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segment average speed (Arnaud Can et al. 2008).
This analytic model is suitable in the freeway
scenario, which has relatively continuous traffic
flow and less traffic characteristic variation.
In recent years, many researchers have focused on
dynamic models (Ruffin Makarewicz et al. 2011),
which can output not only hourly equivalent sound
level, but also instantaneous noise emission.
Dynamic models such as MOBILEE and
ROTRANOMO (Volkmar, H. 2005) are based on
different microsimulation methods, which can give
position, speed and acceleration of each vehicle.
When the values of these variables are substituted
into a noise emission law and sound propagation
algorithm, instantaneous sound pressure can be
calculated. Microsimulation models are well suited
for complex traffic situations such as cross
intersections and roundabouts, where traffic
characteristics are quite variable. However the
massive amounts of data involved necessitate large
amounts of computing power and calculation time.
This paper offers a refined classic noise
prediction method (analytic model) based on the
classic FHWA noise prediction model and using the
VISSIM traffic microsimulator to analyze the sound
level contributed by the traffic on the Nanjing Lukou
airport connecting freeway before and after its
widening. The aim of this research is to (i) assess the
traffic noise impact on the Nanjing University of
Aeronautics and Astronautics (NUAA) campus
before and after freeway widening, (ii) compare the
prediction results with the field data to test the
accuracy of this method, and (iii) analyze the
relationship between traffic characteristics and
sound level.
The organization of this paper is as follows: (i)
the first part describes the geometric layout of the
experimentation site, then discusses the traffic
microsimulation and noise prediction model
selected, and (ii) the second part demonstrates the
results and analyzes different traffic characteristics
and their impact on noise level.
2 METHODOLOGY
2.1 Case Study
2.1.1 Geometric Design
The selected study site is located on the Nanjing
airport connecting freeway, in a suburban district of
the city. It contains three lanes in the North to South
direction as well as in the opposite direction before
widening (current scenario). After widening, lane
number will be doubled in each direction, with the
new lanes being located in the middle of the origin
site (space was pre-reserved). The detailed
geometric design is shown in Figure1: (i) the overall
length of the studied freeway section is 400m,
including a 3.5m high barrier on the side where
noise levels are of interest; (ii) the width of the
traffic lanes is 3.75m, while the shoulder width is
3.3m; (iii) the tree zones after widening have two
different widths: 2.7m and 6.5m.
2.1.2 Field Data Collection
The experiment included traffic and acoustic
measurements, which were carried out before the
widening in two one-hour periods (7:30-8:30,
9:30-10:30) on a weekday. The two time periods
cover peak and normal traffic flows respectively.
The recorded traffic accounts for all traffic flow in
the freeway section as there are no access ramps or
intersections. Overall peak hour traffic flow
(7:30-8:30) was 6401 veh/h, comprised of 3376
veh/h in the north to south direction and 3025 veh/h
in the opposite direction. Normal traffic flow was
4833 veh/h, comprised of 2579 veh/h travelling
north to south and 2254 veh/h travelling in the other
direction. Three vehicle categories were recorded:
cars (including light trucks), heavy goods vehicles,
and buses (including mid-size trucks). The detailed
traffic composition is given in Table 1.
Acoustic recordings are L
,
(A-weighted
equivalent sound level for 1 second) for the points
P1, P2 (Figure 1) selected for sound pressure level
estimation. P1 was near the NUAA gym, and P2 was
in front of the student dormitory. Both were in the
barrier-contained section at the same cross section,
with receivers set 1.5m high.
2.2 Traffic Microsimulation
In this paper, the chosen traffic microsimulator
VISSIM (PTV. Ltd. 2007) was used to refine
dynamic speed calculation of the FHWA noise
prediction model. VISSIM is a microscopic, time
step and behavior based simulation model developed
to be applied in a variety of transportation problem
settings. The essential elements of traffic modeling
is the car following and lane change model which
directly affects vehicle interaction, especially
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