You mean on top of the gridlock already caused by human-driving cars? To drive in NY is insanity.
From Jennifer Ouellette at Arstechnica: "Immediately after a vehicle is hacked, the traffic slows down around the hacked vehicle," said Vivek. And when multiple vehicles are hacked, there is a critical slowdown, which can turn into complete stoppage. "This is a concerning scenario, because when emergency vehicles want to get through, they can't," said Vivek. "You need to wait for a tow truck to come and move one of the hacked self-driving cars."
I find this to be one-dimensional thinking. Why presuppose a hacked vehicle to be only a blockage for gridlock? In this age of horrific vehicle carnage wrought by Jihadis, why couldn't a hacked vehicle be weaponized to run down pedestrians, smadh store fronts, or other vehicles?
And don't get me started on drones or flying cars. Our robot task masters will always be susceptible to criminal minds.