1. No category

The Method about How to Decide the Geographic
Area of Study in Traffic Impact Analyses
Wang Li, Liu Xiaoming, Ren Futian
Abstract. At present, many countries have adapted Traffic Impact Analysis (TIA) to assess
the additional demands for road capacity imposed by new developments. This article
discusses when such a traffic impact analysis is needed, and what factors should be
considered in the study. The factors relate to the issues involve the geographic area of study;
time frame for the phases of the project; non-site traffic; programmed transportation
improvements; study methodology; and analyses and recommendations.
The main purpose of the study was to put forward diffusion model that decide the geographic
area of study, which adapt to different types of developments and different development size.
The model is founded according to site traffic dissipative character and based on the principle
of synergetic theory, one of System theory. The model found the biggest study area function
on the condition of different traffic impact extent around road network of new developments,
and then validate this model by how to decide the study area of Beijing Legend science zone.
The model makes up for the disadvantage that local officials and highway agencies decide the
geographic area of study arbitrarily. The objective of the model is to develop a more efficient
and understandable means, by which the traffic impact of a new or expanded development
can be identified, coordinate the relationship between land uses and road supply.
INTRODUCTION
Transportation needs have traditionally derived from land use decisions. One of the major
transportation issues addressed in recent years has been the growing concern about
transportation infrastructure not being able to keep pace with development. This imbalance
between transportation supply and demand has resulted in increased congestion, delay and
safety hazards at many locations throughout the big cities of China, such as Beijing. In
addition, increasing demand on transportation network results in high noise and exhaustive
emission pollution without any planning and regulations. Estimating and taking precautions
for several years later network is important as much as for network at the time being. As a
result urban transportation planning is the macroscopical planning tool to solve these question,
and traffic impact analysis (TIA) are becoming more popular as a microcosmic planning tool
compare to urban transportation planning, so that the traffic impacts of new developments
can be foreseen and effective mitigation measures can be taken using optimal allocation of
available funds.
Site impact traffic analysis is a formalized process that attempts to quantity the need for
transportation improvements on the roadway network in response to development. The TIA is
prepared as part of an application for development approval for a specific proposed land
development project, typically during the site plan review process. The primary purpose of
the TIA is to identify total roadway improvements needed to ensure that the roadway system
will be adequate after completion of the proposed project. The TIA includes a significant
amount of data and analysis, generally prepared to meet local standards and local adequate
facilities requirements. These include future traffic volumes, anticipated improvement needs
and improvement costs, and various measures of development impact.
When we develop a procedure for requesting, preparing and reviewing a traffic impact
analysis for a proposed development, we need recognize the critical relationship between
transportation planning and land use system. In developing the needs of the long-range plans,
virtually every metropolitan planning organization relied on Standard Urban Transportation
Modeling Structure (SUTMS). Based on SUTMS forecasts, roadway deficiencies are
assessed, and roadway capacity are added to the network until the system can better
accommodate new development demand. Based on this land use scenario, the model
performs trip generation, trip distribution, and network assignment of future travel demand.
The result is that forecast roadway capacity deficiencies drive the planning process and put
forward programmed transportation improvements.
Traffic Impact Analysis, in use for decades in the Western world, has also been recognized in
many China Metropolises such as Beijing, Shanghai and Guangzhou, making local agency
aware of its advantages as an alternative in solving their chaotic traffic problems. At present,
abroad range of local agency requirements and technical procedures for conducting site
traffic impact analysis has begun in China. This has led to inconsistencies and disagreements
on how such analysis should be performed. During this period of extremely fast growth, it
became apparent that a uniform method of assessing the traffic impacts of new development
was necessary in China. Traffic studies submitted to the Institute of Urban Transportation
Planning were varied in format and methodology and were inconsistent in the types of
mitigation measures proposed by the traffic engineer preparing the study.
When we develop a procedure for preparing and reviewing a traffic impact analysis for a
proposed development, what elements should be addressed? We should consider the key
elements required for preparing and reviewing traffic impact analysis for new and expending
land developments, the key elements include threshold for study, geographic area of study,
study horizon, peak traffic hours and so on. Geographic area of study is the basis of a
procedure for preparing a traffic impact analysis, however, the definition of study area varies
significantly from locality to locality because of research depth and emphases are different,
experience has shown that study area used by most of the localities are arbitrarily--there have
no reasonable method which can decide study area objectively. So this study was to put
forward which decides the geographic area of study, which adapt to different types of
developments and different development size. The model is founded according to site traffic
dissipative character and based on the principle of synergetic theory, one of system theory.
