a given segment of
A
highway has a volume of 2,000 vehicles per hour and is 10 miles in length,
-27-
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then it experiences 20,000 VMT during that hour. However, if the steady-
state speed during that hour is 60 mph, then its equivalent vehicle miles
traveled (EVMT) will be 3.5 x 20,000, or 70,000 EVMT. The advantage of
this approach in macro analysis is that the EVMT for all segments of a
highway being considered can be summed, even though operating speed may
vary from segment to segment. Hence EVMT may be stated as follows:
n
EVMT = E c(si) * di x V1
1=b
Where c(s.) is the correction factor corresponding to speed s. in segment
i for a particular pollutant. Conveniently, the CO and HC factors are
1.0 above 30 mph.
Studies have shown very typical patterns in the relationship between speed
and volume, which can perhaps be most clearly understood in terms of
traffic density. As traffic density increases flow increases in a linear
fashion until density becomes such as to cause a reduction in speed.
This phenomenon can be observed in Figure 10. Curves for highways in
Detroit and Los Angeles show that speed is not affected for low.volumes.
As density increases volume continues to increase, despite a lowering of
average speed, up to a point of critical density, beyond which flow and
speed both decrease, while density continues to increase. This relation-
ship is illustrated by Figure 11. It is at this point of critical density
that volume is maximized--that is, the highway's capacity is reached.
Traffic flow beyond critical densities is widely varying, ranging from
near capacity down to zero.
From the above it is apparent that capacity is a key variable in analyzing
the flow along the highway. Traffic flow demands which are well below
capacity will not cause a forced flow (or stop-and-go) situation, although
speed may be diminished for higher flows. The highway's capacity may be
derived according to well defined guidelines established in the Highway
2
Capacity Manual, utilizing the design characteristics of the highway.
Such design characteristics include number and width of lanes, lateral
-28-
-------
clearance, and other variables outlined in the previous section. These
guidelines fpr establishing highway capacity are outlined in the Results
section.
RATE OF FLOW (100 VPM/L4NE)
Figure 10. SPEED-FLOW RELATIONSHIPS FOR THREE DIFFERENT
HIGHWAYS9'10'11
40 80 120
DENSITY (VEH/MILE)
Figure 11. EXAMPLE OF FLOW-
DENSITY RELATIONSHIP IN LIMITED-
ACCESS TRAFFIC FLOW (HOLLAND TUNNEL,
NEW YORK)12
-29-
-------
If the capacity corresponding to a segment i is determined to be C., then
the volume to capacity ratio is V./C-. If this ratio is less than 1, and
the critical density has not been reached (the solid lines in Figure 12),
then Figure 12 illustrates how the operating speed S. corresponds to a
particular design speed for segment i.
As the critical density is reached and exceeded, the volume begins to
decrease and the speed continues to decrease; this phenomenon is repre-
sented in approximate form by the dashed curve in Figure 12.
These curves are of course interpretable, and useful, in a variety of
ways, and represent the distillate of a vast amount of information from
a number of sources, adapted from the Highway Capacity Manual.2 Another
interpretation is found by examining the range of speeds which may be
associated with a given volume of traffic, where the higher speeds repre-
sent conditions of low density, free flow; intermediate speeds represent
light to moderate restrictions, and the lower speeds (still at the same
volume) represent increasingly higher densities and severe restrictions,
ranging finally to stoppages of traffic for long periods of time, and
maximum densities. These conditions, called levels of service, are
designated conditions A through F, as described in the Appendix. It is
of use to note that one study (reference 6) has addressed the question
of traffic density under stopped conditions on freeways by means of still
photography, with a result indicating a "jam," or stopped density (the
maximum expected) of the order of 200 vehicles per mile. While this is
useful in micro-analyses of extreme cases of congestion, it is not
intended to imply that such conditions are important in impact analyse:;
rather, this figure (approximately 200 vehicles per mile) may be useful
in examining maximum possible emission rates, by combining it with the
"at idle" emission rates from reference 7 as follows: CO, 16.19 gm/min.;
HC, 1.34 gm/min.; and NO , 0.11 gm/min. These data, incidentally, fit
A
precisely with the steady-state emissions of Figures 1, 2, and 3, when
the latter are converted to grams per minute at various speeds, and
extrapolated to zero speed.
