Which of the following best defines special mobile equipment?

Special mobile equipment is best defined as a vehicle that is not primarily designed for transportation purposes but can be used incidentally on highways. This definition encompasses a range of equipment typically involved in activities such as construction, agriculture, and maintenance, where the primary use is not to transport goods or passengers.

When considering the context of special mobile equipment, it's essential to recognize that while these vehicles may occasionally traverse public roadways, their main function pertains to other tasks, such as excavation, lifting, or providing services. Hence, their design and capabilities prioritize those functions over standard transportation.

The other definitions do not encapsulate the unique nature of special mobile equipment. Vehicles primarily used for transporting passengers or goods have a distinct operational focus on transportation. In contrast, a vehicle used for construction that does not move on highways does not align with the incidental use of the highways that is characteristic of special mobile equipment.