“Special mobile equipment” means, except as provided in subsection (c) of this section, a vehicle that: Is not used primarily for highway transportation of people or property; and Is operated or moved on a highway only as an incident to its nonhighway use. “Special mobile equipment” includes a road construction or maintenance machine, mobile crane, ditchdigger, well driller, concrete mixer, jobsite office vehicle, or portable power generator. “Special mobile equipment” does not include a farm tractor or any farm equipment.