This thesis represents the development and pilot application of a novel methodology for the speculative qualitative assessment (or "prototyping") of new infrastructural systems. Its core aim and guiding principle is to make infrastructure legible: to reveal and narrate its role in everyday life from a more human perspective than that of the paradigmatic technology-focussed approach. Or, more simply, the project aims to understand how infrastructures develop, how they evolve and entangle over time. The methodology is centred on a novel model of sociotechnical change, known as the infrastructural trialectic. The trialectic makes a unique relational distinction between infrastructural systems and the technologies through which infrastructural functions are accessed, traces vectors of influence between focal actors in the model, and provides a framework for mapping the articulatory institutions which are enrolled in the formation and mutation of infrastructural assemblages. The methodology has two modes of application: the historical mode, and the speculative. In the historical mode, the trialectic model becomes the lens of a situated longue duree analysis which explores the historical dynamics of sociotechnical change in the assemblages underpinning a particular everyday practice. In the speculative mode, the findings from the historical mode are used as the basis for an extrapolative and speculative analysis of a novel technological intervention into the practice previously analysed. Drawing on techniques from strategic foresight and critical design, the prospective technology is "prototyped" against the context of a suite of four divergent near-future scenarios, so as to "stress test" the plausibility of its deployment under difficult circumstances. This thesis presents and applies a novel model of sociotechnical change, and in doing so demonstrates that the shortcomings of paradigmatic models of change might be addressed through such an approach. It further demonstrates a unique hybrid method for the assessment and critique of new technologies and practices alike, which provides a more human perspective upon infrastructure (and indeed upon change itself) than prevailing approaches to assessment.