Tolling overweight vehicles on highways

By Rish Malhotra Regional Manager International Road Dynamics Inc.,

The concept of Weigh-In-Motion (WIM) has a long history of application in data collection for use in planning transportation infrastructure. This has expanded to use in enforcement in sorting compliant from violating vehicles to increase the throughput and ease congestion at inspection stations. The wear and tear on pavements, bridge structures, and other components of the transportation infrastructure caused by an individual vehicle is directly proportional to the weight of that vehicle or axle. In fact, as gross vehicle and axle weights increase, the damage to highway and bridge infrastructure increases exponentially, as was concluded by the American Association of State Highway Officials (AASHTO) Highway Study (1). An increase in loading thus results in an exponential increase in the acceleration of road wear. In other words, if pavement loading is miscalculated by as little as 10%, it could lead to acceleration of road wear by over 40%. WIM systems installed by International Road Dynamics Inc. (IRD) combat road-wear by providing accurate data for traffic planners and by identifying overloaded trucks.

Where WIM is applied, both axle and Gross Vehicle Weights (GVW) are monitored.  A truck may be GVW compliant and still have individual overweight axles causing significant and costly damage to infrastructure.  Traditional methods of recouping the costs, in the form of either fuel taxes or tolls based on vehicle type can only factor in weight on a cost averaged basis, spreading the charge across all vehicles in a class.

This process thus overcharges under-loaded vehicles and undercharges over-loaded vehicles, relative to the actual cost of their consumption of the transportation infrastructure.  This in turn promotes overloading, as the cost per mile decreases with each extra unit of weight carried.  There is also an additional cost of biasing the system to encourage overloading from the increased accident rate; overloaded vehicles have a greater stopping distance, reduced maneuverability, increased risk of rollover, and an increased rate of mechanical failure.

Applying Weigh-in-Motion (WIM)

WIM has a long history of application in data collection for use in planning transportation infrastructure.  This has expanded to use in enforcement in sorting compliant from violating vehicles to increase throughput and ease congestion at inspection stations.  The incorporation of radio frequency identification (RFID) for automatic vehicle identification (AVI) and various video imaging technologies has increased the scope of WIM into one of the main elements of what is now known as Intelligent Transportation Systems (ITS).  Most recently, in the ongoing search for ever more efficient methods of funding the building and maintenance of roadways and bridge structures, both public and private sector leaders are turning to the concept of tolls based on vehicle weight, also known as WIM@Toll^TM.

Weighing Strategies

In order to change to a toll strategy that charges by weight and penalizes overloading, it is necessary to weigh and classify vehicles.  There are three main options for doing this. 

The first is to have static weigh stations on the toll way.  While being the most accurate, this method has a number of drawbacks.  Vehicle throughput is very slow compared to other options, resulting in delays and congestion.  It requires an additional large infrastructure investment and staff to build and man the weigh station.  The departure from the weigh station and merging of heavy vehicles with the main traffic stream poses a safety hazard.  Thus, the GVW accuracy of the static weigh stations, though being the best, comes at a high initial investment and equipment cost to the buyer.  The added cost to the user also includes the delay and slow down caused by the time taken by the traditional scales to calculate the GVW.

A second option is to have high-speed WIM lanes on the roads or toll way.  This method is the least intrusive as it involves no impediment to traffic flow.  The challenge is that it requires a way of identifying all heavy vehicles at highway speeds.

The third option is to have low-speed WIM lanes at toll plazas.  This method integrates the weighing operation into the normal toll collection process at the plaza.  The WIM operation requires little additional delay.  When combined with Electronic Toll Collection (ETC), this system can process the majority of vehicles without stopping, while still allowing manual collection from vehicles that have not subscribed to the ETC program (the efficiency is significantly increased by having some WIM toll lanes reserved for ETC vehicles only).

IRD has implemented low speed WIM in toll lanes in both China and India.  In China, IRD has installed its low-speed WIM solution using IRD Bending Plates® at several toll plazas.  Two Bending Plates® are installed in a staggered format on each lane of the toll plaza.  Test results conducted on the Bending Plate® systems in China has proved that it is possible to achieve accuracies of at least ± 97.5% at 90% confidence levels.  In India, IRD has installed its Slow Speed WIM (SSWIM) scale solution on the two exterior extra wide lanes in toll plazas.  On average, installing IRD's Bending Plate® solution or the slow speed WIM scale solution for slow speed operations costs anywhere between US$25,000 to US$35,000 per lane including the electronics depending on the configuration chosen.  Introducing low-speed WIM lanes at toll plazas assists the overall weighing operation by offering a significant reduction in delay in comparison to using static weigh stations on the toll ways which is why this method is now gaining popularity.

Weighing Solutions Offered by IRD

Various vehicle weighing solutions offered by IRD include the usage of high speed and slow speed WIM sensors.  The WIM technology deployed, installed and maintained by IRD in various locations worldwide include Single Load Cells, Bending Plates®, slow speed WIM (SSWIM) scales, Lineas® Quartz and Piezoelectric sensors.  The systems in operation use a range of video imaging technologies including color, low light and infrared imaging; license plate recognition and Electronic Vehicle Identification (EVI) to assist in vehicle identification.  Most systems installed by IRD also include various vehicle dimensioning sensors such as height sensors, light curtains, tire width sensors and axle sensors.

Future Trends

The future trends towards the usage of universal EVI in commercial vehicles thus leading to further integration of transportation management functions via ETC of overloaded vehicles.  IRD specializes in system integration of EVI based solutions for ETC, Access Control, Security and related applications based on the customer's specific requirements.

References

1.     Special Report 61G, The AASHO Road Test: Report 7 - Summary Report, Highway Research Board, National Research Council, Washington, D.C., 1962.