-30-
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i
CO
70
60
50
Operating
Speed,
MPH 40
30
20
10
0.1
0.2
0.3 0.4 0.5 0.6 0.7 0.8
Volume/Capacity (V/C) Ratio
1.0
Figure 12. RELATIONSHIPS AMONG V/C RATIO AND OPERATING SPEED, IN ONE DIRECTION OF TRAVEL,
ON FREEWAYS AND EXPRESSWAYS, UNDER UNINTERRUPTED FLOW CONDITIONS (Adopted from Reference 2)
-------
Public Transit Systems
Figure 13 shows the anticipated impact of a transit system on miles
traveled for a new highway. This impact is measured by A VMT, the reduc-
tion in vehicle miles traveled due to the transit system. The magnitude
of the reduction will obviously depend on a number of factors, such as
the effectiveness of the transit system and the amount of traffic on the
highway which is due to intra-region travel (particularly commuter and
shopper traffic). Obviously if most of the traffic on the highway is due
to through traffic, then the existence of an urban mass transit system
which uses the highway will have little effect on total VMT.
Frequently mass transit systems will utilize one lane of a multilane artery,
thereby reducing significantly the capacity of the highway to carry other
traffic. If the mass transit system is not successful in attracting users,
this reduction in highway capacity will result in congestion and potential
air quality impact.
This special situation may be treated by considering the lane restricted
to mass transit and the remainder of the highway separately. The schedule
and routes of the mass transit system may be utilized to calculate VMT and
speed for macro analysis, and maximum density and associated speed for
micro analysis. The expected usage of the remainder of the highway must
be calculated in the normal fashion, with capacity based only on lanes
available, and input traffic demands reflecting decreases due to the mass
transit system. The resulting EVMT for macro analysis can be obtained by
adding the values for the mass transit and usual traffic. Micro analysis
would utilize one line to represent pollutant source intensity due to mass
transit, a second to represent the line source intensity due to the remainder
of the traffic (or the two intensities could be added to be represented
by one line). Emission factors for the mass transit system will reflect
the usage of heavy-duty vehicles. The factor used will depend on the
type of pollutant, the year and age of the vehicle, and weather it is diesel
or gasoline fueled. The appropriate tables are contained in reference 3.
-32-
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a
•H
•a
0)
4J
n)
B
•H
4-1
CO
W
Normal and
Development
Growth
Normal Growth
Without Mass
Transit
Growth With
Mass Transit
Temporary Increase
Due to Generated
Traffic
Years
Figure 13. EXPECTED GROWTH ON NEW FACILITY SHOWING IMPACT OF MASS
TRANSIT PROGRAM
-33-
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SECTION VII
RESULTS
THE METHODOLOGY
As indicated earlier, we wish to obtain as end product of this study procedures
to evaluate the impact of new or improved highway facilities from both macro-
analytic and -micro-analytic approaches. To reiterate, macro-analysis
utilizes proportionate modelling to determine the impact of new facilities
on the overall air quality of a designated area, while the micro approach
utilizes diffusion modelling to predict pollutant concentrations and their
detailed spacial distribution.
MACRO-ANALYSIS
The objective of macro-analysis is to determine the increase in total
equivalent vehicles miles travelled for a designated area. Since the net
result of the analytic approach outlined here is the number of EVMT due
to the new highway, the base EVMT (and any resulting reduced EVMT in the
road network excluding the highway) must be available from other studies.
Obviously the impact of the new highway (in terms of percentage increase
in EVMT) is highly dependent on how the designated area is defined. If
we consider the width of the designated area to be the dimension perpen^
dicular to the direction of the highway, then the greater the width, the
smaller will be the measured impact of the new highway. However, this
report does not attempt to address the difficult question of how to define
a designated area, except to suggest that such a definition should closely
align with meaningful urbanization or geographic boundaries.
-34-
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The first step in impact analysis is to obtain a map (or maps) showing
the spatial design of the highway, through the designated area. Other
input should include cross-sectional design specifications, anticipated
traffic demands, and land use maps. The steps to be carried out for
macro-analysis should proceed as follows:
1. Consider each direction of travel separately. Divide the highway
into segments having uniform levels of service, as described in the Analysis
Section. Within each of these segments, operating speed and capacity should
be uniform, and there should be no inflow or outflow of traffic within the
segment (that is, exits and/or entrances should define endpoints of segments).
