$\textbf{Answer}$

Left. (Or right, if you assume it is in the countries where driving is on the left side of the road, such as UK).

$\textbf{Analysis}$

The picture only shows windows, but no doors. Therefore, the door must be on the other side, i.e. it is a bus on the opposite side of the road to the observer.

$\textbf{Note}$

Be creative and think out of the box.