Jeep and others have it but drive like a bowling ball with guardrails up.
Has anyone here successfully retrofitted Audi a4 with ACC? I hear Cruise started like that, would be awesome to get a kit like that.
Gas, brake, steering in cars are controlled by each separate computers with each own firmware - there's no centralized keyboard controller or USB root hub to take over. The computers have to be designed and built to accept driving commands, or replaced by one supporting it, or mechanically actuated. The latter two paths are rarely taken.
Regular cruise works fine on highways, and my experience with adaptive is that it makes passing more difficult as it slowed down way too early (though other implementations might be better), plus the focus of smooth / comfortable slowdowns means it can be hard to realise if you’re watching the road and your surroundings, at which point you’re quite a ways below speed for the passing lane and need to accelerate back up.
Also doesn’t deal well with passing trucks for lack of anticipation.