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I am afraid that under WA law as long as the sign was posted, they do have the legal right to impound the load. The fact it was at night and you did not see the sign, or perhaps did not look for the sign or missed the sign would legally not be a defense to the impound. Under the legal theory applied by the courts, "ignorance of the law is not a legal defense," which means just because you did not know the law or missed the sign does not mean you did not commit the violation and that you could not be penalized for the violation. Any violation occurring while driving
an employer's vehicle the employer can hold the driver liable for paying I am afraid.
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