4Exemptions from duty of certain descriptions of vehicle
(1)No duty shall be chargeable under this Act in respect of mechanically propelled vehicles of any of the following descriptions, that is to say—
(a)fire engines;
(b)vehicles kept by a local authority while they are used or kept on a road for the purposes of their fire brigade service ;
(c)ambulances;
(d)road rollers;
(e)vehicles used on tram lines, not being tramcars used for the conveyance of passengers ;
(f)vehicles used or kept on a road for no purpose other than the haulage of lifeboats and the conveyance of the necessary gear of the lifeboats which are being hauled;
(g)vehicles (including cycles with an attachment for propelling them by mechanical power) which do not exceed eight hundredweight in weight unladen and are adapted, and used or kept on a road, for invalids;
(h)road construction vehicles used or kept on a road solely for the conveyance of built-in road construction machinery (with or without articles or material used for the purposes of that machinery);
(i)vehicles constructed or adapted, and used, solely for the conveyance of machinery for spreading material on roads to deal with frost, ice or snow or for the conveyance of such machinery and articles and material used for the purposes of that machinery;
(j)local authority's watering vehicles ;
(k)tower wagons used solely by a street lighting authority, or by any person acting in pursuance of a contract with such an authority, for the purpose of installing or maintaining materials or apparatus for lighting streets, roads or public places.
(2)In this section—
" road construction vehicle " means a vehicle constructed or adapted for use for the conveyance of built-in road construction machinery and not constructed or adapted for the conveyance of any other load except articles and material used for the purposes of that machinery;
" road construction machinery " means a machine or contrivance suitable for use for the construction or repair of roads and used for no purpose other than the construction or repair of roads at the public expense;
" built-in road construction machinery ", in relation to a vehicle, means road construction machinery built in as part of the vehicle or permanently attached thereto ;
" local authority's watering vehicle " means a vehicle used solely within the area of a local authority by that local authority, or by any person acting in pursuance of a contract with that local authority, for the purpose of cleansing or watering roads or cleansing gulleys;
" tower wagon " has the same meaning as in Schedule 4 to this Act;
" street lighting authority " means any local authority or Minister having power under any enactment to provide or maintain materials or apparatus for lighting streets, roads or public places.