This chapter discusses the major themes in the evolution of accessibility over the twentieth century. The idea of accessibility is not new, either to urban and regional planning or to the social sciences in general. The concept dates back at least to Richard Hurd's 1903 analysis of urban growth, Robert Haig's 1926 “ease of contact,” and John Stewart's 1948 “demographic energy.” In Stewart's analysis, the first of the three to quantify the accessibility concept, it was a good predictor of outcomes, including observed income at the state level. Stewart also recognized the potential of accessibility as a normative goal early on: if energy or accessibility can predict important outcomes such as income, then surely it could also be seen as a policy variable to be directly manipulated by central planners. Echoing earlier authors, Walter Hansen applied the term “accessibility” to Stewart's “demographic energy” and broadly introduced the concept into the urban and regional realm with three ideas central to the planning use of the tool. First, like Hurd's and Haig's analyses early in the century—and unlike Stewart's nationally scaled research—Hansen's analysis was metropolitan, not the continental. Second, the outcome variable for Hansen was residential development, a central concern of the urban-planning profession. And finally, where Stewart had implicitly treated peoples' inclination to travel as a constant value, Hansen showed that it was a variable subject to empirical investigation.
