This 'generalism' is just that - a general rule. It doesn't work, the faster you go and if you do the maths you will find that just shy of 60mph, the 2 second rule does not work. At speeds above 57 - 58mph ish - the 2 second rule will cause you to hit the car in front (albeit at a snails pace), but you will collide. And the faster you go above this speed, the harder you will hit the car in front...and around 80+ mph, you will cause severe damage during the collision.
So be very very careful when discussing this 'general' rule, it isn't a rule perse, it is a suggestion, a guideline and many factors come into play....not least....speed.