The need for weigh-in-motion technology

weigh-in-motion technology

The increase in freight traffic and the free movement of goods across European borders is creating significant impacts on the infrastructure, environment and resources (petroleum), in conflict with EU aims to encourage more sustainable modes of transport.

Q-Free is a leading global supplier of tolling, parking and info-mobility solutions. This includes the supply and installation of cost-effective high speed and low speed weigh-in-motion (WIM) systems to over 50 countries throughout the world. Applications include asset management/protection (highways and bridges), bridge specific load monitoring and vehicle weight enforcement systems. Each of these can be combined with automatic number plate recognition (ANPR) technology.

The life expectancy of the asset – namely roads and bridges – can be severely impacted by illegally loaded vehicles, whether that be in total vehicle weight or by poorly distributed loads that impact on individual axles. Overloaded trucks pose serious threats to road transport operations, with increased risks for road users, deterioration of road safety, severe impacts on the durability of infrastructure (pavements and bridges) and are considered an environmental hazard.

In addition, economic issues are a factor with overloading leading to large distortions in freight transport competition, between transport modes (e.g. rail, waterborne and road), and between road transport companies and operators. In France, it was estimated that a 5-axle articulated truck, operating at 20% overload all year round, generated an additional €25 000 benefit per year.

Q-Free weigh-in-motion solution

Q-Free weigh-in-motion systems and traffic counters/classifiers are operational in over 50 different countries worldwide including the UK and Europe, North America, South America, Africa, the Middle East, China, South East Asia and Australia.

We provide customised solutions to meet the exact needs of the end user. This is achieved by combining field proven hardware with modern design techniques along with windows based software to provide its customers with user-friendly and reliable systems.

Detection technologies

Q-Free designs and manufactures its range of high speed weigh-in-motion solutions, ranging from the compact HI-TRAC® UTC-WIM capable of double lane detection to the rack mounted Hi-TRAC® TMU4 capable of monitoring up to eight lanes. Each of these has a long standing and excellent track record in installations worldwide.

Principles of operation

High speed weigh-in-motion (HSWIM) systems are installed in the main highway and weigh vehicles at typical free flow highway speeds (20-120 km/h). However, they will detect, count and classify vehicles from 5-200 km/h. The HSWIM system uses unobtrusive but easily installed piezo electric or linear quartz sensors as the weighing medium.

HSWIM operates on a dynamic basis such that the impact (or downward force) from a passing load (axle) generates an electrical signal measured by the road-installed sensors. The signal output from the sensors is monitored by the HSWIM traffic monitoring unit (TMU) and this, together with developed sophisticated algorithms, is used to calculate the axle weights and resulting gross weight of each vehicle commensurate with other factors including but not limited to vehicle speed, classification and inter-axle separation as the vehicles pass over the array.

In-road equipment options

  • Option one: Piezo-electric (BL) weigh-in-motion sensors are installed approximately 10mm below the road surface and encapsulated within resin. This protects the sensors from damage and the installation is flexible enough to follow the natural curvature of the road surface and in due course any slight pavement rutting.
  • Option two: Quartz weigh-in-motion sensors are extremely robust and are installed in a dedicated channel approximately 75mm wide and 55mm deep in the carriageway surface, coming into direct contact with the vehicle tyres. The typical life expectancy of sensors is in the order of 40 million equivalent standard axles.

An induction loop is also installed sub-surface and covered with a hardwearing encapsulate. The weigh-in-motion detection equipment monitors the inductive loop signal to determine vehicle presence time over the road sensor array and uses the loop signal (loop signature analysis) to determine vehicle chassis length. This is integral in maximising the accuracy of classification when combined with the sensors.

The road temperature recorded by temperature probes – encapsulated in the same resin as the sensors – is analysed by the monitoring equipment to automatically compensate for changes in temperature as this has an impact on the signal generated by passing axles/vehicles. The TMUs have a unique automatic temperature compensation algorithm (auto-TNL) that maintains the high accuracy of the data throughout operation.

Piezo-electric or quartz?

In many applications the standard installation uses BL sensors to collect weigh-in-motion data. This is primarily because enforcement is not ‘direct’ or required and the resulting data is adequate for statistical and soft enforcement purposes.

  • Accuracy: The BL sensor gives accuracy of 15% for 95% of vehicles providing the sensor is installed in a smooth flat road surface (See COST 323 and ASTM E1318-02 for preferred road and surface conditions).
  • Installation: The installation is simple and can be undertaken in approximately three hours per lane, meaning minimal road closure times.
  • Reliability: The sensors are fully encapsulated in resin, do not present a rigid intrusion within road structure and therefore do not exhibit the problem of rigid flush mounted sensors; this being resin break up in the wheel track area causing the sensor to loosen potentially requiring more regular maintenance and repair.
  • Cost: The sensor, installation and associated maintenance costs are considerably lower.

For customers who require more accuracy or where they wish to consider enforcement possibilities, then, the quartz sensor is provided.

