Getting It Right With Direct Enforcement

Direct weight enforcement is the use of Weigh-In-Motion (WIM) to prosecute overweight vehicles directly without confirmation of weights on a second weighing system. Unlike preclearance systems, where vehicles are screened using high speed WIM and overweight vehicles are selected to be weighed on a platform scale, direct enforcement allows fines to be applied based on the WIM weight alone. It is an enforcement process for commercial vehicles that is analogous to radar speed enforcement or red-light camera enforcement. It can be a highly automated system that operates continuously without requiring the involvement of enforcement personnel.

Globally, interest in direct enforcement is growing. It’s not hard to see why — the ability to detect overweight vehicles in live lanes lessens the safety risk caused by disrupting traffic flows and allows legitimate economic activity to carry on unimpeded. But, gaining public acceptance and support for direct enforcement is going to be crucial to its success.

ISWIM’s Guide for Users of Weigh In Motion indicates that the typical WIM accuracy requirement for direct enforcement systems is ±5% of Gross Vehicle Weight (GVW) for 100% of all measurements. However, the actual accuracy required of these systems will depend on the agencies’ objectives, the legislation that is developed, and the technology available. It is likely that some jurisdictions will require even higher accuracies to implement direct enforcement.

Direct Enforcement WIM Concept
IRD Direct Enforcement Concept with VectorSense™ and WIM Sensors

Proving system accuracy and integrity are the key challenges — if enforcement agencies are going to defend themselves effectively against litigation and negative media coverage, they are going to have to prove that in-lane systems perform consistently and meet or exceed legal requirements.

This will always be a challenge in a dynamic environment, as many factors can affect readings. Weather can influence negatively through ambient temperature variations and the presence of snow and ice on sensors and surrounding pavement surfaces. Strong winds or gusty conditions cause vehicles to lean and sway, making readings appear to be outside acceptable tolerances.

Then there is lane position; a vehicle being measured on a static scale can be precisely positioned, whereas a vehicle moving at speed can wander within its lane and pass over an embedded sensor anywhere along its width. A sensor’s uniformity then becomes an issue.

Achieving an installation that in all conditions achieves high WIM accuracy requires several things. Fundamentally, the sensor itself has to be up to the task. Then, there is the challenge of proving ongoing in-specification performance without interrupting traffic operations. Linking the in-lane technology to a nearby static weigh station and frequently cross-referencing measurements is one way of doing this, and specific technology from IRD’s portfolio can help to further improve overall system accuracy.

The VectorSense-based Tire Anomaly and Classification System (TACS) detects tire anomalies in real time and at highway speeds. TACS can identify single or dual tires on an axle, as well as flat, missing or mismatched diameter tires on a dual tire set. TACS can also pinpoint lane position. This is useful because although uniform performance across the width of a linear sensor is desirable, in reality many things can affect this. A sensor’s performance characteristics can be ascertained through testing/monitoring and then combined with highly accurate vehicle positional information provided by TACS to give a truer reading of axle weights.

TACS™ Lane-Position and Tire Anomaly Data
TACS™ Provides Lane-Position and Tire Anomaly Data at Highway Speed

A multi-sensor approach to direct enforcement will improve WIM performance but this will be undone if the technology is not considered in its fuller context — a high-specification WIM sensor might be expected to offer a decade or more of good service, but if it is poorly installed, or the road surface integrity can only be guaranteed for four or five years, that is unlikely to be achieved.

A concrete road surface provides the best longevity and stability but cost and surface noise may militate against this. A concrete raft within a stretch of asphalt road is one solution, however it has to be accepted that for some cost-pointed projects even this might not be possible.

Whichever the solution chosen, good civil engineering practice at the point of installation has to be supported by a credible maintenance and audit regime that extends across the life of the sensor itself. For many deployments, this is at least as important as the choice of technology.

Ultimately, the technology cost is not the only consideration if successful direct enforcement using WIM is the ambition. However, the cost of not getting things right can be considerably greater and two things can help to address that. Engagement with a systems-level supplier with the requisite knowledge to guarantee success is a good first step. DOTs, enforcement agencies, and others with an interest in WIM, can also make major gains by appointing internal champions who have a profound understanding of the technology and its potential.

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