Developments in weigh-in-motion technology

Developments in weigh-in-motion technology

Government Europa looks at how weigh-in-motion technology has developed and analyses the different systems

Weigh-in-motion technology was developed in the 1950’s in Texas, USA and has been used extensively ever since. There a number of new technologies and different types of weigh-in-motion however they all follow the same aim and principle: to monitor traffic on highways. Bridge weigh-in-motion is a recently developed technology and was created as an alternative to pavement-based weigh-in-motion, which is more commonly known as high speed weigh-in-motion. Bridge weigh-in-motion technology systems can be installed without interrupting traffic and can provide better accuracy.

Government Europa investigates key principles of bridge weigh-in-motion technology systems and provides a detailed report around the technological developments over the years. Bridge weigh-in-motion technology systems have provided a lot of benefits to Europe’s road safety and maintenance and GE brings you all of the information around the topic.

High speed weigh-in-motion vs. bridge weigh-in-motion

Bridge weigh-in-motion (BWIM) systems often provide undifferentiated vehicle-by-vehicle data to the high speed, in-road weigh-in motion (HS-WIM) and calibration and connectivity to other types of traffic monitoring equipment is also identical. However, there are a number of differences between the two types of WIM technology.

BWIM does provide some additional information which usually helps with the structural analysis and assessment of bridges, these include: strain measurements, influence lines, load distribution factors and dynamic amplification of loading. BWIM also provides simpler and less intrusive maintenance and implementation, with HS-WIM traffic is often disrupted during the installation process, however with BWIM technology there is no disruption as sensors are not needed on the pavement surface. Having no sensors on the pavement surfaces consequently benefits the system as it means that drivers of overloaded vehicles cannot escape the weighing locations, as the sensors cannot be seen from the vehicles.

How does BWIM technology work?

The weighing scale used to estimate vehicle and axle weights comes from an instrumented bridge and BWIM was originally introduced in the US in the late 1970s by Prof. Fred Moses. The technique which Moses introduced calculated axle weight by minimising the difference between the measured bridge response and predicted bridge response; simply put, the system monitors strains in a bridge deck from the weight of the vehicle crossing overhead.

BWIM can be used on almost any type of bridge and the bridges are generally selected based on the required level of accuracy. The only characteristic a bridge generally needs to be fitted with BWIM is related to the influence line, which is the distance between the two furthest points that affect the measurement, the specification is that the influence line is less than around 30 m.

The accuracy of BWIM is very high as the weighing scales consists of the entire bridge/one bridge span. The only factors that may affect the accuracy of BWIM systems are potholes which mean the road is not smooth and can cause unwanted bouncing and weight pressure.

One of the main advantages of BWIM is the portability of the system, the sensors can be quickly and easily removed and reinstalled onto another bridge. This is a great advantage to saving costs, the technology does not have to be bought and installed at scale, authorities can remove and replace the system to different bridges as they see fit and wherever/whenever necessary.

The European Commission has invested a lot of funding into deciding when to renovate and replace ageing bridges

Overweight vehicles can cause a lot of damage to Europe’s roads and the increase in traffic across the whole of Europe has led the European Commission to take action in improving road safety relating to the ageing of bridges. Overweight vehicles also cause other safety issues such as: increased braking distances, heavier vehicles are slow and can tempt drivers to make dangerous manoeuvres, increased vehicle mass can increase potential impact in the event of a collision.

One of the latest Horizon 2020 funded projects was the BridgeMon project, which was successfully completed in 2014. The project lasted for two years and has led to the development of a number of significant improvements in bridge weigh-in-motion systems and the technologies and services available for carrying out detailed monitoring of bridges. The end of the project posed a successful outlook for European roads and bridges with a ‘virtual monitoring’ technique which can be used to estimate fatigue damage in steel bridges.

In Finland, Tampere University of Technology and Roadscanners Lts have used bridge weigh-in-motion to build a structural monitoring system which can be sued to investigate the effects of seasonal variations on the mechanical behaviour of road structures. Finland is known to be one of the coldest European countries and during the winter months the country sees a lot of snowfall. Due to the extreme weather condition, Roadscanners and Cestel installed instrumentations which enable monitoring of structural responses of the road during vehicle loading and other conditions at different depths below the road surface. As reported by the International Society for Weigh-in-Motion (ISWIM), the lowest measured temperature on the measurement location was -37°C and the system continued operating 24/7 without complications or any other difficulties.

On-board weigh-in-motion technology: a closer look at the vehicle

On-board weigh-in-motion systems are fitted onto vehicles, rather than the infrastructure itself. The system allows communication of weight data from a vehicle directly to authorities. There are two types of onboard weigh-in-motion systems, static and dynamic. The static on-board weigh-in-motion system weighs the vehicle when it is stationary or parked up and the data readings provide an accuracy of 2% within 95% of readings.

The dynamic weigh-in-motion system is less reliable than the static and that is because weight value is monitored continuously with a sampling frequency, the actual weight is then calculated using complex algorithms. However, the dynamic system does allow weight monitoring as the vehicle is moving. Andy Lees, Q Free UK, vice-president of ISWIM explained to GEQ that: “the main drive for WIM technology is to improve the accuracy with dynamic weigh-in-motion because the demand for direct enforcement is growing internationally; it is already introduced in some countries, certainly in Eastern Europe and France is trying to introduce it into their legislation at the moment.” Lees went on to state: “the big problem that you have with dynamic weigh-in-motion is there are so many variables to take into account that to get 100% accuracy for 100% of the time for 100% of the vehicles, it is almost impossible, but the drive for direct enforcement is driving the need for greater accuracy from the dynamic weigh-in-motion systems.”

The International Society for Weigh-in-Motion

ISWIM is an international non-profit organisation which acts as an international network of and for people and organisations active in the field of weigh-in-motion. The society focuses in supporting all advances in weigh-in-motion technology and in the standardisation of weigh-in-motion technologies. ISWIM disseminates knowledge and understanding of WIM through their extensive list of conferences and training courses. The ISWIM organise international conferences on weigh-in-motion every three to four years, with the next being in May 2019 in Prague.

ISWIM state that they initiate, participate and/or monitor weigh-in-motion standardisation through engaging in the development of standards relating to WIM and its applications whilst also supporting tests of systems and publishing scientific results.

ISWIM Young Researcher Award

One of the new ventures for ISWIM is a scholarship programme for 2018, labelled the ISWIM Young Researcher Award. The award ‘recognises young tertiary level students from around the world who are making a contribution to the weigh-in-motion field and are demonstrating a passion through their studies and possible early professional life.’

The society has pledged to sponsor up to four young successful applicants to attend their next international conference in Prague, where they will have the opportunity to present their work, whether that be in poster paper or oral presentation. The opportunity will allow the successful candidates to exhibition their work, network with influential people within the industry and extend their industry knowledge


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