If special bus lanes exist, consider these separately from the remainder of
highway.
2. Based on highway design specifications and percentage use by trucks,
evaluate the capacity of each segment. The capacity for segment i may be
evaluated by use of the formula
C. = 2,000 N W T
Where C. = Capacity in vehicles per hour,
N = Number of lanes,
W = Adjustment for lane width and lateral clearance
taken from Table 5.,
T = Truck factor from Table 7 and includes adjustment
for grade.
Note that more refined tables for evaluating T are available (in reference
2) which take into account steepness and length of grade.
3. Evaluate the demand on each segment i according to the discussion of
Section IV; that is,
V Vi-l + (Vie- V' 1 = 1 ton'
Note that the initial V is known, which, along with the V. and V.
0 16 1X
defines each subsequent V-.
-35-
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Table 6. COMBINED EFFECT OF LANE WIDTH AND RESTRICTED LATERAL
CLEARANCE ON CAPACITY AND SERVICE VOLUMES OF DIVIDED FREEWAYS
AND EXPRESSWAYS WITH UNINTERRUPTED FLOW2
DISTANCE FROM
TRAFHC LANE EDGE
TO OBSTRUCTION
(FT)
ADJUSTMENT FACTOR," W, FOR LANE WJDTJI AND LATERAL CLEARANCE
OaSTRUCTION ON ONF. SIDE OF
ONE-DIRECTION ROADWAY
12-FT
LANES
1I-FT
LANES
10-FT
LANES
9-FT
LANES
OBSTRUCTIONS ON BOTH SIDES OF
ONE-DIRECTION ROADWAV
12-FT
LANES
11 -FT
LANES
10-FT
LANES
9-FT
LANES
(fl) 4-L.ANE DIVIDED FREEWAY, ONE DIRECTION OF TRAVEL
6
4
2
0
1.00
0.99
0.97
0.90
0.97
0.96
0.94
0.87
0.91
0.90
0.88
0.82
0.8!
0.80
0.79
0.73
1. 00
0.98
0.94
0.81
0.97
0.95
0.91
0.79
0.91
0.89
0.86
0.74
0.81
0.79
0.76
0.66
(b) 6- AND 8-L.ANE DIVIDED FREEWAY, ONE DIRECTION OF TRAVEL
6
4
2
0
1.00
0.99
0.97
0.94
0.96
0.95
0.93
0.91
0.89
0.88
0.87
0.85
0.78
0.77
0.76
0.74
1.00
0.98
0.96
0.91
0.96
0.94
0.92
0.87
0.89
0.87
0.85
0.81
0.78
0.77
0.75
0.70
• Same adjustments for capacity and all levels or service.
Table 7. AVERAGE GENERALIZED ADJUSTMENT FACTORS FOR TRUCKS ON
FREEWAYS AND EXPRESSWAYS, OVER EXTENDED SECTION LENGTHS2
FACTOR, T, FOR ALL LEVELS OF SERVICE
TRUCKS, PT
1
2
3
4
5
6
7
8
9
10
12
14
16
18
20
LEVEL TERRAIN
0.99
0.98
0.97
0.96
0.95
0.94
0.93
0.93
0.92
0.91
0.89
0.88
0.86
0.85
0.83
ROLLING TERRAIN
0.97
0.94
0.92
0.89
0.87
0.85
0.83
0.81
0.79
0.77
0.74
0.70
0.68
0.65
0.63
MOUNTAINOUS TERRAIN
0.93
0.88
0.83
0.78
0.74
0.70
0.67
0.64
0.61
0.59
0.54
0.51
0.47
0.44
0.42
-36-
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4. From the design speed (typically 70 mph for Interstate highways)
and the volume demand to capacity ratio (V./C.) evaluate the operating
speed S. for each segment i from Figure 12.
5. From the equation below (developed in the Analysis Section), evaluate
the Equivalent Vehicle Miles Traveled.
n
EVMT =£ c(Si) x dj x V.
i=o
Where c(S.) is the speed correction factor, d^ is the length in miles of
segment i, and V. is the demand on segment i in vehicles per hour.
