How Does Weight in Motion on Freeways Work?

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Weigh-in-motion devices are commonly used to measure the weights of passing vehicles.
Weigh-in-motion devices are commonly used to measure the weights of passing vehicles. (Image: freeway image by Fabio Barni from Fotolia.com)

Weigh-in-motion (WIM) devices are a type of technology capable of recording the weight of a moving vehicle. WIM devices are commonly installed on freeways, specifically to measure the weights of semi-trucks at weigh stations. The WIM sensors are thought of as a newer technology since the passing vehicle does not have to come to a complete stop for the weight to be recorded as was the case at older static weigh stations.

Applications

There are many applications and uses for weigh-in-motion sensors. These devices are used by planners who design and monitor pavement and bridges. Researchers use the WIM-generated data to determine how particular aspects of the freeway system should be designed and maintained. These devices are also used to enforce the laws associated with how heavy a vehicle and its cargo, such as a semi-truck, are allowed to be. Other uses for the systems include legislation and regulation as well as in administrative planning for future projects.

Pros

There are several positive characteristics associated with weigh-in-motion technology, most of which deal with how these systems have overcome the shortfalls of the static weigh stations. Weigh-in-motion technology allows more vehicles to be weighed over a shorter period of time, which means that truck drivers and other motorists are not burdened by the static stops of the previous technology. This decrease in static stops has also improved highway safety as there are not vehicles lined up on the freeway system waiting to access the weight station, which is a potential traffic hazard. For those collecting and monitoring weigh-in-motion data, these systems provide more data over a shorter period of time. After the initial installation, this technology is also cheaper to run and maintain than the static weigh stations. The WIM sensors are able to collect more accurate data as overweight trucks are not able to skip the weigh station as was previously the case with static stops.

Cons

Despite all of the positive technological improvements of the WIM technology, there are still areas for improvement. While more vehicle weight data can be collected over a shorter period of time, there is less data collected per vehicle. At a static weigh station, a number of characteristics related to each truck (type of fuel used by vehicle, state of registry, origin, destination, etc.) can be collected easily. None of this additional data can be collected by WIM devices. These systems are also less accurate than the scales at static weigh stations. The accuracy at static weigh stations is +/- 2 percent while the best WIM systems have an accuracy of approximately 6 percent. Finally, the WIM devices are more susceptible to damage, especially electromagnetic damage caused by nearby lightning strikes.

Weigh-in-Motion Technology

The piezoelectric sensor is the most commonly used weigh-in-motion device. This type of sensor is installed directly into the pavement and measures the weight of a vehicle as it drives over the system. Bending plates and single load cells are other types of weigh-in-motion technology. Both consist of platforms, installed over the pavement in a lane, capable of sensing the weight of passing vehicles.

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