United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park NC 27711
EPA-450/3-78-034
August 1978
Air
Carbon Monoxide
Hot Spot
Guidelines
Volume II: Rationale
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EPA-450/3-78-034
Carbon Monoxide Hot Spot Guidelines
Volume II: Rationale
by
Frank Benesh
GCA Corporation
GCA/Technology Division
Burlington Road
Bedford, Massachusetts 01730
Contract No. 68-02-2539
EPA Project Officer: George J. Schewe
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air, Noise, and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
August 1978
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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 - in limited quantities - from the
Library Services Office (MD-35), U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina 27711; or, for a fee, from the
National Technical Information Service, 5285 Port Royal Road, Springfield,
Virginia 22161.
This report was furnished to the Environmental Protection Agency by
CCA Corporation, CCA/Technology Division, Burlington Road, Bedford,
Massachusetts 01730, in fulfillment of Contract No. 68-02-2539. The contents
of this report are reproduced herein as received from CCA Corporation.
The opinions, findings, and conclusions 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-450/3-78-034
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ABSTRACT
This report presents the rationale used in developing the analytical tech-
niques for the carbon monoxide hot spot guidelines.
Discussed in this report are the technical aspects of the guidelines, in-
cluding the assumptions used in developing hot spot procedures. Since the
guidelines were based largely on EPA's Indirect Source Guidelines, these
are discussed in some detail, as well.
iii
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iv
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PREFACE
This document is the first in a series comprising the Carbon Monoxide Hot
Spot Guidelines. The purpose of this series is to provide state and local
agencies with a relatively simple yet accurate procedure for assessing
carbon monoxide hot spot potential on urban street networks. Included
in the Hot Spot Guideline series are:
Volume I: Techniques
Volume II: Rationale
Volume III: Summary Workbook
Volume IV: Documentation of Computer Programs to Generate Volume I
Curves and Tables
Volume V: Intersection-Midblock Model User's Manual
Volume VI: Modified ISMAP User's Manual
Volume VII: Example Applications at Waltham/Providence/Washington, B.C.
Hot spots are defined as locations where ambient carbon monoxide concen-
trations exceed the national ambient air quality standards (NAAQS). For
both the 1-hour and 8-hour averaging times the assumption is made through-
out these guidelines that a CO hot spot is primarily affected by local
vehicle emissions, rather than areawide emissions. Studies have shown
that for the 1-hour CO concentration, local sources are the dominant
factor. Accordingly, representative urban worst-case meteorological,
traffic, and background concentration conditions are selected as those
corresponding to the period of maximum local emissions — usually the
period of peak traffic. For 8-hour concentrations evidence indicates
that neither the local nor the areawide contributions can be assumed to
be dominant in every case. However, for the purpose of analysis discussed
in these guidelines, local source domination of CO hot spots is assumed
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for 8-hour averages. This allows some consistency between assumptions in
relating the 1-hour and 8-hour CO estimates.
vi
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CONTENTS
Page
Abstract ill
Preface v
List of Figures viii
List of Tables ix
Acknowledgments x
Sections
I Introduction 1
Introduction 1
II Development and Assumptions of the Verification Procedure 3
Introduction 3
Derivation of the Normalized Concentration Curves 5
Derivation of Street Canyon Curve 21
Derivation of Emission Rates 22
III Development of the Hot Spot Screening Process 34
Introduction 34
Assumptions Regarding Background Concentrations 36
Uninterrupted Flow Conditions 39
Interrupted Flow Conditions - Signalized Intersections 43
Interrupted Flow Conditions - Nonsignalized
Intersections 46
Effects of Variation in Parameter Assumptions 47
IV References 52
vii
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FIGURES
No. Page
1 Intersection Geometry 8
2 Normalized CO Concentration Contribution from Excess 9
Emissions on Approach 1
3 Variation of the Normalized CO Concentration with Roadway 10
Length, Road/Receptor Separation, Stability, Wind/Road Angle
4 Normalized CO Concentration Contributions from Excess 12
Emissions on Approaches 2, 3, and 4
5 Intersection Geometry - Crossroad 13
6 Distance Correction Factor for Excess Emission Contributions 15
at Intersections
7 Normalized CO Concentration Contributions from Free-Flow 16
Emissions on Each Lane of Roadways at Intersections
8 Values of Xu/Q (lO"^"1) for Various Roadway/Receptor 17
Separations and Wind/Roadway Angles; Infinite Line Source
9 Distance Correction Factor for Free-Flow Emission Contri- 18
butions at Intersection Locations
10 Normalized CO Concentration Contribution from Each Traffic 20
Lane at Locations of Uninterrupted Flow
11 Examples of Effects on Screening Curves of Variations in 50
Parameter Assumptions
Vlll
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TABLES
No. Page
1 Assumed Operating Speeds, Levels of Service and Demand- 42
Capacity Ratios for Major Streets and Corresponding Emission
Factors for Free Flow Conditions
2 Assumed Operating Speeds, Levels of Service and Demand- 42
Capacity Ratios for Urban Expressways and Corresponding
Emission Factors for Free Flow Conditions
3 Assumptions for CO Concentration Computations 48
4 Variation in Estimated Parameter Values for Sensitivity 49
Analysis
5 Changes in Parameters versus Changes in Allowable Volumes 51
IX
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ACKNOWLEDGMENTS
We wish to acknowledge the significant contributions made early in the
development of the Hot Spot Guidelines by previous GCA/Technology Division
staff members, including Dr. Robert Patterson, Messrs. David Bryant,
Alan Castaline, and Walter Stanley. We are especially indebted to the
EPA Project Officer, Mr. George J. Schewe of the Source Receptor Analysis
Branch, who provided overall project direction and performed extensive
technical and editorial review of the final reports.
x
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SECTION I
INTRODUCTION
INTRODUCTION
This volume, Volume II, is part of a seven-volume report providing guide-
lines for identifying and analyzing carbon monoxide "hot spots"-locations
with the potential for experiencing violations of the National Ambient
Air Quality Standards (NAAQS) for CO. These guidelines are intended for
engineers, planners, and others who must consider the air quality effects
of traffic management decisions and who are responsible for traffic plan-
ning to control CO hot spots.
The guidelines present two levels for screening potential hot spots. One
is a screening procedure to identify potential carbon monoxide hot spots
using only data on automobile traffic volumes, thus obviating time-
consuming and costly monitoring of air quality at potential hot spots.
The other is a hot spot verification procedure that uses more detailed
input data. This procedure provides the capability of accounting for a
number of additional conditions beyond those assumed in the screening
procedure, and it provides a worst case quantitative estimate of hot spot
potential.
Volume I discusses in detail the concepts of hot spot screening and
verification as well as providing analytical techniques and procedures.
Two companion volumes, this volume and Volume III, present the technical
rationale behind the guidelines and a workbook. Volumes IV, V and VI
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provide documentation respectively on an intersection midblock traffic
and dispersion model, the IMM; a traffic assignment and dispersion model,
the modified 1SMAP model; and the computer programs used to generate the
curves and tables in Volume I. Volume VII presents the results of sev-
eral application studies. The remainder of this volume describes the
background and techniques in Volume I.
Section II provided a discussion of the rationale for developing the
verification procedures described in Volume I. Also provides are discus-
sions of the technical aspects of the guidelines including traffic,
emission, and dispersion used and the basis for the assumptions. The
emphasis is on how the various nomographs and tables were constructed
rather than on how to apply the procedure. A similar discussion is
provided in Section III concerning development of the screening procedures,
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SECTION II
DEVELOPMENT AND ASSUMPTIONS OF THE
VERIFICATION PROCEDURE
INTRODUCTION
The verification process in Volume I is a followup to the initial screen-
ing of an urban area. The intent is to perform a more thorough evaluation
of the hot spot potential of a street section or intersection using a tech-
nique that permits input of parameters specific to that location rather
than assumed parameters. While the initial screening process focused on
identifying potential hot spot locations anywhere within a city or town
(thus requiring a very general approach), the verification process involves
analysis of specific locations, and a more detailed analysis of each loca-
tion is feasible. Since the screening curves are developed from the same
methodology as the verification procedure, using a more detailed set of
assumptions, the development of the verification procedure is discussed
first.
