A new way to find speed

I was travelling by bus, when I was keen to do this experiment. I wanted to calculate speed of the bus, obviously without looking in the speedometer. And with the help of a milestone.
Given, that the horizontal distance of the bus by which I am traveling from the roadside where the milestones are laid is 6m.
And I have an exact view angle of 90 degrees.
Also I am looking exactly horizontal I.e. perpendicular to the motion of the bus.
Now if a milestone passes through my view angle in 0.3 seconds. Then calculate the speed of bus in m/s at that moment.

Neglect the width of the milestone.
And assume the distance of the bus from the roadside equal to the distance of my eyesight.