Preventing hijackings is more complicated than that though, since an autonomous vehicle will stop for anyone who gets in its way, not just police officers. The array of cameras, lidar sensors, etc each car comes with is probably a fairly decent deterrent against that threat already though.
I guess eventually police vehicles can just stop autonomous ones with the press of a button, so it doesn't have to rely on its "senses". Some two-way identification between the vehicles; avoid the issue of impersonators completely.
This is an important general consideration. I designed a class of drone/robot I call "gun-hunter": They detect humans holding guns and destroy the forearm.
Now immediately you have to solve "How do you NOT attack LEOs?" Because of course the obvious thing for the badguys to do would be to impersonate LEOs well enough to fool the robots. (This also applies to signals guns might emit to say they're legit/friendly.)
The solution I came up with was that no one should use guns in the presence of active gun-hunters. After all, why would the police need guns if the crooks can't use them?
