The trucking industry can be justly called a ________ of the U.S. economy.

The trucking industry is often referred to as the mainspring of the U.S. economy because it is essential for the movement of goods across the country. As a mainspring is a crucial component that drives the function of a mechanical system, the trucking sector plays a vital role in ensuring that various industries receive the raw materials they need and that finished products reach consumers efficiently. This interconnectedness supports countless jobs and economic activities, highlighting the industry’s influence on trade and commerce.

Additionally, the term "mainspring" emphasizes the idea of being a primary source of energy or movement within a system, which aligns well with the trucking industry's position as a driving force behind economic operations, unlike other terms that might imply additional structural or foundational roles rather than the driving force aspect.