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This set of messages is forwarded via the wireless
communication to the central dispatcher computer, where
they populate a specific database.
Fig 3 The architecture of the central component (Traffic Information
Subsystem)
The architecture of the central component is depicted in
Figure 3. The central component allows for information
storage, reporting, GIS presentation of information and
information regarding the intersection level of service. The
messages received via GPRS from the Data Collection Sub-
System are translated into coherent information which is
afterwards associated with the position of the sensors on a
GIS, so that the operator and the external users can see, via a
web connection, the traffic density in the network. As
mentioned before, the system is able to calculate the level of
service in a junction: it counts the number of vehicles and
compares this information with a specific set of values.
According to the comparison results, a message is produced
and distributed to a VMS installed in the field. The following
table presents the levels of service associated to an
intersection. The level of service is an important indicator of
congestion:
Qv
LS= (1)
C
In Equation (1), LS represents the level of service, Q v the
actual vehicle flow in vehicles per hour [veh/h], or traffic
volume, and C the capacity [veh/h] of the intersection.
Another important indicator that the subsystem uses is
the occupancy degree, measured as percent of time when a
traffic sensor is occupied by a vehicle, divided to the period
of time considered.
O= (2)
T
where O represents the occupancy, τ
v
the duration of time
when a vehicle is in the sensor detection area and T
represents the total measurement interval (e.g. 24 h). Table 1
below presents the standardized levels of service categories,
dependant on the traffic volumes.
τ
v
Level of
service
category
TABLE I.
LEVEL OF SERVICE DEFINITIONS
Specifications
Traffic volumes /
capacity ratio
A Free vehicle flowing 0 – 0,60
Mild congestion, without
B hindering the changing of one 0,61 – 0,70
lane to the other
C
Crowded, but the flow of 0,71 – 0,80
vehicles has a stable motion
The flow of vehicles begins to
have irregularities regarding
D
0,81 – 0,90
speed. Changing lane is more
difficult.
Maneuvering very limited.
E
Unstable flow of traffic. Long
queuing that produces delays
0,91 – 1,00
at transit intersections.
F
Traffic jam. Travel in small
hops. Large delay in transit
intersections. Queues are long
and occupy the upstream
intersections. In extreme cases
traffic volumes fall to zero.
≥ 1,01
Due to the level of service, a specific intersection will be
able to deliver transit times directly dependent on the traffic
volumes. The typical delays recorded, as functions of the
levels of service are presented in table 2.
TABLE II.
DELAY FOR TRANSITING A JUNCTION, ACCORDING TO THE
LEVEL OF SERVICE
Delay experienced for transiting
Level of
the junction
service
[s]
A < 5
B 5 ÷ 15
C 15 ÷ 25
D 25 ÷ 40
E 40 ÷ 60
F > 60
Due to its experimental phase, the system is now
calibrated at selecting three levels of congestion, in-between
A-B levels (”Free flowing”), C-D levels (”Congestion”) and
E-F (”Traffic Jam”).
For the safety monitoring purpose, the SafeTraff system
has also a module for the detection of traffic incidents (like
crossing the red light by a vehicle). For this functionality, a
logical ”AND” has been defined between the moment where
a red light for a certain direction is on and a vehicle crosses a
”virtual loop” marked after the pedestrian crossing. When
these two conditions are true in the same time, a sequence of
video frames is recorded (three seconds before and after the
incident), in order to use them as a proof in post traffic
incident analysis.
III.
FIELD TESTS OF THE COMPONENTS
In order to ensure that the system hardware components
are functioning properly, a series of tests have been
performed, both in the laboratories and in the field. The field
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