These guys did serious research into hacking cars and published the results.
http://www.autosec.org/pubs/cars-oakland2010.pdf
Experimental Security Analysis of a Modern Automobile
“Even at speeds of up to 40 MPH on the runway, the attack packets had their intended effect, whether it was honking the horn, killing the engine, preventing the car from restarting, or blasting the heat. … In particular, we were able to release the brakes and actually prevent our driver from braking; no amount of pressure on the brake pedal was able to activate the brakes. Even though we expected this effect, reversed it quickly, and had a safety mechanism in place, it was still a frightening experience for our driver.”
