The proportional relationship is verified because the velocity (miles per hour) is constant.
So, for evvery pair of points (time, distance) you will verify:
time1 / distance1 = time2 / distance2 = time3 / distance3 = time4 / distance4 = 170 mph.
The proportional relationship means that the ration of the variables is constant, in this case the ratio is the velocity:
velocity = distance / time = constant = 170 mph