CURRENT PRACTICES
In large part, the contents and extent of a traffic impact analysis depend on the location and
size of the proposed development and the conditions prevailing in the surrounding area.
An inappropriately large analysis area will unnecessarily increase costs and time for the
developer, the study preparer, and the reviewer. In determining how large the study area
should be, in many China Metropolises, a general rule is to include in the analysis all roads,
ramps and intersections through which peak hour site traffic composes at least 5% of the
existing capacity on an intersection approach, or roadway sections on which accident
potential or traffic character is expected to be significantly impacted. Traffic that composes
more than 5% of a roadway’s capacity can represent a noticeable impact to most drivers. But
5% of peak hour capacity at an intersection approach is tolerance threshold to driver; the
tolerance threshold is determined arbitrarily. In addition, in areas where site traffic composes
less than 5% of roadway capacity, the intersections adjacent to the site should be analyzed.
Usually the rule adapt to local proposed developments. When all known congested locations
may be impacted by the proposed development, care should be taken to include in the
analysis.
The definition of study area has related to the location and size of the proposed development,
should start from the distribution and assignment of site traffic, and then study area is
determined according to whether the impacts of the proposed project on roadway links and
intersections exceed threshold for study. Mode split in China metropolises is far different
from the Western world, the proportion of car is very low compare to the Western world, if
planners utilize urban transportation planning model to determine study area of single-use
that site traffic is relatively small, it is difficult to assure the accuracy of study. In Shanghai,
planners make use of outward circle method to determine study area, the method assumes the
distribution of site traffic hasn’t direction discrepancy in surrounding road network, namely
the discrepancy of travel points have no impact on the distribution of site traffic, centering on
site location on the surrounding road network, dividing the surrounding road network into
several circles, processing every circle outside, and every link has assigned traffic counts,
then study area borderline is defined based on threshold for TIA. The analysis method based
on link counts and avoids complicated reconnaissance of surrounding intersections, but the
method considered that a lot of intersections have impact on link capacity and link counts,
when determining link capacity, the method considered the impact. In addition, the method is
used easily and simply. But the method adapted only to the cases study that site traffic is
small proportion to traffic zone and recently study and lacking of urban transportation
planning model. As to large size development, assigned traffic count in every link is far
different from each other because of the direction discrepancy of traffic attraction, and it
doesn’t accord with the assumption of the method.
DIFFUSION MODEL
Diffusion model is founded based on the principle of synergetic theory, synergetic theory
regard any research object as system which is made up of many subsystems, these
subsystems interact each other by matter and energy and information exchange, the whole
system will become one kind of new structure. In the layer of system, the new structure has
new properties, and microcosmic subsystems have no the new properties. Synergetic theory
study open system, namely how the system evolve into orderly structure from chaos or from
another orderly structure. Diffusion model has many properties according to synergetic theory,
including:
¯
We regard study area as one system, the system is made up of many subsystems which
include links or intersections, and all subsystems in which site traffic is assigned are
steady.
¯
The system is non-linear system; at the same time the system has the properties of
unbalance or saturation.
¯
Because of the diffusion of site traffic, site traffic break the balance of the original
system and make the system far from balance, because the system has the properties of
unbalance or saturation, the system can form dissipation structure and then reach new
balance at last.
¯
The whole system is open and exchange matter and energy with outside.
Figure 1
Site traffic diffusion process
Site traffic impact on surrounding road network similar to the process that toxic gas enters
into atmosphere. The system is made up of many subsystems; it is open and steady, every unit
is equal to one circle, these circles are assigned in study area and are driven diffuse by site
traffic impact force, site traffic impact force not only relate to site type and site size but also
in direct proportion to size of the proposed development, for example the impact caused by
large size of business development is far bigger than scientific research development. The
impact caused by site traffic equal to the impact caused by all development units distributed
everywhere. Figure 1 indicates the diffusion process of site traffic. The impact is normal
distribution of infinite space diffusion equation.
The site traffic diffusion model is:
C(x,y,z)
q
exp [ ( y 2 ı 2y z 2 ı z2 ) 2]
2ʌȝı y ı z
(1)
Where
C(x, y, z) = the impact extent on point ( x, y, z ) by the proposed development, the
variable has no dimension,
x, y, z = the horizontal, vertical and transverse distance between point ( x, y, z ) and
development unit, x is defined the diffusion direction of site traffic,
q = site traffic impact force,
ȝ = the assigned speed of site traffic,
ı 2y , ı 2z = the distribution parameter of site traffic distributed in the direction of
y and z , both of them are the function of x .
