WIM Accuracy and Reliability at Border

Background

Of the nearly $612 billion of imports and exports exchanged between Mexico and the U.S. in 2018, more than 37 percent of the traffic passed through the Laredo Port of Entry in Laredo, Texas via crossings of the Rio Grande River. Four bridges comprise the Laredo International Bridge System and connect the city with Nuevo Laredo and Tamaulipas, Mexico, permitting the passage of over 6.7 million vehicles annually. The combined trade activity at three bridges in Laredo (two vehicular, one rail) ranks this city as the number-three customs district and the busiest land port in the United States. 

Two of the port’s four vehicular bridges are designated exclusively for commercial traffic — the World Trade International Bridge (Bridge IV) and the Laredo-Colombia Solidarity Bridge (Bridge III). Commercial vehicles traversing these bridges are required to pay a toll and are subject to compliance with established weight limits, classification and inspection — all designed to protect the road infrastructure and public safety. Weight validation, enforcement screening and automated toll collection for vehicles traveling from the City of Laredo to Mexico have been handled by a WEIGH-IN-MOTION (WIM) system for almost 20 years. WIM systems allow vehicles to be weighed at speed, automatically. 

Commercial vehicles detected to be overweight are alarmed on the supervisor’s Real-Time Monitor, and the truck is intercepted and required to turn around on the departure side of the plaza. The truck is then directed to a static scale to confirm vehicle weight. The accuracy range of the WIM system is used to establish an overweight threshold relative to the legal gross vehicle weight of 80,000 lb (36,287 kg).

Challenges

The original WIM system was an older, dual load-cell system with a TRMI treadle installed within the platform. Though strain gauge load-cells are known for their accuracy, the scales displayed other performance concerns. Unlike strip sensors, frame-based systems require digging up the roadway and the installation of a concrete vault, both to support the WIM scale frame and isolate the sensor from roadway rainwater collection. This style of system is inherently prone to water ingress, and requiring heavy equipment to remove the large WIM platform and its multiple components for frequent maintenance, resulting in significant maintenance costs, system downtime and lane closure delays.
Because of the issues associated with the existing system, the City of Laredo sought a solution that would allow for maximum uptime and system accuracy with less maintenance. TransCore, a leading transportation integrator, was selected for the project which consisted of a comprehensive upgrade to the toll collection system on two bridges — the commercial traffic-only Bridge IV World Trade Bridge and the non-commercial and commercial Bridge III Colombia Solidarity Bridge.

Solution

At the center of the new WIM solution, TransCore specified Intercomp Strip Sensors as the enhanced subsystem, because of their straingauge based accuracy, reliability, simplified installation and operation without frames and drains. Toll collection and overweight vehicle screening can occur simultaneously, and truckers no longer need to stop for inspection if their weights have not exceeded specified limits.

The high-performance, strain gauge load cell technology of the Intercomp system operates over a wide range of environmental conditions and roadway surfaces. With its long-term stability and internal temperature compensation, the sensors deliver improved accuracy and consistency of output, day-to-day and season-to-season. Along with enhanced accuracy, the Intercomp Strip Sensor eliminated the maintenance challenges of the existing system. Johnny Redman, Transportation Accounts Manager at Intercomp, noted the sensors easily meet <5% error required by the customer. They deliver a low cost of ownership due to the stability of the sensors, and selfcontained operation minimizes disruptions while vastly reducing installation costs associated with frame-based systems.