If a hacker targeting connected cars in Manhattan could randomly stall 20% of them during rush hour, total gridlock would ensue. “This isn't just bad traffic where you are an hour late. It becomes impossible to get from point A to point B,” says Georgia Institute of Technology researcher Peter Yunker who ran a study on the "cyberphysical risks of hacked internet-connected vehicles." From IEEE Spectrum:
Not all cars on the road would need to be self-driving and Internet-connected for such paralysis to occur. For example, if 40 percent of all cars on the road in Manhattan were online and autonomous, hacking half of those would suffice.
...Cities without large grids—-Atlanta, Boston, and Los Angeles, to name a few—were more vulnerable to gridlock from such attacks.
Yunker and his colleagues cautioned that they considered only static situations where roads were either blocked or not blocked. Future research with more dynamic models would likely show that blocked roads would spill traffic over into other roads. Given such effects, it might be possible to trigger gridlock by stalling much less than 20 percent of all cars, Yunker says.