United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 2771
Research and Development
EPA/600/S8-87/010 May 1987
Project Summary
ROADWAY A Numerical
Model for Predicting Air
Pollutants Near Highways:
User's Guide
Robert E. Eskridge and Joseph A. Catalano
ROADWAY is a finite-difference
model which solves a conservation of
species equation to predict pollutant
concentrations within two hundred
meters of a highway. It uses surface
layer similarity theory to predict wind
and eddy diffusion profiles from tem-
perature at two heights and wind veloc-
ity upwind of the highway. A unique
feature of the model is its use of vehicle
wake theory. It is assumed that vehicle
wakes affect the wind and turbulence
fields in a linear manner with wake
intensity a function of vehicle speed,
downwind distance, and distance from
the wake center. The user has the option
of considering NO, NO2, and O3 chemi-
cal reactions near the road. Output from
the model consists of x-z fields of wind
components, eddy diffusion coeffici-
ents; and concentration of pollutant.
This Project Summary was developed
by EPA's Atmospheric Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Introduction
ROADWAY is a numerical model for
predicting air pollution levels near high-
ways. It solves a conservation of species
equation via finite-difference approxima-
tions. Temperature at two heights and
wind velocity upwind of the highway are
required. With these inputs, surface layer
similarity theory is used to produce wind
and turbulence profiles. A unique aspect
of ROADWAY is its treatment of vehicle
wakes which are superimposed linearly
on the wind and turbulence fields. The
vehicle wake intensity is a function of
vehicle speed, downwind distance, and
distance from the wake center. Addi-
tionally, the user has the option of con-
sidering NO, N02, and 03 chemistry;
reactions of these pollutants are cal-
culated by a two-step mechanism applic-
able to the very near field. Output from
the model consists of fields in the x-z
plane for wind components, eddy diffusion
coefficients, and concentrations of four
pollutant species.
To estimate concentrations for any
simulated hour, information on meteoro-
logy, highway configuration, and emis-
sions are required. The meteorological
information needed for the computation
includes representative roughness length,
temperature at two heights upwind of
the highway, and hourly average wind
speed arid direction at the level of the
upper temperature sensor. If the chemistry
option is exercised, two photochemical
reaction rate constants, background for
each species, and conversion factors
(gm/sec to ppm) are also required.
Since ROADWAY is a numerical model,
it has none of the limitations generally
associated with Gaussian algorithms.
That is,
it is a multilayer model which con-
siders vertical variation of both wind
and diffusivity,
it can treat calm or light wind condi-
tions, and
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it can simulate chemical reactions
of the emitted pollutant species.
Also, the model can include up to ten
traffic lanes and has features to reduce
execution costs (at the expense of ac-
curacy) and to provide intermediate
output. ROADWAY was developed in-
dependent of tracer data, and has been
demonstrated to perform as well as other
highway models currently available.
ROADWAY has several limitations. A
major restriction of the model is the re-
quirement that the vehicle speed be much
greater than the wind speed. This require-
ment, however, should be met in most
instances of significant pollutant impacts.
More importantly, the model is valid for
all vehicle speeds when wind speeds are
light. Another limitation is that ROADWAY
does not consider wind meander which
becomes important when the mean wind
is parallel to the highway. Also, because
its use is restricted to the very near field
(within 200 m of the roadway), other
algorithms would be better suited for
calculating impacts at longer distances.
Finally, since ROADWAY algorithms are
numerically solved, computer execution
costs are greater than those of algorithms
based on the Gaussian simplification.
Discussion
The diffusion equation derived from a
statement of the conservation of mass or
species, forms the basis for the ROADWAY
computational system. This equation is
one of three partial differential equations
used to describe distributed parameter
systems otherwise known as fields.
The conservation of species equation
(i.e., a diffusion equation), is used to
predict pollutant concentration fields near
highways. The finite-difference method
used in ROADWAY represents the time-
space continuum by a set of discretely
spaced points, the grid produced by these
points is not evenly spaced upon the field
in ROADWAY since higher resolution is
needed near the road and lesser away
from it. An algebraic equation approxi-
mating the partial differential equation is
derived for each grid point. The solution
is found by solving these equations for all
points in the grid after applying boundary
conditions and initial values to the field.
