Is it just 18-wheelers involved in the vast majority of trucking accidents?

No, it is not just 18-wheelers involved in the vast majority of trucking accidents. In fact, trucking accidents refer to any accident in which a commercial motor vehicle is involved.

Commercial motor vehicles include, but may not be limited to:

Concrete trucks
Cement trucks
Flatbeds
Box trucks
Oil transports
Dump trucks
Logging trucks
18-wheelers (tractor-trailers)
Refrigerator trucks