Formula for stopping distance can be calculated by
[tex]v_f^2 - v_i^2 = 2 a d[/tex]
here
[tex]v_f = 0[/tex]
[tex]d = \frac{v_f^2}{2a}[/tex]
now we can see from above equation that stopping distance depends on square of the speed.
So as we can see that here 30 mph and 60 mph, if the speed is double then the stopping distance will become four times more.