Since ROADWAY is a numerical model, it
has none of the limitations of Gaussian
solutions to the diffusion equation. That
is,
ROADWAY is a multilayer mode!
which considers vertical variation of
both wind and diffusivity,
it treats calm or light wind conditions,
and
optionally computes chemical re-
actions of source pollutant species.
As mentioned previously, ROADWAY
requires a reduced meteorological input
data set. Realistic wind and turbulence
profiles can be calculated using surface
layer similarity theory. The model was
developed on theoretical grounds and
using wind tunnel experiments and is
independent of tracer studies. It, never-
theless, performs as well as the most
accurate highway models today. Up to
ten traffic lanes can be simulated. The
model can provide intermediate output
and, at the expense of accuracy, has
features to reduce execution costs.
Since the algorithms of ROADWAY are
solved by a computer, the calculations
are subject to truncation and roundoff
errors. In the context of numerical
analysis, truncation errors occur in ap-
proximating infinite series by a finite
number of terms. Roundoff errors, on the
other hand, are machine-dependent and
occur because computations are done on
the precision of a computer which in-
troduces errors by the dropping off of
digits. Another source of error is that
related to computational instability. In
solving the conservation of species
equation, both the time and space vari-
ables must be discretized by means of
finite-difference expansions. A small error
made at one time step of the calculation
can result in a larger error at a later time
resulting in unbounded error growth. A
segment of the ROADWAY code tests
conditions to ensure that calculations
remain stable.
A model limitation is that the vehicle
speed must be much greater than the
ambient wind speed. Considering usual
freeway speeds and meteorological sce-
narios where significant pollutant impacts
would occur, this may not be a limitation.
More importantly, the model is valid for
all vehicle speeds when winds are light
which is when Gaussian approaches
breakdown. ROADWAY does not consider
wind meander; this becomes important
when the mean wind is nearly parallel to
the highway. The use of ROADWAY is
restricted to the very near field-within
two hundred meters of the roadway,
beyond two hundred meters meteorologi-
cal processes that are not accounted for
in the model become important.
Another limitation is costs related to
computer execution. Being a numerical
model, ROADWAY is relatively expensive
to run when compared to Gaussian-based
models. Execution time using the chemis-
try option is on the order of 10 CPU
minutes on a DEC VAX-117780. ROAD-
WAY implementation on a persona I (
computer is entirely possible and execu-
tion costs in this environment would be
much less.
Due to its applicability in only the near
field and because of execution expense,
ROADWAY is recommended for use in
conjunction with a Gaussian model such
as HIWAY-2.
Data Requirements
To estimate concentrations for any
simulated hour, data for program control,
as well as information on meteorology,
highway configuration, and emissions are
required.
The user must indicate whether the
following features are to be employed:
chemistry option,
antidiffusion calculation option, and
intermediate print option.
The meteorological information needed
for the computations are:
roughness length (m),
temperature at two heights upwind
of the high (K), and
hourly average wind speed (m/sec)
and direction (degrees).
The following highway configuration data
are required:
number of traffic lanes,
width of each lane (m),
width of the traffic median (m),
angle between highway and a line
running north-south (degrees),
traffic volume,
average vehicle speed (km/hr), and
average vehicle dimensions (m)
The user must supply the following ail
quality and emission data for each houi
of simulation:
background pollutant concentration;
(ppm),
vehicle emission rates (g/km.veh]
and
factor to convert grams per seconi
(gm/sec) to parts per million (ppm
for the pollutant
If the user exercises the chemistr
option, then background concentration:
vehicle emission rates, and conversio
factors must be provided for nitroge
oxide (NO), carbon monoxide (CO), an
nitrogen dioxide (N02). Also, the bad
ground ozone (O3) must be given.
No sampling grid or receptor informs
tion is required since both are internal
generated by the model. The concentn
tion output is in the form of x-z fiek
which define a plane perpendicular
the highway.
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Joseph A. Catalano is with Aerocomp, Inc., Costa Mesa CA 92626; the EPA
author Robert £. Eskridge is with the Atmospheric Sciences Research
Laboratory, Research Triangle Park. NC 27711.
D. Bruce Turner is the EPA Project Officer (see below).
The complete report, entitled "ROADWAYA Numerical Model for Predicting
Air Pollutants Near Highways: User's Guide," (Order No. PB 87-171 906;
Cost: $ 18.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Onter for Environmental Research
Information
Cincinnati OH 45268
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Official Business
Penalty for Private Use $300
EPA/600/S8-87/010
0000329 PS
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