If you want to foil them, you would put clear, IR-reflective decals next to your license plate with additional letters and numbers so that your license plate no longer scans as a license plate. You might even try to SQL-inject their ALPR ;-)
Yes. It can be considered tampering with evidence in an "ongoing" operation. I too have thought of this for radar guns and laser speed guns. But most of these sensors know when they are being jammed and can actually result in the FCC finding you and fining you.
Radar guns are regulated by the FCC because they are RF sources. They aren't going to come after you for mounting extra lightbulbs on your car.
Infrared bulbs are more similar to polarized plastic covers that obscure the plate when viewed from certain directions. Not sure if they are illegal or not but they are definitely outside the purview of the FCC.
Nowhere in the United States of America are 'citations' sent by - usually private - speed/red light cameras considered criminal offenses.
Know why? They barely stand the requirements for civil infractions in most areas. Typically these 'tickets' are implemented as backwards preconditions-to-vehicle-registration.
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Actively jamming radar is a crime and could possibly net you trouble. Light? IR? No such 'unlawful interference' law exists at the federal level. You might be violating a state statute, but that's the extent of it.
Interfering with a civilly-implemented speed/red light camera via use of plate-obscuring equipment on your vehicle might be a state-level offense but it wouldn't be for long if challenged.
