I've never understood the problem with tailgating!
Surely if you're 50m behind someone, they brake & you don't react, then they'll be doing 0mph and you'll hit them at 70mph - an impact speed of 70mph.
If you're 10m behind someone, they brake & you don't react, then they'll be doing 50mph and you'll hit them at 70mph - an impact speed of 20mph.
However, if you're 0m behind someone (i.e. attached to their bumper), they brake & you don't react, then they'll be doing 70mph and as their braking you'll be braking too - an impact speed of 0mph.