How is the collateral damage from shutting off the wrong car any different than doing the PIT in the wrong place, or spiking the wrong tires - all of which have already happened many times?
If you look at the specification of the device HPEMS system at
http://eurekaaerospace.com/hpems.php , the device generates microwave radiation pulses at 100 Hz with peak pulse widths at 50nS, hence peak power at 2GigaWatt with the average power with the overall design goal to increase the radiation pulses to 1000 Hz, increasing the radiated power by over 10 times. The frequency of the radiated energy will be tunable, (i.e mechanically modifying the length of the transmission line to the antenna load). Total power at the antenna (continous) will be around 10KW and 100KW at full design specification. With that amount of radiated energy being channeled in into an area of a few square metres cross section a person will slowly start to cook from the inside out after a few minutes. You really would not want to be in front of the antenna of the current or future HPEMS device either a few feet or a few miles away.
To shut down an automobiles electronics, which is in effective a faraday cage with a radiation wavelength greater than a few decimetres is impossible. It would be impossible for the radiation to enter dimensional gaps which is less than the wavelength of the microwave radiation if the microwave radiation was of a continuous type. What is being relied on here is something called parametric frequency amplification due to the pulse widths being deployed (only few nanoseconds), whereby spurious higher frequencies of undetermined powerband spectrum are being generated. This is so that much higher microwave frequencies (much shorter wavelengths), which are being generated can enter the seals of the effective faraday cage (radiator grill etc) to damage the electronics of microprocessors and memory circuits due to the huge electric field strengths of 20KV/m still present.
Most of the radiation energy will reflect of the vehicle, causing the large 100-1000 Hz EM pulses to radiate backward to the pursuing vehicle. Some of the energy will reflect skyward. Some of the radiation may reflect of the vehicle and may hit another vehicle possible passing in the opposite direction causing the driver to be completely startled as the top half of his body feels as if it has just caught fire, thus losing control of his vehicle. This will of course also happen to the vehicle being targeted by the HPEMS weapon. If one vehicle drifts left and other drifts right, well I think you can understand the consequeces. Some of the energy may reflect of the target vehicle and destroy other less well protected electronics, which are not prtotected by faraday cages. This could be within a radius of many hundreds of metres.
Then of course we now have the car. After the perp has crashed his vehicle (hopefully not into another car or anything else that would cause collateral damage (other dead civilians to use its correct terminology), we could now have a vehicle in flames because using the HPEMS device could have some other strange effects such as the vehicle fuel pump still pumping fuel into a stalled engine. Vehicles have a strange tendency to catch fire when hit with an EMP.
Have you ever heard of military aircraft initiating anti-radiation missile lock of one of those LEO's RADAR speed detectors (only a few watts power) because I have.
Have you heard of anyone sticking their head between the dish and the waveguide of microwave transmitter for a few seconds. I have and the victim ended up with permanent brain damage.
Have you ever heard of the RADAR set in a MIG25 (RP-25 Smerch radar) killing rabbits at 2km, because I have.
Of course the real question is why on earth is the LA police thinking about deploying electronic warfare weapons against civilian targets. Why not just use high powered laser weapons and deliberately just blind the perpetrator instead. The effects of collateral damage will probably be a whole lot less.
Then of course there is the risk of cancer (for both the target, the colateral damage and the LEO) due to the use high powered microwave devices
Perhaps the authorities could ask James Tatoian to stand in front of his proposed HPEMS device for a good 10 minutes whilst standing 20 feet away. I wonder what his response would be?