3 4
During an earlier project, ' analysis of CO hot spots was initially en-
visioned as a two-step process: screening and detailed dispersion modeling.
Development of the hot spot screening guidelines, originally intended to be
the only hot spot guidelines, involved many assumptions and generalizations
in order to achieve the simplicity that was desired. In these original
guidelines the assumptions were such that the screening was thought to be
overly conservative and thus limit their utility. Therefore, the test
cases in Waltham, Massachusetts of the Hot Spot Guidelines were reanalyzed
utilizing the Indirect Source Guidelines, ISG (1975) to check the degree
of conservativeness.
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The application of the Indirect Source Guidelines to the Waltham study
locations led to the following conclusions:.
• The hot spot screening guidelines are not overly
conservative, as demonstrated by the fact that the
screening guidelines identified only a few more
potential hot spots in the Waltham case study than
did the application of the more detailed Indirect
Source Guidelines.
• The Indirect Source Guidelines are a workable method
for analysis of potential hot spots, and allow the
use of more data specific to conditions at individual
locations. Results are quantitative, rather than
qualitative.
• The Indirect Source Guidelines can, in some cases,
be used for assessment of alternative improvement
measures, in lieu of detailed computer modeling.
These conclusions led to a recommendation for using the Indirect Source
Guidelines as the foundation for a second, verification stage of hot spot
potential. However, the Indirect Source Guidelines have been revised2
and they (the original Guidelines ) are no longer suitable for the
level of analysis desired for hot spot verification. The Revised In-
2
direct Source Guidelines allow much more latitude in the selection of
values of input variables than did the original Guidelines, and their
application is similarly much more complex. In developing revised veri-
fication procedures based on the Revised Indirect Source Guidelines, the
intent was to deep the requirements on the user at about the same level
as in the old verification method.
The remainder of this section discusses how the new verification proce-
dures were derived using the Revised Indirect Source Guidelines, the
12
Automobile Exhaust Emission Modal Analysis Model, and the Compilation
of Air Pollutant Emission Factors (AP-42). In particular, three specific
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Assumptions
Before we begin a detailed discussion of the derivation of the normalized
concentration curves, used in Volumes I and III, we list a number of the
more important assumptions upon which these curves are based. The ratio-
nale for these assumptions are presented in the following sections.
• Queue lengths for each of the four approaches to the
intersection are assumed to be equal to the queue
length on the approach adjacent to the receptor (designated
as approach 1). Criteria for the selection of a given
intersection approach for the placement of a receptor
are presented in Volume I. It must be remembered that
lane volumes are used in the calculation of queue
lengths, but approach volumes are used to calculate
excess or free flow emissions.
• Each of the two intersecting streets are 10 meters wide.
• The distance from the receptor to the centerline of
approach 1 is 10 meters.
• Consistent with worst case urban conditions, the
atmospheric stability is chosen to be category D,
windspeed is assumed to be 1 m/sec, and the initial
vertical dispersion (°"7n) is assumed to be 5 meters.
• The optimum wind angle and location of the receptor along
approach 1 (at a perpendicular distance of 10 m from the
approach centerline) will depend upon the queue length
calculated for this approach. The resultant wind angle
and receptor location is then used for the calculation
of normalized concentration contributions for excess
emissions from the three remaining queues and the free
flow emissions from all four approaches.
Intersections - Queueing Vehicles
The main technical problem in deriving the hot spot verification procedures
was to develop curves relating traffic parameters to normalized CO con-
centrations at the critical receptor location for the interrupted flow,
or intersection, case. The location of the point of maximum concentra-
tion varies along a line parallel to the roadway depending on the queue
length and the wind angle.
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Figure 1 shows the intersection configuration for the determination of
the receptor location and wind angle (6). For the discussion that fol-
lows, all measurements in the direction of the approach in question will
be taken from a reference plane located 20 m behind the receptor. For
a wind perpendicular to the road, the reference plane establishes the
extent of the line source emissions that significantly affect the concen-
tration at the most distant receptor considered. Its use will become
apparent in the following discussion. As shown in Figure 1 the distances
from the reference plane to the downwind and upwind boundaries of the
queue are Yd and Yu respectively, so that:
Yu = Yd + Le
where Le = queue length (m).
-v *
Figure 2 depicts the normalized concentration (X ) contribution from
approach 1 as a function of effective queue length, Le, on that approach.
It is assumed that all lanes comprising approach 1 develop queues of
equal length. The CO concentration at the receptor site is maximized
when the wind angle 6 is such that the contribution from the nearest
lane is maximized. Figure 2 was developed from Figure 3 which is taken
2
from the Revised Indirect Source Guidelines (a = 5m, stability class D).
f-i\J
Figure 3 is based on sequential runs of HIWAY^^ and treats queuing vehicles
*
as finite line sources. X is a function of the distance from the receptor
to the emission source(x), the roadway/wind angle (6), the length Yu, and
* / * * \
the length Yd. X equals ^X (Yu) - x (Yd))) and Yu equals (Le + Yd) where
Le is the effective queue length. For a given queue length Le and dis-
-k
tance x, X is maximized when the following condition is satisfied:
*
dX
*
that is, when X is at a peak value in the curves in Figure 3.
Normalized with respect to wind speed and emissions, such that X = x U
e e
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LEG 4
LEG I
LEG 3
Reference Plane
Yu - Distance from reference plane to upwind end of queue (m)
Yd • Distance from reference plane to downwind end of queue (>0)
(Distance is denoted positive to windward) (m)
iY - Distance between receptor and reference plane (20m)
Le • Effective excess emissions length (m)
V • Wind vector
6 • Wind/roadway angle (acute)
x • 10m
Figure 1. Intersection geometry
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9 20 60 100 200 300 600 10
QUEUE LENGTH , L, (m)
Figure 2. Normalized CO concentration contribution from excess
emissions on approach 1
-------
(A) ROAD/RECEPTOR SEPARATION - 10.0
SIGMA ZO = 5.0 m STABILITY - D
(B) ROAD/RECEPTOR SEPARATION - 15.0
SIGMA ZO • 5.0 m STABILITY - D
(C) ROAD/RECEPTOR SEPARATION = 20.0 m
SIGMA ZO - 5.0 m STABILITY =- D
1000
1000
800
(D) ROAD/RECEPTOR SEPARATION - 40.0 m (£) ROAD/RECEPTOR SEPARATION = 80.0 m (F) ROAD/RECEPTOR SEPARATION •= 160.0 m
SIGMA 2O = 5.0 m STABILITY - D SIGMA ZO = 5.0 m STABILITY = D SIGMA ZO = 5.0 m STABILITY - D
600
o
p
6
400 -
a
200
0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90
WIND/ROAD ANGLE — degrees WIND/ROAD ANGLE — degrees
10 20 30 40 50 60 70 80 90
WIND/ROAD ANGLE — degrees
SA-4429-15
Figure 3. Variation of the normalized CO concentration with roadway length, road/rectptor separa-
tion, stability, wind/road angle
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In terms of Figure 3a, one would move the receptor downwind of the queue
(subject to the constraint of a 10 m road receptor separation), thereby
decreasing the wind/road angle and increasing both Yu and Yd. This max-
imization procedure is actually a two-step process in which the distance
Yd and therefore Yu are first selected and then the wind angle which
i\ if
maximizes (X (Yu) - X (Yd)) is determined. A larger value of Yd is then
obtained. In practice, a value of Yd of approximately 20 m was found to
if
maximize X in most cases.
e
*
Figure 4 is used to determine the x contributions from legs 2, 3, and
4 of the intersection. This graph was also developed from Figure 3.
