Technology Profile |
Certifying weigh-in-motion
solutions
maximum permissible error
(MPE), where if a single field test
value falls outside of the allowed
range, the system fails. The
system undergoes a stringent
type certification test, with inservice
(validation) testing
required for WIM sites. This
testing all uses MPE, generating
a high degree of proven
performance for the systems.
The type of vehicles and
conditions a type certified
system will experience are
reflected in the requirements for
the testing. Three different
vehicle classes, including rigid
and segmented vehicles must be
included in the testing, making
multiple passes over the WIM
site in the left, right, and centre
of the lane. Loaded and
unloaded vehicles are tested, as
is performance across the speed
ranges for which the system is
looking to get certified. All this
translates to 60-90 vehicle runs
of varying speed, weights, lane
positioning, and classifications,
none of which can be outside the
MPE. It is not a simple
undertaking to conduct this
testing, and it is quite difficult
wheel placement, and vehicle
configurations to verify the
device’s capabilities.
International differences
WIM standards have different
applications. ASTM E1318 in
North America, and COST 323
from Europe are used to test the
performance of WIM systems
in many different countries
worldwide. These methods are
excellent tools to evaluate the
performance of a WIM device
or site(s), with different levels
of performance accessible
within the standards. For
example, E1318 Type III
performance requires 6%
error for Gross Vehicle Weights
(GVW), whereas COST 323 A(5)
or B+(7) have 5% and 7% GVW
respectively. Both standards
operate on a 95% confidence
interval, giving assurance but
not a guarantee that values fall
within a certain error as some
outliers are allowed.
Certification to WIM
standards takes a similar path
but with notable differences.
COST 323 and ASTM E1318
are used for field testing, where
metrological standards have
laboratory and field testing
requirements for components
and the entire system. The most
notable is the international
OIML R134 WIM metrological
standard, with national or
regional standards such as
GOST (Russia), INMETRO
(Brazil) and NMi (Netherlands)
being examples which typically
draw content from OIML, COST,
and include local preferences.
What differs here is that the
standards are used for legal
applications, such as weight
based enforcement.
Stringent testing
To ensure the WIM systems
are appropriate for legal
applications, testing applies
Any device used to measure
something is built and
tested to standards. Scales
used for weighing vehicles
have in-country or international
standards which define how
testing is done. They illustrate
factors like pavement conditions,
laboratory testing for operation
in adverse weather, electrical
interference, and field testing
conditions such as varying
vehicle configurations and
dynamics while monitoring
scale accuracy and stability.
WIM challenges
For static weighing, testing
under field conditions is more
straightforward than with
weigh in motion (WIM).
Applying weights and waiting
for the result to evaluate
performance benefits from
allowing time for measurement
while eliminating vehicle
dynamics, as the vehicle is not
moving. The challenge WIM
devices face are to perform
accurately in spite of vehicle
dynamics, and testing protocols
which reflect different speeds,
| Need to know
WIM certification
standards around
the world
> OIML R134 is an
international standard
> Continent-wide
certifications include
ASTM E138 (North
America), while COST 323
(Europe), GOST (Russia),
INMETRO (Brazil), NMi
(Netherlands) are
examples of national
standards
to achieve the required
performance.
Wide variety available
As a result of years’ of testing
and certification, US-based
Intercomp offers a wide range of
certified portable and in-ground
scales and sensors. From the
manufacturing headquarters and
offices throughout the world,
they provide fixed, in-ground
WIM scales and sensors capable
of gathering vehicle weights at
low (3km/h) to high speeds
(100km/h). Intercomp offers
scales or sensors to help comply
with almost any application and
regional standard.
The WIM Strip Sensor is used
in various applications including
data collection, mainline
enforcement, tolling, and
industrial applications at both
low and high speeds. Installed
in pairs across a vehicle lane and
grouted in place into 3” (75mm)
channels cut into pavement,
installation takes a single day
with minimal civil works to
place sensors into WIM systems.
Certified to OIML R134
performance standard, no other
Traffic Technology International March/April 2020
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