Boundary Discontinuities in Journey-To-Work Data: a Transportation-Based Explanation (Commuting, Transit, Walking Distances).

Abstract

The impetus for this study was the description, appearing in a 1969 Census Bureau working paper on metropolitan area definition, of boundary discontinuities contained in the 1960 census commuting data. These discontinuities, occurring along certain political boundary lines, indicated that residents on either side of the boundary tended to work in the same area in which they resided. There appeared to be a strong bias against crossing the political boundary line. The working paper's authors contended that these discontinuities were fictitious effects, reflecting data deficiencies. The present author proposed an alternate hypothesis: that boundary discontinuities were actual phenomena which are observable in non-census commuting data of comparable location and date. That hypothesis is confirmed: boundary discontinuities reflected actual trip-making by commuters. To explain the phenomenon, four groups of elements are identified, (setting, locations of employed persons, person movement, areal units), which combine to produce four transportation factors: very short trip lengths, political boundary lines affecting transit systems, physical barriers impeding movement across the boundary line, and mode shift time lag effects on trips by auto. Boundary discontinuities result from the interaction of the four factors and five basic transportation modes (at home, walk, bicycle, transit, automobile). The factor/mode interrelations are expressed as four hypotheses. Evidence is presented substantiating the four hypotheses: (I) Most walk-only trips to work, as well as most walk trips to access transit modes, terminate very close to the residence; (II) Home-to-work trip-makers using transit modes tend to avoid crossing political boundary lines where such lines restrict the spatial continuity or availability of those modes; (III) Physical barriers along or near political boundary lines accentuate the lines' barrier effects by increasing minimum walking distances and by blocking or otherwise distorting transit lines; (IV) A considerable portion of commuting by automobile appears to reflect a barrier effect resulting from residence-workplace pairings originally chosen by commuters who initially made the trip by transit. Each mode's role in discontinuities, and degree of boundary effect exhibited, is quantified. The use of 1970 census place-of-work data by mode independently supports the 1960 census place-of-work data, and the boundary discontinuities they reveal.