18.1. For the purposes of calculating the number of axles of an apportioned road vehicle or combination of road vehicles, the word “axle” means an assembly of a vehicle consisting of 2 or more wheels whose centres are in one horizontal plane, by means of which a portion of the weight of a vehicle and its load is continually transmitted to the roadway, even if such assembly is load-bearing only part of the time.
Sections 16 to 18 do not apply to those vehicles.