Generally guidance for “safe” following distance is to be able to stop before you hit a car that is also stopping with the assumption that the car ahead is stopping at the same rate. So 2 seconds of headway between cars (roughly reaction time alone). Obviously this does not give enough time if the car ahead has a head on collision or similar (but the third car will collide at lower speed and the fourth might stop).
Most traffic is a little closer together than this (hence the prevalence of pile ups), but there is also uneven speed and gaps at traffic lights and similar
Generally guidance for “safe” following distance is to be able to stop before you hit a car that is also stopping with the assumption that the car ahead is stopping at the same rate. So 2 seconds of headway between cars (roughly reaction time alone). Obviously this does not give enough time if the car ahead has a head on collision or similar (but the third car will collide at lower speed and the fourth might stop).
Most traffic is a little closer together than this (hence the prevalence of pile ups), but there is also uneven speed and gaps at traffic lights and similar