Smart pedestrian

A truck of width $l$ is moving with a uniform speed ${v}_{0}$ along a straight horizontal road. A pedestrian starts to cross the road with a uniform speed $u$ when the truck is $d$ away from him. The minimum value of $u$ so that he can cross the road safely is of the form $\frac{m}{\sqrt{n}}$ where $m$ and $n$ are coprime and $n$ is square free. Find $m-n$ .
Details
$l=2m\\{v}_{0}=8m{s}^{-1}\\d=4m$