Although the NLRA limits a union’s picketing locations, the NLRB and the courts have developed rules to accommodate union activity directed at a primary employer, also known as primary activity, in several specific settings.

I. The Ambulatory Situs 

An ambulatory situs is a mobile worksite that moves from place to place. One example would be a delivery company whose trucks make stops at neutral third-party locations. In the Moore Dry Dock case, the NLRB held that a union may lawfully...