With weigh-in-motion (WIM), there are sensors in the road that weigh each axle of vehicles travelling at highway speeds. They are very accurate - however the weights can be subject to some variation due to changing forces as vehicles in motion bounce around. This is usually mitigated by ensuring the road is as smooth as possible before and just after the WIM sensors. The system also uses sensors to measure the distance between each axle and the vehicle itself. Often the rules allows some axles to carry more weight than others. It also can take a picture of the vehicle and measure whether it is overheight or not.
The transponder (Pre-pass etc.) then correlates the particular vehicle with the one that just drove over the scale. A mile before the truck passes the weigh station the operator in the weigh station can see a picture of the truck, its WIM weights on each axle and anything out of allowance will be red flagged.
The transponder (or electronic message signs on the road) then can signal to driver if they need to go into the weigh station or whether they can stay on the road. The computer determines this instantly and the operator can vary the settings as desired depending on how busy they are (i.e. allowing a large tolerance all the way to bringing everyone in). Additional loops/sensors can further track where the truck went and let the operator know if someone is running the scale as well as which lane(s) they went down.
The goal here is to lighten the load for the operators so that they are not overwhelmed with thousands of trucks slowly driving through the scale and can focus attention on the trucks that are either likely violating axle weights or are otherwise flagged in the system via their transponder ID’s. Also trucks that are within weight and good carriers save time by not having to drive through the scale.