6. If some of the segments defined in (1) above correspond to lane-
restricted public transit, then the EVMT corresponding to these segments
should be evaluated and added to the value derived above. Obtain the
schedule and routes of transit system. From the schedule evaluate the
number of trips during a given time frame (1 or 8 hours) and the expected
operating speed. Then use the following equation to evaluate EVMT due to
transit trips for a highway.
n
EVMT =£ N. x d. x c (S^
i=o
where
N. = Number of transit trips over segment i during time period,
d. = Length in miles of segment i.,
c(S.) = Speed correction factor for heavy duty vehicles (assume
equal to 1 until data becomes available).
The demands for use of non-transit lanes in Step 3 above should be adjusted
for the impact of the urban transit system, if this has not been done
already. This can be done by evaluating the expected number of passengers
to be carried by the transit system. It is also necessary to know the
percentage of these which would normally be riding in a private automobile
and the average number of passengers per automobile. The reduced demand
is given by:
P x N
-37-
-------
where
D = The reduced demand in vehicles per hour,
P = The percentage of passengers who would normally ride in a
private auto,
N = The number of passengers to be carried by the transit
system during time period T,
A = The average number of passengers per auto,
T = The time period being considered (1 or 8 hours).
MICRO-ANALYSIS
Micro-analysis centers about diffusion m..del ing of a highway as a line
emission source or a series of line emission sources. The objective of
this methodology is to provide a means of obtaining the line source
intensity, which is normally expressed in units of grams of pollutant per
unit distance per unit time. While the geometry of the line source is
determined by highway design, the actual source intensity varies with
location and time. Since traffic density is reasonably constant within
segments, it is reasonable to limit our consideration of location to
differentiation between segments.
For a given segment, volume demand and operating speed are obtained as
outlined in Macro-Analysis. Density can be derived by the following
equation:
where
D. = Density in vehicles per mile,
V. = Volume demand in vehicles per hour_,
S. = Speed in miles per hour.
-38-
-------
The line source intensity is the product of emission rate per vehicle and
density. Thus:
I. = e, c(Sj J-
Si
where
I. = Line source intensity in gms per hour per mile,
e. = Pollutant rate of emission in gms per hour for one vehicle,
c(S.) = Speed correction factor,
and V. and S. are as defined above.
In summary, the traffic parameters derived for macro-analysis can be
directly translated for use in micro-analysis. Diffusion modeling can be
used to reflect spatial distribution of pollutant concentrations and to
reflect more sensitively the actual geometric design of the highway,
surrounding terrain, and anticipated weather conditions.
METEOROLOGICAL ASPECTS
The meteorological characteristics which most importantly affect atmospheric
dilutive capacity are mixing height, wind speed and atmospheric stability.
A convenient summary of mixing height and wind speed characteristics which
affect air pollution potential is given in the Office of Air Programs
Publication No. AP-101 (Holzworth 1972). Atmospheric stability may be
determined in terms of cloud cover, solar radiation and wind speed by a
method proposed by Pasquill and shown in Table 8. For ground level sources,
such as automobiles on highways, the ground level concentrations, both
in the vicinity and downwind of the sources will be inversely proportional
to wind speed and mixing height, and directly proportional to atmospheric
stability (i.e., the more stable the atmosphere, the higher the concentration)
The seasons of peak use of highways have been cited as the winter months
in the southern part of the country, and the summer months in the northern
part. The peak 1-hour and 8-hour periods will occur during Friday afternoon
-39-
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Table 8. KEY TO STABILITY CATEGORIES (after Turner 1970)
Surface Wind
Speed (at 10 m),
m sec"1
<2
2-3
3-5
5-6
>6
Day
Incoming Solar Radiation
Strong
A
A-B
B
C
C
Moderate
A-B
B
B-C
C-D
C
Slight
B
C
C
D
D
Night
Thinly Overcast
or
>_ 4/8 Low Cloud
E
D
D
D
< 3/8
Cloud
. F
: E
D
D
The neutral class, D, should be assumed for overcast conditions during
day or night.
NOTE: Class A is the most unstable, class F the most stable class. Night
refers to the period from 1-hour before sunset to 1-hour after sunrise.
Note that the neutral class, D, can be assumed for overcast conditions
during day or night, regardless of wind speed.
