My car's "smart cruise control" leaves what seems to be around a car length for every 10 mph, which is what I remember hearing in driving school. Feels a bit excessive in practice, but I also never feel like I'm being an asshole so I'm okay with it.
I was rear ended, HARD once. That distance gave me the space to control both my vehicle, and the one that hit me. It turned a potential multi car, multi lane pileup into a 2 vehicle wreck, either 1 more dinged car.
The space isn't for the 99.999% of the time, but that 0.001% OH FUCK time.
I almost had that happen to me a year ago. Traffic had stopped on the interstate and I hear skrrt, skrrt, skrrt, skrrt getting closer and closer to me. I looked up in my mirror and see a Silverado rapidly closing the distance and thought "god damn it". I fortunately left enough room between me and the car in front of me that I was able to give him some more space and turned a full on collision into a tap that didn't do any damage. Pretty sure my foot was actually off the brake when he hit me and that helped absorb some of it as well. I was still pretty pissed though because I had more than enough time to think about all these things and consider moving onto the shoulder but I thought he would do that instead and still hit me with how fast he was going. Being an idiot, he did not and hit me anyway.
I've dodged similar collisions a couple of times, over the years. Unfortunately, this time, she didn't even touch the brakes. I was hit with a 50 mph differential, and no warning whatsoever.
Oh I'm not saying anything against it! I love that it leaves as much space as it does. Sometimes I just have to give it a little umpf to pass a semi because it sees the car waaay ahead and slows down.