It's fortunate that terrorists are both very incompetent and very low in number. How else do you explain the fact that there's been exactly 1 very serious and successful foreign terror attack on US soil (the highest value target in the world) in the past 50-ish years?
Currently there's the very possible opportunity of a power grid/infrastructure attack that could kill tens of thousands. But nobody should be truly worried about it.
Terrorists just aren't that good at what they do. Why would they somehow be better at hacking cars than they are at anything else?
The Oklahoma bomber killed 168, for example. The first one that came to my mind.
https://en.wikipedia.org/wiki/Terrorism_in_the_United_States
Most news organizations report on foreign and domestic terror differently. When the average American thinks of a terrorist it isn't a white guy with a U-Haul.
I contend that 40 years from now our grandchildren will be astounded that we got into such dangerous vehicles before self-driving cars.
[1] - I'm being facetious, of course.