The MIRT, or Mobile Infrared Transmitter, is a device that can be installed in emergency vehicles (police, fire trucks, etc) that change red lights to green for them. This way, in an emergency, they don't have to worry about going through a red light and getting in an accident.
Of course, a few years after this device showed up, and before a law was enacted against doing this, everyday people found ways to get their hands on these devices, so they didn't ever have to wait at red lights. I wouldn't be shocked if people were still doing this even though it's illegal.
Schematics to build the device are even available freely online!!
The problem in a system like this is this - there is no authentication. In other words, in many places, if the infrared controller receives a certain signal, regardless of what device sends it, it will turn the light green. What really should be happening is that it should reject any unauthorized devices, but that's harder to do and has not been enacted in many areas.
Even though there are laws against this, how many police officers would even recognize a device like this if it were in someone's car? How many people would even notice that someone enough to establish a pattern and call the police? Is a fine of $200 (let's say) even a deterrent?