"Strong" incoming solar radiation corresponds to a solar altitude greater
than 60° with clear skies; "slight" insolation corresponds to a solar
altitude from 15° to 35° with clear skies. Table 170, Solar Altitude
and Azimuth, in the Smithsonian Meteorological Tables (List 1951) can be
used in determining the solar altitude. Cloudiness will decrease incoming
solar radiation and should be considered along with solar altitude in
determining solar radiation. Incoming radiation that would be strong with
clear skies can be expected to be reduced to moderate with broken (5/8 to
7/8 cloud cover) middle clouds and to slight with broken low clouds.
-40-
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and evening, when home-bound commuter traffic combines with weekend departures,
The single hour on any given weekend is generally during departure, say,
5 to 6p.m. The peak 8-hour period would encompass the total combined
period and thus would run about, say, 2p.m. to 10p.m.
Mean afternoon wind speeds and mixing heights for the winter months are
shown in Figures 14 and 15, and for the summer months in Figures 16 and 17,
taken from Holzworth (1972). During the afternoon and into the early
evening, atmospheric stability classes B, C and D may occur, with classes
C and D being the most prevalent. As periods further into the evening
are considered, class D becomes even more prevalent, with class E beginning
to occur.
The period when meteorological conditions are least favorable for diluting
pollutants is the period when highways are generally in periods of lesser
use. This would be the period from very late evening until approximately
sunrise. It is most often during this period that mixing heights are lowest,
wind speeds are lightest, and atmospheric stability is greatest.
Special attention should be paid, depending on the location, to the possible
coincident occurrence, say on the last business days before Thanksgiving
and Christmas, of the normal homeward bound commuter load, together with
shoppers and holiday bound travelers, as potentially creating the highest
peak load encountered.
THE NINE QUESTIONS
Hhile the specific information called for by the task work statement
has been provided in the sections from Highway Characteristics and
Parameters through Meteorological Aspects, the nine questions spelled
out as part of the statement warrant specific response. This is given
concisely here, with the question abbreviated.
1. Area allotted to or occupied by a single vehicle? Not relevant to
highways, except in the "stopped" or "jam" condition (approximately 200
vehicles per mile).
2. Percentage of highway potentially occupied by vehicles? The usual
percentage? Treated in related sense as vehicle density in Analysis and
Results sections.
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Figure 14. ISOPLETHS (m sec-1) OF MEAN WINTER WIND SPEED AVERAGED THROUGH THE AFTERNOON MIXING LAYER
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CO
I
10-
Figure 15. ISOPLETHS (m x 102) OF MEAN HINTER AFTERNOON MIXING HEIGHTS
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Figure 16. ISOPLETHS (m sec'1) OF MEAN SUMMER WIND SPEED AVERAGED THROUGH AFTERNOON MIXING LAYER (Figure 5)
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I
45.
cn
Figure 17. ISOPLETHS (mxlO2) OF MEAN SUMMER AFTERNOON MIXING HEIGHTS
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3. Typical and peak va'iu;-s (absolute or fractional) of vehicles running
for one- and eight-hour periods? These data are developed in Section V.
4. Typical and worst case (slowest) vehicle speeds? Treated in
Sections VI and VII, Typical spaeds correspond to normal flow. Worst
speeds (idling) corresponu to completely congested traffic flow.
5. Vehicle distribution within the complex? Ultimately defined by
spatial design of highway. Vehicle density considered variable from
segment to segment.
6. Design parameters of the complex likely to be known before hand?
See section titled Highway Characteristics and Parameters.
7. Design parameters in question (6) which can be most successfully
related to traffic, and hence emissions? See section titled Analysis.
8. Relationships of parking lot design to parking densities and
vehicle circulation? What is typical design? Design with highest parking
densities, lowest vehicle speeds, longest vehicle operating times? Not
relevant to highways.
9. Meteorological conditions likely to occur during peak use? Use
level during periods of worst meteorology? See section titled Meteorological
Aspects.
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Section VIII
REFERENCES
1. Personal communication with Mr. Robert Probst, Federal Highway
Administration.
2. Highway Capacity Manual* Highway Research Board Special Report 87,
1965. National Academy of Sciences, National Research Council,
Washington, D.C.
3. Traffic Information Requirements for Estimates of Highway Impact on
Air Quality. Air Quality Manual Vol. Ill, FHWA-RD-72-35. Prepared
for Federal Highway Administration, Office of Research, Washington, D.C.
