How many accidents have you had of that type in the past, say, 300000km of driving?
Because you say it legitimately prevented an accident, but this is a routine thing that happens (drivers slam on brakes at the last second) and if you aren't routinely driving into the rear of cars who brake at the last minute, then ... no, the accident wouldn't have happened anyway.
As an aside, trying to quickly search for these sorts of statistics leads to an awful lot of law firm websites.
But that number is not relevant. What is relevant is the ratio of humans avoiding these accidents to humans not avoiding these accidents.
Like I said, having to avoid the car in front which braked hard is routine. It happens all the time. It's why the blame and culpability for a rear-end accident is always on the rear driver by default - safe driving distance and all that.
Which is why I phrased my question to the GP the way I did - if he managed to go 300000km without rear-ending someone even though unpredictable braking happens so routinely ...
> What % reduction in these would make it make sense to "require" this?
I doubt there'll be a decrease, if any. A better system that beeps if a safe minimum following distance is not adhered to will almost certainly prevent more accidents than auto-braking, because the auto-braking system will sometimes get it wrong and cause the jackass tailing you to rear-end you.
An alert won't cause any accidents.
I don't have an auto-brake feature, only ridden with people who do. I believe that most of them do have some sort of warn prior to brake - but to your point, there are so many situations where this could falsely trigger that it would likely end up being tuned out, at least as I imagine it. Actually, my truck has one of the "warning" things, and I ultimately turned it off for this reason. The number of "immediate danger" warnings that would flash when I was driving, e.g. alone on a country road at night, made it not worthwhile to me. I can imagine an auto-brake would be even worse that way though, lol.
Anecdotally: I and a number of friends have held this same opinion for years, but recently it has changed. Not because of lane marker following, but because of this situation: “idiot in front of me slammed on their brakes at the last second while I was scanning left or right”; the system kicked in and legitimately prevented an accident. If there was some way to just enable that feature I’d be pretty happy.