The ones I've seen are not EMP generators, they're basically remote-controlled cars with a wire sticking up and another sticking down to touch the ground. Cops can drive them under cars they are chasing, and when the wire brushes the underside of the car, they dump a few thousand volts into the chassis, simulating a small lighting hit. It's possible it uses some frequencies other than just a DC current dump, I don't know.
Certainly not EMP though. Even if directed, you'd burn out every cell phone, stop light controller, and other electronics in that direction for a few hundred feet. Not to mention the fact that in order to generate enough power in the EMP, it would probably have to be pumped with explosives.
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