Answer
$v=3.9\times10^{-38}m/s$; This is significantly lower than normal freeway speeds.
Work Step by Step
For diffraction effects to be important, the wavelength is roughly the size of the opening. Use equation 27–8.
$$\lambda=\frac{h}{p}=\frac{h}{mv}$$
$$v =\frac{h}{m \lambda}$$
$$v=\frac{6.63\times10^{-34}J \cdot s }{(1400kg)(12m)}=3.9\times10^{-38}m/s$$
This is about a factor of $10^{39}$ below ordinary highway speeds of 30 m/s.