Agricultural machinery is considered a **special vehicle**. This is because **tractors** and **self-propelled machines**, which are types of agricultural machinery, are designed to perform specific tasks in the field and construction. These vehicles have special features, such as large engines and the ability to transport or tow heavy materials, which distinguish them from common vehicles.

In addition, **agricultural trailers** and **towed machines** are also considered part of special vehicles, as they need to be towed by tractors or other special vehicles to perform their functions in the field or on construction sites.

Therefore, both agricultural machinery and their trailers are classified as special vehicles due to their specific use and characteristics.