This curve was formulated by assuming the road center/receptor seperation, x
of the leg 2 approach is 15 m and Yu = Yu + Le + 10 m.
approach 2 approach 1
Inherent in the latter assumption is the fact that the queue which de-
velops on approach 2 is the same length as that which develons on
approach 1 (Le). Yd = Yu - Le, and 6 is the same as de-
approach 2 approach 2
termined previously for approach 1, at the corresponding Le. Hence, all
*
variables are specified and x for approach 2 can be derived from Figure 3(B)
The procedure is slightly different for determining the excess emissions
from the crossroad, approaches 3 and 4. The reference plane must be
rotated 90° to yield the intersection geometry depicted in Figure 5.
The actual receptor location (where the contribution from approach 1 is
maximized) remains unchanged, as does the wind direction. However, the
roadway/wind angle 0' is 90° - 6. Yu = 27.5 m and Yd = 0 for approach 3.
The roadway/receptor separation distance x = Yu - 12.5 m. Thus,
approach 1
x varies as Le changes on approach 1.
11
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140
130
120
110
100
?E 90
o
^_« eo
-•—- TO
60
50
40
30
20
10
50 100 190 200
EFFECTIVE QUEUE LENGTH, L, (m«t«rt)
50O
Figure 4. Normalized CO concentration contributions from excess
emissions on approaches 2, 3, and 4
12
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LEG 4
LEG I
RECEPTOR
REFERENCE PLANE
LEG 3
XrYu, -12.5
V
LEG 2
(o) APPROACH 3 CONTRIBUTION
Yu =27.5
Yd =0
X=Yu-l7.5
LEG I
-Le
LEG 4
•J RECEPTOR
REFERENCE PLANE
LEG 3
LEG 2
. 5
(b) APPROACH 4 CONTRIBUTION
Figure 5. Intersection geometry - crossroad
-------
* o
For approach 4, x is calculated at 0' = 90 - 9, Yu = Le + 37.5 m,
Yd = 37.5 m, and x = Yu - 17.5 m. Note that slight errors will
approach 1
result for crossroad flow configurations and lane widths which would yield
values of x that differ from that of the estimated values. However, this
*
error will be extremely small since the xo value at large 9 changes very
*
slowly with changes in x; i.e., dxe (9) « Q at e>45°
dx ~
*
Figure 6 gives the distance correction factors (Cxe) for the X terms
previously determined. These curves were derived by calculating
the X values in the HIWAY model at road/receptor separation distances
other than 10 m, thus at different optimum wind angles. The ratio of
these X values to X at x = 10 was plotted. The curves are given for
e e
queue lengths only up to 180 m, since longer queue lengths produce curves
that vary only slightly from the 180 m queue length curve.
Intersections - Through Vehicles (Free-Flow)
In making CO concentration estimates at a receptor near an intersection,
the contribution from free-flowing vehicles, that is, those that do not
stop, on each leg must also be considered.
Figure 7 from the Volume 1 procedures depicts the normalized concentration
contribution from free-flowing traffic, Xf as a function of queue length
Le. Actually X is a function of the 6 which maximizes the excess emis-
sion contribution, X at that Le. Excess emissions contributions are
e,
generally higher than free-flow, hence the use of the 6 and Le that max-
imizes X . Figure 8, from the Revised ISG, was used in generating the
curves by taking a given Le and the associated roadway/wind angle, 0,
as determined previously in constructing Figure 3, and finding xf- Curves
are plotted for both the main road and crossroad at each intersection.
Distance correction factors similar to those derived in Figure 6 are
shown for free-flow traffic contributions at intersections in Figure 9.
14
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10
20
30 40
50
60 70 80 90 100 110 120 130
x ROAD/RECEPTOR SEPARATION, meters
140 ISO 160 170 ISO
Figure 6. Distance correction factor for excess emission
contributions at intersections
-------
in
z
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in
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700
600
500
400
300
250
200
150
100
90
80
70
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50
40
30
20
10
MAIN ROAD
^
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*••'
—
0 20 30 40 50 60 70 8090100 150 2C
EFFECTIVE QUEUE LENGTH, Le ( meters )
Figure 7. Normalized CO concentration contributions from free-flow
emissions on each lane of roadways at intersections
16
-------
NEUTRAL STABILITY (D)
10
Figure 8.
20 30 40 50 60 70 80 90 100 200
ROADWAY/RECEPTOR SEPARATION __m
Values of
"1
(HT"1) for various roadway/receptor
separations and wind/roadway angles; infinite line
source
17
-------
00
O
(J
1.0
-------
Uninterrupted Flow
Curves for concentration estimates from uninterrupted flow roadways
(expressways, midblock locations, etc.) in Volume I are derived similarly
to the above from the infinite line source curves (Figure 8) in the
2
Revised Indirect Source Guidelines. Figure 10 gives the normalized
*
concentration contributions from each traffic stream. In this case X^
is a function of the road/receptor distance, x, and the road/wind angle,
0, and are taken from the maximum concentration estimates in Figure 8.
#
Because Xf is a function of x no further distance correction is needed.
Further Comments on Curves
The procedure just described is based upon a number of limiting assumptions.
The most important of these is that the position of the critical receptor
location at an intersection is determined by the queue length on approach 1,
Ideally, the receptor should be located so as to maximize the joint con-
tributions from the free flow emissions and the queue emissions on all
approaches but this approach would be quite difficult to implement on a
graphical basis. Using the queue on approach 1 to locate the critical
receptor is a reasonable approximation for the following reasons:
• The relative concentrations of approaches 2, 3 and 4 are
small with respect to approach 1. For example, a queue
length of 60 meters (approach 1) will contribute approxi-
mately 70 percent of the contribution of all queues combined.
• Although the normalized contribution of queue and free
flow emissions are comparable, the excess emissions
assigned to the queue are often several times higher
than the free flow emissions.
• If free flow emissions were allowed to influence the
choice of the critical receptor and the wind angle,
the wind angle which would have been selected for the
10 meter road-receptor configuration would have been
too small in terms of finite queues or finite line
sources. The assumption of an "infinite" line source for
free-flow traffic does not apply very well near inter-
sections, hence the use of the limiting wind/road angle
dependent on queues.
19
-------
LJ
(J
IT
tO 00
900
800
= 700
ac
o
E
10
i
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o
4
o:
UJ
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UJ
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o
600
500
400
300
200
100
-I-
10
15 20 25 30 35 40 45 50 60
ROADWAY/RECEPTOR SEPARATION, meters
70 80 90 100
Figure 10. Normalized CO concentration contribution from each traf-
fic stream at locations of uninterrupted flow
20
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The green phase length is a fraction of the signal cycle time minus the
total .amber time. ,A:3-second amber time is assumed for all green phases.
The green phase length of phase j is given: by the following equation:
Max (V /Cs. )
j x r j 1 »J
G. = Cy ± 3 (6)
J E Max (V. ./Cs^ .)
all j i *•* *>*
where Max (V. ./Cs. .) is the maximum V/Gs ratio on all approaches i
i '•* moving on greeii phase j.
3 is an assumed 3-second amber time.
2 (V^ ./Cs. .) is the sum of the V/Cs ratios that control
all j 'J >J the green phase durations.
The approach capacity, C, is found by multiplying the approach capacity
service Volume by the appropriate green to cycle ratio and summing for
all applicable phases. The capacity of an approach is given as follows:
Ci = j Csi,j Gj/Cy (7)
where j are those green signal phases that allow traffic to move on inter-
section approach i.
Unsignalized Intersection Traffic Movement
At unsignalized intersections, the number of queued vehicles, N, is given
simply by
N -
The queue length is found using equation (3) or 40 meters as before, and
the idle time is
360 ON
Rq = ~ n - (9)
25
-------
For an unsignalized intersection, the approach capacity, C, is estimated
differently than for a signalized intersection. Instead of depending on
the G/Cy ratio, it is a. function of the traffic flow on the cross street
and the time gap between cross street vehicles that is acceptable to a
driver wanting to cross or turn onto the cross street. In this case C
is found from
-T(V + V )/3600
e m
i , -T(2V)/3600
1 - e
where V is the volume in one direction on the cross street in vph.
2V is the assumed two-way volume on the cross street.