The
equation
Assuming ı y
C(x,r)
is
ız
deduced
ı ,r2
from
the
diffusion
phenomenon
of
infinite
space.
y 2 z 2 then the site traffic diffusion model can be shown as:
q
exp ( r 2 2ı 2 )
2ʌȝı 2
According to Robert equation, ı
(2)
2kx where k is site traffic diffusion coefficient, and
then the site traffic diffusion model can be formulated:
C(x,r)
q
exp ( r 2 4kx)
4ʌxkȝ
(3)
Where q,ȝ and k is the coefficient which need to be determined. Under normal
circumstances, the determination of k relate to the type and size of the proposed
development, the determination of q relate to the type of the proposed development and the
conditions prevailing in surrounding road network, and the determination of ȝ relate only
to surrounding road traffic conditions. In order to estimate site traffic impact by diffusion
model, these coefficients need to be assumed in order to simplify computation. Because
diffusion coefficient k is only the function of x , according to dimension analysis: k ax
Where a is constant, the calibration of a related to the type and size of the site developed,
a  (0.15, 1). The model can be shown as:
C(x,r)
q
exp ( r 2 4ax 2 )
4ʌax 2 ȝ
(4)
q
4ʌaȝR02
ȗQ
S
C
q
4aSȝ
(5)
Where Q is the impact force which one development unit has impact on surrounding road
network. ȗ is diffuse rate of site traffic diffuse outside. The determination of ȗ relate to the
type and size of the proposed development, ȗ  (0, 1). In order to simplify the model again,
assuming the impact force which development unit has impact on links and intersections after
site traffic diffuse is equal to the average impact force of development units before site traffic
is assigned.
C * = the average impact force of development units,
ȗQ = the impact force of diffused development unit,
S = the development unit area,
R0 = the unit circle radius of development unit area.
Equation (6) can be deduced from equation (5):
q
aȝ
4ȗQ
(6)
Put equation (6) into equation (4), equation (4) can be shown as:
C(x,r)
ȗQ
r2
(
exp
)
ʌx 2
4ax 2
(7)
The diffusion model considers only site traffic assignment in the direction of x and y ,
namely z = 0. r 2
y 2 . Where r is the diffusion radius, the value of r is not much too
bigger in order to ensure precision, the distribution of r is normal distribution, the range of
r is
interval
a x 2 a x 3 a x 4 a x 5 a x 6 a x , if credibility of r is 0.95 and confidence
of
r is
2
exp( r 2 4ax )
e 4
2.807
ax
4.193
ax
,
when
0.0183 .
In order to simplify the model again, the model can be shown as:
r =4
ax ,
then
C ( x)
1
2S 4
]Q
4 aSx 3
]Q
|
4 aSx 3
]Q
4 S x2
³
ax
4 ax
0
³
4 ax
³
2S
0
e ( r
2
C ( x, r )drdT
4 ax 2 )
dr
0
³
f
0
(8)
exp( r 2 4ax 2 )dr
Where C is the average value of C , and C shows the average impact which development
units have impacts on point ( x, r ), r  (0,4 a x )
In order to compute the biggest impact radius, the model can be shown as:
C
]Q 4 S x d2
Cd
(9)
Where C d is the utmost impact force which the proposed has impact on surrounding road
network. xd is the biggest impact radius.
The definition of study area depends on local policy and economy as well as the location and
type and size of the proposed development. The population and industry size and commerce
and finance of the proposed can be regarded as the standard that can measure the economy
strength of the proposed development. The impact force can be simulated by way of
gravitational field theory; the impact forces of development units can be formulated:
Q
kP
X d2
(10)
Where K is diffusion coefficient. P is potential energy, the value of P depends on many
factors, such as business area, turnover, employee number, plot ratio and so on, at present it is
difficult to find one appropriate expression, assuming that the potential energy of
development depends on employee number in order to simplify the expression. The biggest
impact radius can be formulated:
Xd
3
]Pa
4 S Cd
(11)
Where P is employee number pop . When the peak hour level of service without the site
developed deteriorates to a LOS D or LOS E as a result of the site being developed, the
biggest impact radius x d can be formulated:
Xd
E3
]Pa
4 S Cd
(12)
Where E is modulation parameter, the biggest impact radius corresponding to different site
traffic impact can be determined by modulating parameter E , E  (0, 1).
The utmost impact force C d relate to the location and type and size of the proposed
development, C d can be determined by use of transportation planning software, such as Trips
and TransCAD, it is determined by comparing the peak hour traffic volumes and level of
service with the site developed with without the site developed.
