They actually do weigh trucks when they are open. For two reasons.
1) Is the truck overweight for the roads they are traveling.
2) Is the truck itself rated for the weight it is carrying.
To the casual observer a big truck is a big truck but in reality they are widely different in the weight they are designed to carry.
A house moving truck and trailer is really large but has light duty axels, wheels and brakes. It could be easily overloaded if someone uses it to move some heavy product.
They also inspect truck driver logs, brakes, frames whatever they really feel like.
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