It is your responsibility as a driver to maintain a safe distance behind the car you are following. The car in front of you should be able to spontaneously turn into a brick wall, and you should have enough space to stop before hitting it
Why is it unrealistic, besides the fact that you're following too close behind? If your line-of-sight is shorter than your braking distance, you are driving wrong.
Google shows two-second rule when looking for "safe following distance" - I was taught 3 seconds and it seems more reasonable assuming slightly over a second reaction time.