  • Accuracy: The quartz sensor gives accuracy of +/-5% for 95% (COST323) of vehicles providing the sensor is installed in a smooth flat road surface.
  • Installation: The installation is more complex than for the BL piezo electric sensors. The installation of quartz WIM sensors is fast and easy, requiring just a few steps. Depending on the ambient temperatures, the lane is usually closed for just 6-8 hours.
  • Reliability: The sensor element is fully encapsulated and mounted in a rigid alloy profile. The upper surface of the sensor can be ground flat with the road surface. Special grouting is used to secure the sensors in place and they can give years of dependable use with minimum maintenance.
  • Cost: The sensor cost, installation cost and associated maintenance costs are higher than piezo electric sensor WIM technology.

In summary, a multiple system roll-out where statistical data is sufficient for typical asset management is cost-effective using the BL sensor system. A sizeable network of asset management and protection weigh-in-motion sites can be deployed at a significantly lower cost than a similar number of quartz sensor sites although if specific accuracy or enforcement was required then a select number of sites using this methodology could be considered.

Calibration

Post installation and commissioning a calibration of each lane of high speed weigh-in-motion must be undertaken after allowing a brief period of time for the new installation to settle and for the TMU to gather intelligence via the auto-TNL temperature compensation algorithm.

A process is then undertaken using suitable vehicles with known weights making a number of passes over the HSWIM arrays. The first phase of the calibration is used to determine a mean impact factor. Adjustments are made to these settings in the TMU and then a second phase of passes is undertaken to confirm the accuracy of the calculated weights. It is recommended the calibration process is repeated at least once annually.

Accuracy

For high speed weigh-in-motion systems the recognised accuracy requirement: COST323 Class B (10) is usually required.  This is a European standard for accuracy that being ± 10% weight accuracy for 95% of all vehicles. Q-Free’s HSWIM system in the UK is based on the use of Class 1 BL piezo-electric sensor technology and readily meets this specification and this can be confirmed by customers, including the Department for Transport, Driver and Vehicles Standards Agency (DVSA) and Transport Scotland (TS) where approximately 75 systems are operational.

In other countries quartz sensors have been specified by customers who wish increased accuracy, ±7%, (COST323 Class B+ 7) for direct enforcement of offending vehicles. Q-Free has installed these in the Kingdom of Saudi Arabia and has started to see demand of this type of detection requirement increase with a recent request by Highways England for a trial site in the UK to be installed during 2018.

Integration with ANPR and other systems

Q-Free can and has integrated the weigh-in-motion detection equipment with specialist ANPR providers and is happy to develop any required data protocols. For example, in the UK, Q-Free works closely with Jenoptik to deliver an integrated WIM/ANPR solution for the UK DVSA and Transport Scotland in partnership with other agencies to enforce weight infringements at key, strategic locations including major river crossings such as the iconic Forth bridges. The weight information is synchronised with the Jenoptik Vector camera to match ANPR records and the data is filtered to subsequently target offending drivers or operators.

In other situations, the WIM system can be co-located at a tolling gantry such as the system in Stockholm, Sweden ­and used to manage the tolling concession as well as the resulting data being integral for enforcement purposes. This system has also been delivered in Saudi Arabia as a pre-selection tool, with offending vehicles directed to an enforcement station downstream from the free-flow weigh-in-motion site for further investigation.

Choosing Q-Free

The illegal operation of heavy goods vehicles can, in summary, shorten the life of infrastructure, degrade the quality of travel and increase costs of operation and maintenance. Protecting road assets is fundamental to economic stability, growth and the social well-being of a region. Investing in intelligent transport systems to monitor and manage weights provides long term benefits including cost savings for local and strategic governments, increased productivity and improvement in the quality of life.

Q-Free’s HSWIM with BL sensors provides a cost effective solution for asset management, protecting a customer’s strategic road network including bridges. The resulting data is of sufficient accuracy to make informed decisions about current network condition and to plan for future improvements. Where enforcement is a factor Q-Free can install the quartz sensor system but this impacts on cost and perhaps therefore the volume of sites that can be deployed.

With remote monitoring capability, integration with ANPR and other systems such as weather stations, a comprehensive weigh-in-motion network can be deployed.

WIM/ANPR provides effective vehicle weight enforcement by providing a highly effective targeting tool and ensures that a significant percentage of vehicles are identified as being overloaded. In addition to this, recent studies have shown that a significant number of the drivers caught were also in breach of driver’s hours and various mechanical defects.

Without effective weight enforcement operators can take full advantage of the benefits of overloading, in contrast to compliant operators who could be economically disadvantaged and the road/bridge maintenance contractors who are left to pick up the costs of repairing the damaged roads.

International Society for Weigh-In-Motion (ISWIM)

Q-Free have been active in the field of WIM for over 20 years and are one of the founder members of ISWIM. ISWIM is an international non-profit organisation founded in 2007 and legally incorporated in Switzerland. It is an international network of and for people and organisations active in the field of WIM. Its purpose is to disseminate knowledge and understanding of weigh-in-motion and to support advances in WIM technologies promoting widespread use of WIM globally.

ISWIM has over 700 members from 70 countries and is run by a board of officials elected by the membership at the global conferences held every three years or so. Q-Free currently holds position of vice-president of the society responsible for end-user engagement and is the chair of the ISWIM vendors’ college where the views of 20 vendors on the board are represented.

Colin Reekie

Commercial Manager

Q-Free UK

+44 (0)7530 642 004

colin.reekie@q-free.com

www.q-free.com

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