The capacity on the cross street is assumed to be equal to the free flow
capacity on that street. It is also assumed that no vehicles on the
cross street stop at the intersection. The parameter T is the acceptable
time gap (seconds) between cross street vehicles. It was assumed to be
4 seconds in developing Table 11 of Volume I. While this value is lower
than that given in the Revised Indirect Source Guidelines and hence produces
a higher capacity and fewer queued vehicles, it was chosen as being more
appropriate for congested, potential hot spot locations where more aggres-
sive driver behavior would previal.
Excess Emission Calculation
To calculate the excess emissions produced due to the queue length, Le,
and the idle time, Rq, emissions must be known as a function of driving
mode as well as speed. Thus, rather than using average speed as in AP-42,
an emissions estimating technique called the Modal Model is used. The
Modal Model calculates total emissions over a user-specified driving sequence
by adding the emissions from each 1-second time interval of the driving
sequence. The emissions during each interval are found as the product of
the "instantaneous" emission rate and the 1-second time interval. The
instantaneous emission rate, e, during deceleration and acceleration modes
is a function of speed, v, and acceleration or deceleration, a:
26
-------
e (v,a) or e (v,a) = b- + b v + b a + b.av + b,-v
2 2 2 22
+ b,a + b,v a + b_a v + b.v a . (11)
o / o y
where for this application it is assumed a = 2.5 mph sec" , a typical value.
During cruise mode it is a function of speed only:
ec (v,o) = b1Q + buv + b12v2. (12)
These two equations were modified to calculate modal emissions for hot
spot analysis as described below.
All Modal Model emission estimates above are from a user-specified vehicle
age mix. The effect of different vehicle age mixes is to change the b. co-
efficients of equations (11) and (12). These empirical coefficients were
derived using 1975, warmed-up light-duty vehicle emission data deteriorated
to 1977 emission levels.
Excess emissions are those occurring over and above those which would have
occurred had the vehicle not stopped. This may be expressed by:
EE = V ED - EC + EID Rq (13)
where E = total excess emissions per vehicle
E
E = total emissions due to acceleration per vehicle
A
E = total emissions due to deceleration per vehicle
E = total idle emissions per vehicle/per second
R = average queueing time (seconds)
q
E = total cruise emissions per vehicle
c
The emissions during acceleration or deceleration are found by relating
speed to acceleration and time. Under constant acceleration or deceleration
the speed can be expressed as a function of time as:
27
-------
v = v + at (14)
o
where v is the initial speed of the vehicle .
v is the vehicle speed after time, t
a is the rate of acceleration or deceleration
t is the time of travel
Substituting Equation (14) in Equation (11), and integrating over the time
to come to a stop or to reach cruise speed:
/T
e (v(t),a) dt, grams per vehicle
A
/T
e_ (v(t),a) dt, grams per vehicle
..
(15)
•T
Implicit in these equations is that the initial approach and the final de-
parture speeds are assumed to be equal.
The emissions due to idling are not estimated using the Modal Model, but
are calculated using AP-42 (1978) mobile source emission factors since the
estimate of idle emission from the Modal Model, that is the use of b^g, is
less accurate than the AP-42 factor. This is because AP-42, is from observed
data and the Modal Model from an empirical fit. The cruise emissions may be
estimated by using the vehicle speed in Equation (12). This is the estimated
emissions per vehicle on an uninterrupted roadway. The cruise emissions
used in Equation (13), however, must only be over the acceleration and
deceleration distance so that only those emissions had the vehicle not
stopped are subtracted.
When 'constant acceleration is assumed as in this analysis, the time (T)
to cover the distance a vehicle travels accelerating from 0 mph to cruise
speed, is traveled by a cruising vehicle in %T. Hence, to calculate the
equivalent emissions of a cruising vehicle during acceleration and decelera-
tion, Equation (12) is modified so that v is a function of time (as in
Equation (14)) and integrated with respect to i>T:
28
-------
E
.T/2
c
/'
e£ (v(t),0) dt, grams per vehicle
(16)
This is two times the integral to allow for both the acceleration and de-
celeration portions.
In deriving the excess emissions for the hot spot guidlines, the estimated
idle emissions from AP-42 times the idle time plus the integrated Modal
Model estimates in Equations (15) and (16) are summed as in Equation (13).
-1 -1
Converting to units of gm sec by considering the number of vehicles that
stop, N, the total cycle time, C , the average running time for all vehicles,
y
Rq, the volume demand, V, and the queue length, Le, (and letting
E = E + E - E ) Equation (13) becomes:
AJJ A JD L»
(EN
AD
~
R
EE-
which gives the emissions occurring per unit length of queue from acceler-
ating, decelerating, and idling vehicles.
Free Flow Emissions
Similarly, the cruise emissions, e , in Equation (12) may be used to esti-
mate free flow emissions for all vehicles travelling through an intersection
or for uninterrupted flow. Considering the number of vehicles per hour, V,
and the distance traveled by those vehicles in 1 hour, X, (i.e., the speed,
mph), the following equation results:
e V , .
-c ,__ _-l ___-!) (lg)
c X 1609
where 1609 is the conversion from miles to meters.
29
-------
Derivation of Emission Correction Factors
As stated in the last section, the Modal Model is used instead of AP-42
to generate emissions estimates for these guidelines for the acceleration,
deceleration, and cruise modes of vehicle travel. The Modal Model is used
because it can more accurately estimate emissions over variable driving
sequences, such as occur at intersections, than can AP-42. One drawback
to using the Modal Model is that the emissions are only applicable to one
set of emission conditions, viz:
(100 percent stable)
calendar years = 1977
75 F ambient temperature
0 percent cold starts )
0 percent hot starts (
low altitude
non-California
light duty vehicles.
These will be called base conditions. In order to combine the best features
of the Modal Model (variable driving sequences) with the best features of
AP-42 (variable average speed, cold starts, hot starts, temperature, calen-
dar year, and region) it is necessary to make an assumption relating the
two procedures. The assumption is essentially that the ratio of estimated
emissions under other than base conditions to those estimated under base
conditions are equal for AP-42 and the Model Model; i.e.:
(AP-42) Scenario = (MM) Scenario
(AP-42) Base (MM) Base (19)
where AP-42 estimates are calculated assuming the average vehicle speed
of the driving sequence in the Modal Model
and (MM) Base are calculated using any driving sequence under base
conditions in the Modal Model
and (MM) Scenario is the unknown being solved.
Hence, to correct for calendar years, temperatures, cold starts, hot starts,
average speed, and regions other than the base conditions, total emission
30
-------
factors using AP-42 (henceforth called composite emission factors) are
calculated ,for (AP-42) Scenario, and divided by the AP-42 emissions for
light duty vehicles for base conditions. This ratio is multiplied by the
Modal Model composite emission factor under the same base conditions and
thus solves for (MM) Scenario. This emission factor reflects adjustments
for both variable driving sequence and variable environmental and calendar
year conditions.
In the following subsections the above procedure will be discussed as it
applies to each vehicle category and finally how to apply a total correction
factor to the Modal Model emissions using vehicle proportions as weighting
factors.
Light Duty Vehicles (LDV)
To adjust the Modal Model emissions estimates to reflect the user specified
light duty vehicle emissions conditions, the ratio technique discussed
above must be applied. The AP-42 emissions may be more specifically defined
by:
(AP-42) = S E . M. R. (20)
cy cy,i i i
where (AP-42) is the composite emission factor for a given calendar
^ year (composite meaning total of all model years still
running in that calendar year)
E . is the emission factor for each model year in the given
cy>1 calendar year
M is the fraction of annual vehicle travel by model year
i
R. is the correction factor for cold starts, hot starts,
1 temperature, and speed by model year.
Thus (AP-42) Scenario and (AP-42) Base are estimated and multiplied by the
Modal Model emissions for the base conditions to yield the AP-42 adjusted
Modal Model estimate for the desired scenario. For convenience this AP-42
adjustment will be called R*DV Qr.
31
-------
(AP-42) _ . £E . M R
_ . .
