It is illegal in Pennsylvania to have things hanging from the rear view mirror which "materially obstruct" the driver's view. The officer must have reasonable suspicion that this law has been violated before she stops a car.
One defendant got pulled over for this. During the stop, the officer determined the driver was under the influence of a controlled substance and made an arrest. The driver was convicted but appealed, claimiing the officer had no reasonable suspicion to make the stop and therefore all evidence obtained from the stop should be ignored (like the driver's condition?).
Unfortunately for the defendant, the officer opined in court that foam dice and at least 4 pine air fresheners hanging from the rear view mirror would hinder the driver's view. The court agreed. The dice rolled the wrong way for this driver.
A search of the California Vehicle Code (under "rear view mirror" and "mirror") turns up no similar law. It is widely believed in that state that hanging things from the rear view mirrior is illegal but I don't see the law. (Probably they would cite you under a general obscured vision section.)
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