Weigh in motion

Weigh-in-motion is a technology that can be used for various private and public purposes (i.e. applications) related to the weights and axle loads of road and rail vehicles. WIM systems are installed on the road or rail track or on a vehicle and measure, store and provide data from the traffic flow and/or the specific vehicle. For WIM systems certain specific conditions apply. These conditions have an impact on the quality and reliability of the data measured by the WIM system and of the durability of the sensors and WIM system itself.

WIM systems measure the dynamic axle loads of the vehicles and try to calculate the best possible estimate of the related static values. The WIM systems have to perform unattended, under harsh traffic and environmental conditions, often without any control over the way the vehicle is moving, or the driver is behaving. As a result of these specific measurement conditions, a successful implementation of a WIM system requires specific knowledge and experience.

Knowing that the installation, operation, calibration and maintenance of WIM systems are difficult, the question arises: “Why use WIM systems in the first place?”

The answer is simple: “Only a WIM will provide detailed vehicle weight information!”

The weight information consists of the gross vehicle weight and axle (group) loads combined with other parameters like: date and time, location, speed and vehicle class. For on-board WIM systems this pertains to the specific vehicle only. For in-road WIM systems this applies to the entire vehicle traffic flow.

This weight information provides the user with detailed knowledge of the loading of heavy goods vehicles.[1] This knowledge will replace the assumptions and estimates that had previously been used; as a result, margins of uncertainty are reduced. This means, for example, that the match between the heavy goods vehicles and the road/rail infrastructure can be optimized. This leads to more efficient goods transportation and better economic infrastructure management and productivity [2](Moffatt, 2017).

Weigh in motion location on the A28 motorway (Netherlands)

Axle load sensor

Especially for trucks, gross vehicle and axle weight monitoring is useful in an array of applications including:

• Pavement design, monitoring, and research
• Bridge design, monitoring, and research
• To inform weight overload enforcement policies and to directly facilitate enforcement[3]
• Planning and freight movement studies
• Toll by weight
• Data to facilitate legislation and regulation

The most common road application of WIM data is probably pavement design and assessment. In the United States, a histogram of WIM data is used for this purpose. In the absence of WIM data, default histograms are available. Pavements are damaged through a mechanistic-empirical fatigue process[4] that is commonly simplified as the fourth power law. In its original form, the fourth power law states that the rate of pavement damage is proportional to axle weight raised to the fourth power. WIM data provides information on the numbers of axles in each significant weight category which allows these kinds of calculations to be carried out.

Weigh in motion scales are often used to facilitate weight overload enforcement, such as the Federal Motor Carrier Safety Administration's Commercial Vehicle Information Systems and Networks program. Weigh-in-motion systems can be used as part of traditional roadside inspection stations, or as part of virtual inspection stations.[5] In most countries, WIM systems are not considered sufficiently accurate for direct enforcement of overloaded vehicles but this may change in the future.[6]

The most common bridge application of WIM is the assessment of traffic loading. The intensity of traffic on a bridge varies greatly as some roads are much busier than others. For bridges that have deteriorated, this is important as a less heavily trafficked bridge is safer and more heavily trafficked bridges should be prioritized for maintenance and repair. A great deal of research has been carried out on the subject of traffic loading on bridges, both short-span,[7][8][9] including an allowance for dynamics,[10][11][12] and long-span.[13][14][15]

Recent years have seen the rise of several "specialty" Weigh-in-Motion systems. One popular example is the front fork garbage truck scale. In this application, a container is weighed—while it is full—as the driver lifts, and again—while it is empty—as the container is returned to the ground. The difference between the full and empty weights is equal to the weight of the contents.

The accuracy of weigh-in-motion data is generally much less than for static weigh scales where the environment is better controlled. The European COST 323[22] group developed an accuracy classification framework in the 1990s.[23] They also coordinated three independently controlled road tests of commercially available and prototype WIM systems, one in Switzerland,[24] one in France (Continental Motorway Test) and one in Northern Sweden (Cold Environment Test).[25] Better accuracy can be achieved with multiple-sensor WIM systems[26] and careful compensation for the effects of temperature. The Federal Highway Administration in the United States has published quality assurance criteria for WIM systems[27] whose data is included in the Long Term Pavement Performance project.

Weighing in motion is also a common application in rail transport. Known applications are[33]

• Asset protection (imbalances, overloading)
• Asset management
• Maintenance planning
• Legislation and regulation
• Administration and planning
• Hi – Speed Road Weigh in Motion ( Bending Plate Technology )

Some airports use airplane weighing, whereby the plane taxis across the scale bed, and its weight is measured. The weight may then be used to correlate with the pilot's log entry, to ensure there is just enough fuel, with a little margin for safety. This has been used for some time to conserve jet fuel.

Also, the main difference in these platforms, which are basically a "transmission of weight" application, there are checkweighers, also known as dynamic scales or in-motion scales.

The International Society for Weigh-In-Motion (ISWIM, www.is-wim) is an international non-profit organization, legally established in Switzerland in 2007. ISWIM is an international network of, and for, people and organisations active in the field of Weigh-In-Motion. The society brings together users, researchers, and vendors of WIM systems. This includes systems installed in or under the road pavements, bridges, rail tracks and on board vehicles. ISWIM organises periodically the International Conferences on WIM (ICWIM), regional seminars and workshops as part of other international conferences and exhibitions.

In the 1990s, the first WIM standard ASTM-E1318-09[36] was published in North America, and the COST 323 action provided draft European specifications of WIM[23] as well as reports on Pan-European tests of WIM system. The European research project WAVE [37] and other initiatives delivered improved technologies and new methodologies of WIM. These first tests were done with the combination of WIM systems with video as a tool to assist overloading enforcement controls.[38]

In the early 2000s, the accuracy and reliability of WIM systems were significantly improved, and they were used more frequently for overload screening and pre-selection for road side weight enforcement controls (virtual weigh stations). The OIML R134 [39] was published as an international standard of low speed WIM systems for legal applications like tolling by weight and direct weight enforcement. Most recently, the NMi-WIM standard [40] offers a basis for the introduction of high speed WIM systems for direct automatic enforcement and free flow tolling by weight.

• 6. WEIGH-IN-FREE-FLOW
• "Weigh in Motion Systems"
• International Society for Weigh-In-Motion