R* = _ cy. Scenario cy,i i i _ ( 1}
*LDV (AP-42), 2E,. . M. R . ^
by by,i i by,i
where the subscript, by, refers to base year.
Light Duty Trucks (LPT) and Motorcycles (MC)
The derivation of correction factors for light duty trucks and motorcycles
is similar to that for LDV's. The only difference of note is that the E
cy,i
AP-42 estimates are for trucks (or motorcycles).
Heavy Duty Trucks (Gas-HDG and Diesel-HDD)
Correction factors for heavy duty trucks only apply to adjustments for
speed and model year. The correction factors (C*) are calculated by
taking the ratio of the composite AP-42 emission factors for the calendar
year of interest and the composite AP-42 emission factors for LDV's under
the base conditions. Thus:
2(EHDG. M. V .)
r-.- = cy,i i s,i (22)
C'HDG - LDV v }
v by, i i s,i
-I-
and a similar expression for heavy duty diesel trucks (c" ).
HDD
Application of Correction Factors
A composite correction factor must be calculated combining all vehicle types
at the roadway under analysis in order to make the CO concentration esti-
mates reflect all vehicle categories. Thus a total composite correction
factor C may be given by:
C = P R* + P R* + P C* + P C* + P R*
T rLDV KLDV LOT *LDT HDG HDG HDD HDD *MC MC
where PLDV, PLDT> PHDG> PHDD' PMC are the Pr°Portion of each vehicle type
R* R* R* C* C*
LDV LDT' MC: HDG' HDD are composite correction factors as
described previously.
32
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Since the verification methodology employs precalculated tables of cruise
and excess emission rates to compute emissions on each link, a separate
Crp must he applied to both the cruise emissions and the excess emissions.
With these basic correction factors in hand, base emissions may now be
varied to other scenarios. Since the cruise component of emissions (i.e.,
free flow) has a known speed, s, the corrected cruise emission can be
calculated, viz :
(C ) (E ) = (E ) corrected (24)
J- S (j \_»
where E is the cruise emission rate estimated from the Modal Model, speed,
LI
in
discussed above for given cruise speed.
and volume of traffic and (C ) is the composite correction factor as
The corrected excess emissions is the difference over the queue length (or
acceleration and deceleration distance), between the cruise emissions and
the emissions to decelerate, idle, and accelerate. The average speed of
the cycle of deceleration, idling, and acceleration is very low - in almost
all situations less than 5 miles per hour. For the purposes of screening
and hot spot verification the R-factors of AP-42 are calculated at 5 mph
to correct the excess emissions at intersections.
Thus, corrected excess emissions are:
CT,5(EAD + EC + V ~ CT,S(y = (V C°rreCted (25)
Note that the correction factor is applied to total queue emission rates
(E is added back into E ) and not to excess emissions. This allows the
C AJJ
cruise emissions to be separately accounted for at the actual cruise speed
and then subtracted to yield corrected excess emissions.
It is important to note that the cruise emissions in Equation (25) is the
cruise emissions that would have occurred for the vehicles that stop, if
they had not stopped, and not the total cruise emissions. It is for this
reason that the tables of excess emission rates in Volume I list two
numbers, the total queue emissions and the cruise emissions that would
have occurred had the vehicles in the queue not stopped.
33
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SECTION III
DEVELOPMENT OF THE HOT SPOT SCREENING PROCESS
INTRODUCTION
This section describes the development of the hot spot screening techniques
presented in Volume I. In essence these screening procedures graphically
protray the results of repeated applications of the verification procedure.
The primary input parameter is traffic volume. For each roadway/receptor
situation, there is a critical traffic volume above which the potential
for a violation of the CO standard exists (according to the model implicit
2
in the Indirect Source Guidelines). Thus, utilizing traffic volumes and
several simplifying assumptions about traffic and dispersion, a determina-
tion can be made as to whether a given location has hot spot potential.
Separate techniques have been developed for analyzing three broad cate-
gories of roadway facilities, including: signalized intersections, non-
signalized intersections, and uninterrupted flow locations. Furthermore,
separate provisions are provided for considering particular types of loca-
tions within each category, such as freeways versus nonfreeways for un-
interrupted flow locations, two-lane versus four-lane approaches in the
intersection categories, and so on.
Because the effort here was directed toward development of a general guide-
line for identifying carbon monoxide hot spots, consideration had to be
given to several issues that have an influence on the methodology being
utilized. First, the guidelines will be used to evaluate literally
hundreds of street sections and intersections within any municipality;
34
-------
therefore the parameters considered must be general enough to require the
absolute minimum of data input, yet the process must yield a reliable as-
sessment of hot spot potential. Second, the process should be relatively
simple and capable of being accomplished quickly, utilizing data that are
ordinarily available from state or city agencies. Third, the process is
intended to be applicable to any city or town where the existence of a hot
spot problem is suspected. These factors, plus the fact that traffic op-
erating characteristics are often highly varied among similar locations
(for example, among signalized intersections), indicated that the screening
process had to involve a very general approach, relying to a large extent
on the validity of applying an assumed set of conservative traffic emission
and dispersion parameters in order to reduce to a minimum the number of
variables considered in the process.
Consequently, the screening guidelines were developed utilizing generalized
assumptions regarding several of the variables in the verification pro-
cedure, thus simplyifying the amount of data and computing needed for CO
*
assessment. A sensitivity analysis described below verifies the reason-
ableness of the simplifying assumptions and shows the direction of the
effects on air quality of variations in the parameter assumptions.
The screening guidelines were first presented in previous documents de-
3 4
scribing a procedure for identifying hot spot locations. ' Subsequent
to the publication of these volumes, the Indirect Source Guidelines, and
AP-42 emission factors were revised. These revisions are reflected in
both this volume and Volume I of the Guidelines.
The screening curves are generated by a computer program described in
Volume IV of these Guidelines. The program is set up so that new curves
may be computed for assumptions other than those made in this section.
35
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ASSUMPTIONS REGARDING BACKGROUND CONCENTRATIONS
As mentioned previously; ambient concentrations of carbon monoxide at any
location on an urban highway network are actually a result of emissions
from the nearby source (highway) plus a background concentration resulting
from emissions generated at more distant sources. Consequently, it was
necessary to account for background concentrations in the screening guide-
lines. Various methods are suggested in the Indirect Source Guidelines
and Volume I of the Hot Spot Guidelines for determining the background
concentration for a particular area; however, these methods require a certain
amount of short-term, local air quality monitoring and also assume that
historical ambient air quality data are available from a permanent monitor-
ing station within the general area of the site. Given the cost and time
requirements for short-term monitoring, and the extent of current long-term
air quality monitoring programs, it is highly unlikely that these procedures
could be used in the context of the hot spot screening. In an attempt to
develop a simple yet reasonable method for identifying background concentra-
tions, the results of diffusion modeling (using the APRAC diffusion model)
efforts in three New England cities were analyzed. The data available for
analysis included estimates of background concentrations at 20 receptors in
•each city computed from local traffic data, and local meteorological data
for a 1-year period. The averages of the maximum background concentrations
3 3
computed for the 20 receptors in each city were 2.9 mg/m (2.5 ppm), 3.3 mg/m
2
(2.9 ppm), and 5.9 mg/m (5.1 ppm) (averaged over 8 hours). These data were
developed for conditions during 1973-74. If these averages are projected
for 1982-83 conditions, the results (conservatively) are 1.4 mg/m3 (1.2 ppm),
1.6 mg/m3 (1.4 ppm), and 2.9 mg/m3 (2.5 ppm). Clearly, this shows that there
may be significant variations among cities with regard to background concen-
trations. However, it can be postulated that a value of about 2.9 mg/m3
(2.5 ppm) may be representative of the upper portion of the range in which
8-hour average background levels occur. Assuming that this is a reasonable
conclusion, then the corresponding 1-hour average background can be estimated
36
-------
by applying the correlation factor (0.7) developed for relating 8-hour to
1-hour average concentrations. This results in an estimated 1-hour average
background concentration of about 4.1 mg/m3 (3.6 ppra).