CASE STUDY
The Fourth Ring Expressway
BaiYi Road
LEGEND
SCIENCE
NORTH
AREA
LEGEND
SCIENCE
SOUTH
AREA
QingHua South Road
ChengFu Road
ZhiChun road
N
Highway
Figure 2
Map of area around Legend science zone
In order to verify the effect of diffusion model in practice, the study prepared Traffic impact
analysis for Beijing Legend science zone and defined study area by the model. Beijing
Legend science zone locate in the central district of Beijing Zhongguancun Science City, the
northern part of the project adjacent to the Fourth Ring expressway, the eastern and western
and southern part of the project near to city main arterial road, Figure 2 indicates the project
position.
Xd
3
ȗPa
4 ʌCd
Where
P = 6000 (according to the project planning in 2010).
a = 0.9.
The project is mixed-use development, different land uses are combined at a site, such as
office and residential uses, conference center and other auxiliary facilities, the position of the
site developed correspond to CBD of Beijing Zhongguancun Science City, as a result ] =1,
the biggest impact radius can be formulated:
Xd
9.13
3
1
Cd
Figure 3 indicates the relationship between the biggest impact radius and the utmost impact
force:
¯
C d =2.17u 10 4 , X d =152 m .
The peak hour LOS B or C without the site developed deteriorates to a LOS E or F as a result
of the site developed, the level of service of surrounding road network decline a large amount,
and the area is impacted most seriously.
¯
C d =1.66u 10 6 , X d =771 m .
By comparing the level of service with the site developed with without the site developed, the
peak hour level of service of some links and signalized intersections in surrounding road
network deteriorates to a LOS E or F, site traffic have impact on these sections seriously,
other links and signalized intersections are impacted also, the peak hour level of service of all
links and intersections in the area deteriorates to different extent.
¯
C d = 5.03u 10 7 , X d = 1148 m .
By comparing the level of service with the site developed with without the site developed,
there have few links and intersections impacted on the borderline of the area, and site traffic
impact extent is minor on these sections, only several sections of Chengfu Road in the north
of the area are impacted seriously.
The Biggest Impact Radius (m)
1.4E+03
1.2E+03
1.0E+03
8.0E+02
6.0E+02
4.0E+02
2.0E+02
0.0E+00
0.0E+00
5.0E-05
1.0E-04
1.5E-04
2.0E-04
2.5E-04
3.0E-04
3.5E-04
Utmost Impact Force
Figure 3
The relationship between the biggest impact radius and the utmost impact force
The peak hour level of service of all links and intersections can be computed by use of
transportation planning software; different utmost impact forces correspond with different
area. By comparing the impact extent on all links and intersections in different area according
to peak hour level of service, the study area can be determined on the condition that the site
traffic has minor impact or have no impact on the borderline of the area. By comparing the
three cases, the impact on the borderline of the third case is the smallest; as a result the radius
of the biggest study area is 1148 meter.
CONCLUSIONS
This paper presents a methodology that can be used by localities in determining the study
area in which a formal TIA is required for a given development. In developing the
methodology, we have employed four major assumptions; i.e., the impact force which
development unit has impact on links and intersections after site traffic diffuse is equal to the
average impact force of development units before site traffic is assigned; the potential energy
of development depends on employee number.
Diffusion model not only adapts to different types and different sizes of new development,
but also near future and long term traffic impact analysis, especially the model adapts to the
locality in which background traffic vary greatly, it is easy to calculate the biggest impact
radius by the model, this methodology can make up for the deficiency of present method. It is
the author’s expectation that this methodology will be beneficial to localities in developing
requirements for TIA submission and encourages additional studies relating to
standardization of TIA requirements.
But C d is calibrated according to specific site developed, this make the application of the
model have somewhat error, and the calibration of C d need to be studied deeply.
ACKNOWLEDGEMENTS
The authors would like to express their appreciation to the teachers and graduate students
from the Transportation Research Center at Beijing University of Technology for the support
during field work. A very special thanks to Dr. Yiying Wu for her assistance and technical
support in developing the diffusion model.
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Author’s Information:
Wang Li
Traffic Engineer
Transportation Research Center
Beijing Polytechnic University
Beijing, 100022 China
Tel: +8610-67392082
Fax: +8610-67391532
Email: Lwang@bjpu.edu.cn
Liu Xiaoming
Professor
Transportation Research Center
Beijing Polytechnic University
Beijing, 100022 China
Tel: +8610-67392082
Fax: +8610-67391532
Email: Lxming@bjpu.edu.cn
Or Liuxm@bjpu.edu.cn
Ren Futian
Professor
Transportation Research Center
Beijing Polytechnic University
Beijing, 100022 China
Tel: +8610-67392082
Fax: +8610-67391532
Email: renfutian@bjpu.edu.cn