4. Compilation of Air Pollution Factors, publication AP-42 of the
Environmental Protection Agency, April 1973.
5. Lynch, Kevin. 1962. Site Planning. The M.I.T. Press, Massachusetts
Institute of Technology, Cambridge, Massachusetts.
6. Bern'off, Barry and Ahmad Moghaddas. February 1970. "Stopped Vehicle
Spacing on Freeways." Traffic Engineering.
7. Automobile Exhaust Emission Surveillance. May 1973. Prepared for
Environmental Protection Agency by Calspan Corporation. PB-220-755.
8. U.S. Department of Commerce, Bureau of Public Roads, Highway Statistics,
GPO, Washington, D.C.
9. May, A.D., "Traffic Characteristics and Phenomena on High Density
Controlled Access Facilities." Traffic Engineering, 31:No. 6, 11-19, 56
(March 1961).
* Figures and tables from the Highway Capacity Manual should not be repro-
duced in formal publications without permission from the Highway Research
Board.
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10. Keefer, L.E., "The Relation Between Speed and Volume on Urban'
Streets." Quality of Urban.Traffic Service Committee Report, HRB,
37th Ann. Meeting (1958) (unpubl.).
11. Webb, G.M., and Moskowitz, K., "California Freeway Capacity Study--1956.
Proc. HRB, 36:587-642 (1957).
12. Edie, L.C., Foote, R.S., Herman, R., and Rothery, R., "Analysis of
Single-Lane Traffic Flow." Traffic Engineering, 33:No. 4, 21-27
(January 1963).
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Section IX
SELECTED DEFINITIONS2
Functional Types
Arterial highways - A highway primarily for through traffic, usually on
a continuous route.
Expressway - A divided arterial highway for through traffic with full or
partial control of access and generally with grade separations at major
intersections.
Freeway - An expressway with full control of access.
Major street or major highway - An arterial highway with intersections at
grade and direct access to abutting property, and on which geometric
design and traffic control measures are used to expedite the safe movement
of through traffic.
Operations
Design speed - A speed selected for purposes of design and correlation of
those features of a highway, such as curvature, superelevation, and
sight distance, upon which the safe operation of vehicles is dependent.
Average highway speed - The weighted average of the design speeds within
a highway section, when each subsection within the section is considered
to have an individual design speed.
Operating speed - The highest overall speed at which a driver can travel
on a given highway under favorable weather conditions and under prevailing
traffic conditions without at any time exceeding the safe speed as deter-
mined by the design speed on a section-by-section basis.
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Volume - The number of vehicles that pass over a given section of a lane
or a roadway during a time period of one hour or more. Volume can be
expressed in terms of daily traffic or annual traffic, as well as on. an
hourly basis.
Average annual daily traffic - The total yearly volume divided by the
number of days in the year, commonly abbreviated as AADT.
Maximum annual hourly volume - The highest hourly volume that occurs on
a given roadway in a designated.year.
Tenth, twentieth, thirtieth, etc., highest annual hourly volume - The
hourly volume on a given roadway that is exceeded by 9, 19, 29, etc.,
respectively, hourly volumes during a designated year.
Peak-Hour Traffic - The highest number of vehicles found to be passing
over a section of a lane or a roadway during 60 consecutive minutes.
Rate of Flow - The hourly representation of the number of vehicles that
pass over a given section of a lane or a roadway for some period less
than one hour. It is obtained by expanding the number of vehicles to an
hourly rate by multiplying the number of vehicles during a specified
time period by the ratio of 60 min to the number of minutes during which
the flow occurred. The term "rate of flow" will normally be prefixed by
the time period for the measurement. For example, a 15-min count of N
vehicles multiplied by 60/15 or 4 would produce a "15-min rate of flow
of 4N vehicles per hour."
Interrupted Flow - A condition in which a vehicle traversing a section of
a lane or a roadway is required to stop by a cause outside the traffic
stream, such as signs or signals at an intersection or a junction. Stop-
page of vehicles by causes internal to the traffic stream does not
constitute interrupted flow.
Uninterrupted flow - A condition in which a vehicle traversing a section
of a lane or a roadway is not required to stop by any cause external to
the traffic stream although vehicles may be stopped by causes internal
to the traffic stream.
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Density - The number of vehicles occupying a unit length of the through
traffic lanes of a roadway at any given instant. Usually expressed in
vehicles per mile.