The need for this "standard value" is again stressed because of anticipation
that sufficient local data will not be available in all instances to permit
a determination of actual background concentrations. Therefore, a standard
value of background will be used in the generation of the screening curves.
2
Data and procedures in the Indirect Source Guidelines are oriented to the
maximum 1-hour average concentration of carbon monoxide while the 8-hour
average concentration is of interest here. Analysis of air quality data
8 9
from a number of continuous monitoring stations ' indicate that the re-
lationship between the maximum 1-hour and 8-hour average concentrations
can be expressed by:
X8 = PXX (26)
where x0 = highest expected 8-hour average concentration
o
X, = highest 1-hour average concentration (in same 8-hour period)
P = 8-hour correlation factor.
While the value of the correlation factor can be expected to vary depending
on local traffic and meteorological characterists, data analysis described
below indicates that a value of 0.7 may be considered appropriate, es-
3
pecially in the range of 1-hour average concentrations of from 10 mg/m ,
3
to 20 mg/m . In this analysis, monthly summaries of carbon monoxide con-
centrations measured at several monitoring stations in three cities were
reviewed. The analyses consisted of determining the ratios of the maximum
daily 8-hour average concentrations to the maximum daily 1-hour average
concentrations for various monitoring sites in each city. The maximum
8-hour average concentration used in the analyses included the maximum
Discussed later in this section.
37
-------
daily 1-hour average concentration in its averaging period. Further,
these ratios were examined for three ranges of maximum daily 1-hour average
concentrations - where the maximum 1-hour average was (1) between 0 and
3 33
7 mg/m (6 ppm), (2) between 7 mg/m (6 ppm) and 11.5 mg/m (10 ppm), and
3
(3) greater than 11.5 mg/m . Considering the higher range separately, the
analysis indicated that for moderately active downtown locations (locations
typified by occasional traffic congestion) with apparently good atmospheric
ventilation (that is, fairly wide streets and sidewalks and building heights
generally not exceeding five or six stories), that the ratio of the maximum
8-hour average concentration to the maximum 1-hour average concentration
ranges from about 0.6 to 0.7. For downtown areas where heavy traffic con-
gestion occurs throughout much of the day and where ventilation is somewhat
restricted by narrow streets and tall buildings, values of from 0.7 to 0.8
were indicated for this ratio. Conditions in most areas where hot spot
analyses are conducted are like to be somewhat less severe (with regard to
traffic conditions and ventilation) than those in the congested area re-
ferred to here. Given this assumption, the value of 0.7 is considered
"reasonable" for use as a "standard value" to describe the ratio of the
maximum 8-hour average concentration to the maximum 1-hour average concen-
tration, for cities and towns lacking sufficient data to permit development
of a more specific value. The verification procedure does allow the use
of a locally-derived 8-hour correlation factor.
The maximum 8-hour average concentration of carbon monoxide for a particular
location, then, can be expressed by the general equation:
Xft = (X, + Xn) P (27)
o ID
where x0 = tne estimated maximum 8-hour concentration
o
X, = the total estimated concentration contributed by a nearby
source (roadway)
XT, = the 1-hour average background concentration, assumed to be
3.6 ppm (4.1 mg/m3)
P = 8-hour correlation factor =0.7
38
-------
If Xg is set equal to the National Ambient Air Quality Standard for 8-hour
concentrations (10.0 mg/m (9.0 ppm)) then the only unknown in the equation
becomes x-, > or :
_ X8 " XB(P) (10.0) - (4.1)(0.7) .- _ /3
X1 p = Q-^ = 10.2 mg/md
Therefore, for every roadway condition where the calculated 1-hour average
concentration contributed by the roadway is about 10.2 mg/m3, there is a
potential for violations to the 8-hour standard, given the above
assumptions.
UNINTERRUPTED FLOW CONDITIONS
General
For conditions of uninterrupted flow on streets and highways, vehicle speed
is a key determinant of emissions intensity. Because average travel speeds
vary to such a large extent throughout a highway network, it is necessary
to separately consider facilities where speed- characteristics are expected
to be quite dissimilar. For this analysis, two facility-types were con-
sidered, these being (1) freeways, expressways, or other limited access,
high speed highway classes; and (2) arterial streets and highways.
The verification procedure presents a technique for expressing an empirical
relationship between air quality and various combinations of roadway volume
and capacity. Again, other parameters, mainly meteorology, emission factors,
and certain operating characteristics, have been included, but as nonvari-
ables (the reader is referred to the previous section for a discussion of
the assumptions made regarding these parameters). An implication of the air
quality-volume-capacity relationships expressed by the verification procedure
is that for every given value of lane capacity there is a critical volume
demand which, once reached, will generate concentrations of carbon monoxide
that are in violation of the National Ambient Air Quality Standards.
39
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Representative vehicle operating speeds are needed to estimate the emissions
from free-flowing traffic. The assumptions used here are based on relation-
ships between volume-capacity ratios and operating speeds on various types
of roadways with specified average highway speeds as estimated from the
Highway Research Board's L965 Highway Capacity Manual.11 Traffic on the
roadway is assumed to be accommodated at the maximum level of service con-
sistent with the indicated v/c ratios. This procedure is followed in de-
riving the screening curves for freeways and for arterials. For the free-
flow curves in the screening procedures, demand-capacity ratios are
assumed identical with volume-capacity ratios. This assumption implies
that, under free-flow conditions, traffic _is_ moving at the maximum
level of service consistent with the specified volume-capacity ratio.
Thus, the operating speeds on a roadway are possible to estimate given
the demand-capacity ratio. The combinations of operating speeds and
demand-capacity ratios derived for arterial streets and expressways,
along with the corresponding levels of service appear in Tables 1 and 2.
To be representative of as large a portion of the country as possible and
to insure usability under representative adverse emissions conditions,
about 20 percent of the vehicle population is assumed to be operating under
cold-start conditions and 0 C is assumed appropriate for worst case temper-
ature in the winter months (the season where CO concentrations have most
commonly been high).
An additional adjustment was to adjust the curves to apply to the winter
of 1982 to 1983, to correspond with the statutory attainment date for the
carbon monoxide NAAQS. The adjustment is identical to that described pre-
viously for the verification procedure.
Curve Generation and Discussion
The result of the analyses performed was the identification of the critical
(minimum) ADT which could result in a violation to the National Ambient
Air Quality Standard for 8-hour average carbon monoxide concentrations.
41
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Table 1. ASSUMED OPERATING SPEEDS, LEVELS OF SERVICE AND DEMAND-CAPACITY
RATIOS FOR MAJOR STREETS AND CORRESPONDING EMISSION FACTORS FOR
FREE FLOW CONDITIONS
Assumed
operating
speed
(mph)
30
25
20
15
15
Demand -
capacity
ratio
^0. 60
0.70
0.80
0.90
1.00
Level
of
service
A
B
C
D
E
Description
Completely free flow
Stable flow (slight delay)
Stable flow (acceptable delay)
Approaching unstable flow
(tolerable delay)
Unstable flow (congestion,
intolerable delay)
Table 2. ASSUMED OPERATING SPEEDS, LEVELS OF SERVICE AND DEMAND-CAPACITY
RATIOS FOR URBAN EXPRESSWAYS3 AND CORRESPONDING EMISSION FACTORS
FOR FREE FLOW CONDITIONS
Assumed
operating
speed
(mph)
57
55
53
50
47
45
42
40
37
30
Demand -
capacity
ratio
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Level
of
service
A
A
A
A
B
C
C
C
D
E
Description
Completely free flow
Completely free flow
Completely free flow
Completely free flow
Stable flow (upper speed
range)
Stable flow
Stable flow
Stable flow
Approaching unstable flow
Unstable flow
Average highway speed assumed to be 60 mph.
42
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Figures in the screening guidelines show these critical volumes for various
configurations of limited and uncontrolled access facilities. The figures
provide the basis for screening roadways where conditions of uninterrupted
flow prevail. The resulting procedure provides a "go/no-go" type of anal-
ysis. The procedure simply indicates that a hot spot potential exists or
does not exist.
