To get to the power output that will stop a vehicle from distances usually seen in car chases would require a massive arrangement, capacitor bank, and a dedicated power supply to keep the HERF pulses sustained. This certainly will not be the kind of device that will be mounted on police cars any time soon.

I have to also wonder how effective it would be in an actual car chase (assuming they could find as way of making it mobile). They would typically be shooting it at the rear of the car where the bodywork would act as shielding for the engine computer, and there is nothing to stop portions of the RF pulses reflecting off the metal bodywork and disabling chasing police cars.

Strap it to a police helicopter (or a police UAV, in 10 years).