Average density - The average number of vehicles per unit length of roadway
over a specified period of time.
Critical density - The density of traffic when the volume is at capacity
on a given roadway. At a density either greater or less than the critical
density, the volume of traffic will be decreased. Critical density occurs
when all vehicles are moving at about the same speed.
Levels of Service - Traffic operational freedom on a highway of a particular
type is considered equal to or greater than level of service A, B, C, or
D, as the case may be, when specified values of the two separate conditions
previously described are met. These conditions require that: (1) operating
speeds or average overall speeds be equal to or greater than a standard
value for the level considered, and (2) the ratio of the demand volume
to the capacity of any subsection not exceed a standard value for that
level. Level of service E describes conditions approaching and at
capacity (that is, critical density). Level F describes conditions under
high-density conditions when speeds are low and variable; it is not
effectively described by combinations of speed and volume-to-capacity
ratios, because these may vary widely.
Level of service A describes a condition of free flow, with low volumes
and high speeds. Traffic density is low, with speeds controlled by
driver desires, speed limits, and physical roadway conditions. There is
little or no restriction in maneuverability due to the presence of other
vehicles, and drivers can maintain their desired speeds with little or
no delay.
Level of service B is in the zone of stable flow, with operating speeds
beginning to be restricted somewhat by traffic conditions. Drivers still
have reasonable freedom to select their speed and lane of operation.
Reductions in speed are not unreasonable, with a low probability of
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traffic flow being restricted. The lower limit (lowest speed, highest
volume) of this level of service has been associated w'ith service volumes
used in the design of.rural highways.
Level of service C is still in the zone of stable flow, but speeds and
maneuverability are more closely controlled by the higher volumes. Most
of the drivers are restricted in their freedom to select their own speed,
change lanes, or pass. A relatively satisfactory operating speed is still
obtained, with service volumes perhaps suitable for urban design practice.
Level of service D approaches unstable flow, with tolerable operating
speeds being maintained though considerably affected, by changes in
operating conditions. Fluctuations in volume and temporary restrictions
to flow may cause substantial drops in operating speeds. Drivers have
little freedom to maneuver, and comfort and convenience are low, but
conditions can be tolerated for short periods of time..
Level of service E cannot be described by speed alone, but represents
operations at .even lower operating speeds than in level D, with volumes
at or near the capacity of the highway. At capacity, speeds are typically,
but not always, in the neighborhood of 30 mph. Flow is unstable, and
there may be stoppages of momentary duration.
Level of service F describes forced flow operation at low speeds, where
volumes are below capacity. These conditions usually result from queues
of vehicles backing up from a restriction downstream. The section under
study will be serving as a storage area during parts or all of the peak
hour. Speeds are reduced substantially and stoppages may occur for short
or long periods of time because of the downstream congestion. In the
extreme, both speed and volume can drop to zero.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-450/3-74-003-f
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Vehicle Behavior In and Around Complex Sources and
Related Complex Sources Characteristics
Volume VI - Major Highways
5. REPORT DATE
November 1973 (Date of issue)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Scott D. Thayer
Jonathan D. Cook
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORG'XNIZATION NAME AND ADDRESS
10. PROGRAM ELEMENT NO.
Geomet, Inc.
50 Monroe Street
Rockville, MD. 20850
11. CONTRACT/GRANT NO.
68-02-1094
12. SPONSORING AGENCY NAME AND ADDRESS
Office of Air Quality Planning & Standards
Environmental Protection Agency
Research Triangle Park, North Carolina 2.7711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A general methodology is presented for relating parameters of traffic behavior
on major highways, including traffic volume and average speed, to more readily avail
able characteristics of highways, including design speed and capacity. Such
relationships are to be used to relate major highway characteristics to air quality.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. cos AT I Field/Group
Air pollution, highways, transportation
management, urban planning, urban develop-
ment, urban transportation
models, land use, regional
vehicular traffic,
highway planning
transportation
planning,
Indirect sources
Indirect source review
13 B
traffic engineering,
8. DISTRIBUTION STATEMENT
Release unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
20. SECURITY CLASS /Thispage)
Unclassified
21. NO. OF PAGES
57
22. PRICE
EPA Form 2220-1 (9-73)
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INSTRUCTIONS
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