The procedure for screening highway sections where uninterrupted flow con-
ditions prevail requires only very basic data regarding the roadway net-
work. Essentially, the data required involve traffic volumes and traffic
flow characteristics, and general physical data including number of travel
lanes, and estimates of other capacity determinants, such as lateral lane
clearance. The data required allow estimates to be made of the facility's
lane capacity which, through the use. of the curves presented in Volume I,
can be related to a corresponding "critical" ADT which, potentially, would
result in a violation to the National Ambient Air Quality Standard (NAAQS)
for 8-hour average concentrations of carbon monoxide. This critical ADT
is then compared with the estimated ADT on the facility, and a potential
hot-spot is indicated when the estimated ADT for the facility equals or
exceeds the critical ADT.
INTERRUPTED FLOW CONDITIONS - SIGNALIZED INTERSECTIONS
General
Near signalized intersections, emissions intensity is affected by vehicle
operating characteristics including acceleration and deceleration rates,
time in idle mode, volume of vehicles that stop, and total volume. These
operating characteristics, in turn, are influenced by such elements as
intersection capacity, amount of green (signal) time allocated to each
approach, location of the intersection (e.g., rural, residential, or down-
town), and the proximity of other signalized intersections. Emissions in-
tensity is also related to the physical layout of the intersection with
respect to lane configuration as well as receptor location relative to the
43
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emissions source. Since there is a Large number of variables that can
significantly affect carbon monoxide concentrationsj 40 prototypical inter-
sections were developed as the basis for the screening process.
A number of assumptions in addition to those in the verification procedures
were required in developing a general procedure for assessing the hot spot
potential of signalized intersections. These concern mainly traffic volume
distribution, receptor distance, and general intersection operating charac-
teristics, as discussed below.
Assumptions
It was assumed, again, that the basic parameter of traffic volume would be
expressed as ADT. Also, the following assumptions were used regarding ADT.
It is recognized that conditions vary widely in urban areas; the following
conditions are assumed to be typical:
• peak hour traffic represents 8.5 percent of the ADT
• an even directional distribution occurs on two-way
facilities during the peak hour
• for multilane facilities, the volume on the lanes are
equally distributed.
The distance from the edge of the roadway to the receptor was assumed to
be 5 meters.
Two different cruise speeds were used in developing the screening procedure
the first was 15 miles per hour used for conditions where congestion is
highly likely (e.g., within urban business districts), while 30 miles per
hour was used for noncongested areas.
Assumptions regarding cold operation and temperature correction for sig-
nalized intersections were the same as those described in the previous
section dealing with uninterrupted flow conditions. These assumptions
44
-------
were £hat the ambient temperature representative of winter operation is
0 C and that 20 percent of the vehicles passing any point in the highway
network would be operating under cold conditions. These issues are dis-
cussed in detail in a previous portion of this section. Data were also
adjusted to 1982 to 1983, as before.
Curve Generation and Discussion
The results of the analyses were the identification of the critical (mini-
mum) ADT's for several basic configurations of intersecting streets that
could result in violations to the National Ambient Air Standard for 8-hour
average carbon monoxide concentrations. In the analysis, eight general
intersection approach configurations were considered, including:
(a) 4-Lane, 2-Way in congested areas
(b) 4-Lane, 2-Way in noncongested areas
(c) 3-Lane, 2-Way in congested areas
(d) 3-Lane, 2-Way in noncongested areas
(e) 2-Lane, 2-Way in congested areas
(f) 2-Lane, 2-Way in noncongested areas
(g) 2-Lane, 1-Way
(h) 3-Lane, 1-Way
For each of these, five configurations of intersection street-types were
analyzed and corresponding "critical" ADT's determined. Thus, a total of
40 separate intersection configurations were evaluated; results are presented
in the screening procedures.
For screening signalized intersections, traffic volume data, physical lay-
out and traffic operational characteristics are of primary importance.
The traffic volume and physical/operational characteristics of each approach
to the intersection are then related to the physical/operational charac-
teristics of the opposing roadways and, from these, the "critical" volumes
are determined for the opposing traffic. If the actual volumes on these
cross streets are greater than or equal to the "critical" volumes, a hot
spot potential is indicated for the approach being analyzed.
45
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INTERRUPTED FLOW CONDITIONS - NONSIGNALIZED INTERSECTIONS
General
Although nonsignalized, at-grade intersections are by far the most common
type of intersection, they are also the least studied with regard to op-
erational characteristics and capacity. Nonsignalized intersections are
very seldom a critical factor of capacity or level of service on through
routes although they are of great significance with regard to the minor
cross route. Typically, when a point is reached where traffic flow on
the major route is affected to any degree by traffic from a minor cross
•,<
street, a signal is installed. To date, most of the research that has
been conducted on capacity and operational aspects of nonsignalized inter-
sections has tended to produce data representative of local conditions
only.
In perspective, however, nonsignalized intersections are also probably the
least likely hot spot locations since they are generally characterized by
relatively low volumes on the minor cross streets, and produce little, if
any, interference to main street flow. This being the case, the main
stream legs of many nonsignalized intersections can be assessed for hot
spot potential using the technique described earlier in this section for
uninterrupted flow conditions. For the minor street legs, however, a dif-
ferent analysis must be performed.
The concept, then for develping a screening procedure for nonsignalized
intersections was the same as that used in developing the screening pro-
cess for both uninterrupted flow conditions and signalized intersections.
In essence , this involved identifying volume relationships which result
in concentrations (based on the verification procedure) high enough to po-
tentially violate the NAAWS for 8-hour average concentrations of carbon
monoxide.
Four-way stop sign controlled intersections are excluded to keep the scope
jf these guidelines manageable. Such intersections, with their typical low
traffic volumes, are unlikely hot spots.
46
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Assumptions
Assumptions used regarding peak hour volume, directional split, and lane
distribution were the same as were used for development of the screening
procedure for signalized intersections. The minimum distance from the edge
of the nearest lane to a receptor was assumed to be 5 meters.
Curve Generation
The verification procedure and the assumption noted above were utilized
to develop critical volumes for various configurations of STOP-sign con-
trolled intersections, as shown in the guideline volume. Included are the
following:
• 2-lane, 2-way minor; 2-lane major (congested area
• 2—lane, 2-way minor; 2-lane major (noncongested area)
• 2-lane, 2-way minor; 4-lane major (congested area)
• 2-lane, 2-way minor; 4-lane manor (noncongested area)
• 4-lane, 2-way minor; 4-lane major (congested area)
• 4-lane, 2-way minor; 4-lane major (noncongested area)
• 2-lane, 1-way minor; 2-lane major
• 2-lane, 1-way minor; 4-lane major
EFFECTS OF VARIATION IN PARAMETER ASSUMPTIONS
The analysis presented here is performed to show the sensitivity of the
computed CO concentrations to changes in the input parameters for the
screening curves. Assumptions for six traffic and air quality parametert
were among those required to compute the values for the plotting of the
hot spot screening curves. These assumptions are calculated in Table 3
for the analysis of a signalized intersection composed of a 4-lane/2-way
roadway in a noncongested area which is crossed by a 2-lane/2-way facility.
47
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Table 3. ASSUMPTIONS FOR CO CONCENTRATION COMPUTATIONS
a
Parameter
Directional volume split on main street
Roadway edge to receptor distance
Percent ADT during peak hour
Background concentration, 1 hour
Acceleration-deceleration
8-Hour correlation factor
Value assumed in
screening curves
50% - 50%
3
5 meters
8.5%
5.8 mg/m
2.5 mph sec"
0.70
Analysis of a 4-lane/2-way roadway in a noncongested area
crossed by a 2-lane/2-way roadway. Intersection is fully
signalized.
The estimates for the parameters are regarded as producing an accurate
model of the proposed generalized conditions. Since all estimates, how-
ever, are subject to some amount of uncertainty, a sensitivity analysis,
discussed in the following text, helps in analyzing and understanding
the effects of variations in the values of the control parameters.
