The ethnographic research reported here reveals patterns of authority and learning on an experimental construction site that are significant for the promotion of a safety culture. It seeks to display the methods of understanding used by site personnel to constitute the construction site as a local work site. In making these explicit, an alternative is offered to recent suggestions that critical studies of situated learning demand recourse to historical or macro resources. Findings confirm insights from previous studies, detailing in addition: the role of trial and error; alternative bodies of knowledge underpinning competing authority structures; and complex and subtle patterns of the informal authority of elite manual workers, its ambiguity and its limitations. The experiential knowledge valued by site personnel forms a basis for the recognition of authority on site that can conflict with that of construction professionals. The conflict between forms of authority and knowledge can inhibit the dissemination of good safety practice: initiatives will meet significant resistance if they contradict the experiential knowledge of site operatives; if they do not make use of this experiential knowledge, they may fail to address hazards fully; methods of site learning, particularly in the development of innovative practice, are inherently hazardous.