Does it really mean 13ft 9inches ?.... NO it doesn't ! This is not a measure of the height, it is a measure of the clearance, which has strict rules for measurement, including..... On a horizontal road, the height is measured accurately, and rounded down to the nearest 3 inches, ANOTHER 3 inches is then deducted. This is the CLEARANCE height displayed on the bridge. On a bridge with an incline approach there are more complicated rules, because an approaching/leaving vehicle roof will be at an increased height, measurements will vary depending on the gradient and it's proximity to the bridge .