The process selected for the sensitivity analysis was to vary one param-
eter at a time and recompute the CO concentration screening curve for
that new set of parameters. The variations in the parameter values se-
lected for use in the sensitivity analysis are listed in Table 4. The re-
sults of the analyses show that most of the selected changes (Table 4) in
most parameters can have substantial effects on the curves. The only excep-
tion is acceleration, in which a change of 0.5 mph sec'1 had no effect.
The basic screening curves and the seven variations are plotted on Figure 11
for comparison. The percentage variation in the allowable volumes are
presented in Table 5, along with the differences in the parameter values.
48
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Table 4. VARIATION IN ESTIMATED PARAMETER VALUES FOR
SENSITIVITY ANALYSIS
Parameter
Value used in
sensitivity analysis
Directional volume split
Roadway edge to receptor distance
Percent ADT during peak hour
Background concentration, 1 hour
Deceleration-acceleration
8-hour correlation factor
60% to 40%
2 meters
10%
4.8 mg/nf
2 mph sec
0.9
-1
The significance of this analysis of possible variations in the parameters
is that it exhibits the effects on expected CO concentration of differences
between the assumed conditions and conditions at an actual site. In ad-
dition, it shows the direction and relative magnitude of the effect on air
quality of changes in site conditions that might, be brought about through
some of the control measures.
The results shown in Table 5 and plotted on Figure 11 exhibit the variations
possible with example changes in the estimated values of the parameters. It
is considered that the set of values for the generalized conditions used in
the screening analysis yield satisfactory results for average conditions.
Uncertainty in the use of these estimated values will undoubtedly occur at
unusual or complex locations. The uncertainty for these special cases,
however, would be eliminated during application of the verification process.
49
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<" g
CO g
O 2
SI
z •-
O ~*
o
<
CURVE
LEGEND
VARIED PARAMETER
BASE CASE
BACKGROUND
8 »o I hour FACTOR =0.9
ACCELERATION, DECELERATION =2.0, -2.0
ROAD-RECEPTOR = 2 meteri
DIRECTIONAL SPLIT = 75% , 25 %
PEAK HOUR FACTOR = 10%
NOTE -REFER TO TABLES 3,4 AND 5
2 3 4 5 6 7 8 9 10 II 12 13 14-
ADT ON STREET OF ANALYSIS : 4-lone/2-way (NON-CONGESTED AREA)
(in thousands of vehicles)
15
Figure 11. Examples of effects on screening curves of variations in parameter assumptions
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Table 5. CHANGES IN PARAMETERS VERSUS CHANGES IN ALLOWABLE VOLUMES
Parameter
Difference in value
of parameter between
screening curve
assumption and
sensitivity analysis
Approximate change
in allowable volumes
(percent)
Background concentration
8-Hour correlation factor
Acceleration, deceleration
Roadway edge to receptor distance
Directional volume split
Percent ADT during peak hour
- 1 mg/m
0.2
0.5 mph/sec
-3 meters
25%
+2.5%
+35%
40%
no change
-15%
-25%
-25%
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SECTION IV
REFERENCES
1. Carbon Monoxide Hot Spot Guidelines. Volume I: Techniques and
Workbook. GCA Corporation, GGA/Technology Division, Bedford,
Massachusetts. Prepared for U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina. GCA Report No.
TR-77-02-GU). June 1978.
2. Guidelines for Air Quality Maintenance Planning and Analysis.
Volume 9 (Revised): Evaluating Indirect Sources. OAQPS No. 1.2-
028R, EPA-450/4-78-001, U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina. September 1978.
3. Guidelines for Identification and Evaluation of Localized Violations
of Carbon Monoxide Standards. Final Guideline Report. GCA Corpora-
tion, GCA/Technology Division. Bedford, Massachusetts. Prepared for
U.S. Environmental Protection Agency, Region I Office, Boston,
Massachusetts. Publication Number EPA-901/9-76-001. January 1976.
4. Development of Guidelines for Identification and Evaluation of
Localized Violations of Carbon Monoxide Standards. Final Summary
Report. GCA Corporation, GCA/Technology Division, Bedford, Massachu-
setts. Prepared for U.S. Environmental Protection Agency Region I
Office. Boston, Massachusetts. Publication Number EPA-901/9-76-002.
January 1976.
5. Guidelines for Air Quality Maintenance Planning and Analysis.
Volume 9: Evaluating Indirect Sources. U.S. Environmental Pro-
tection Agency, Research Triangle Park, North Carolina 27711.
Publication Number EPA-450/4-75-001. January 1975.
6. Mobile Source Emission Factors. U.S. Environmental Protection
Agency. Washington, D.C. EPA 400/9-78-005. March 1978.
7. Midurski, Theodore P., et al. GCA/Technology Division. Development
and Evaluation of a Transportation Control Plan for the Massachusetts
Portion of the Hartford-New Haven-Springfield Air Quality Control
Region. Volume I: Proposed Transportation Control Plan. Prepared
for U.S. Environmental Protection Agency, Boston, Massachusetts.
Publication No. EPA-901/9-75-002a. September 1975. p. 321.
52
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8. Massachusetts Department of Public Health, Bureau of Air Quality
Control. Computer Summaries of Air Quality Data from the Waltham,
Massachusetts Monitoring Site, 1-Hour and 8-Hour Average Carbon
Monoxide Concentrations for 1972 through 1974.
9. Massachusetts Department of Public Health, Bureau of Air Quality
Control. Computer Summaries of Air Quality Data From the Monitoring
Sites in Springfield, Massachusetts, 1-Hour and 8-Hour Average
Carbon Monoxide Concentrations for 1972 through 1974.
10. Kunselman, P. et al. Automobile Exhaust Emission Modal Analysis
Model. U.S. Environmental Protection Agency Report No. EPA-460/3-74-
005. January 1974.
11. Highway Capacity Manual. Highway Research Board, National Academy
of Sciences, National Research Council. Special Report No. 87. 1965.
12. Users Guide for HIWAY. U.S. Environmental Protection Agency.
Research Triangle Park, North Carolina. EPA/650/4-74/008.
February 1975.
13. Mancuso and Ludwig. Users Manual for the APRAC-1A Urban Diffusion
Model Computer Program. Stanford Research Institute, Menlo Park,
California. September 1972.
53
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1 RfcPORT NO. 2.
EPA-450/3-78-034
-J. TITLE AND SUBTITLE
Carbon Monoxide Hot Spot Guidelines
Volume II: Rationale
7. AUTHOR(S)
Frank Benesh
9. PERFORMING ORGANIZATION NAME AND ADDRESS
GCA/Technology Division
Burlington Road
Bedford,, Massachusetts 01730
12. SPONSORING AGENCY NAME AND ADDRESS
Office of Air Quality Planning and Standards
Environmental Protection Agency
Research Triangle Park, N.C. 27711
15. SUPPLEMENTARY NOTES
3. RECIPIENT'S ACCESSI ON- NO.
5. REPORT DATE
August 1978
6. PERI-ORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
GCA-TR-78-32-G(3)
10. PROGRAM ELEMENT NO.
2AF643
11. CONTRACT/GRANT NO.
68-02-2539
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
16. ABSTRACT
This report presents the rationale used in developing the analytical techniques
for the carbon monoxide hot spot guidelines.
Discussed in this report are the technical aspects of the guidelines, including
the assumptions used in developing hot spot procedures. Since the guidelines
were based largely on EPA's Indirect Source Guidelines, these are discussed in
some detail, as well.
17 KEY WORDS AND DOCUMENT ANALYSIS
.i DESCRIPTORS b.lDENTIFI
Carbon
Air Qua
Transpc
•i - - r ^ < i JTI O\ STATEMENT 19.SECURI
Pelease Unlimited UNCLASE
20. SECURI
UNCLAS5
ERS/OPEN ENDED TERMS C. COSATI Field/Group
Monoxide
ility Models
>rtation Planning
TY CLASS (This Report) 21. NO. OF PAGES
5IFIED 64
TV CLASS (This page) 22. PRICE
JIFIED
EPA Porrn 2220-1 (9-73)
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