by Marius Thériault & François Des Rosiers
Accessibility to goods and services is a complex notion which completely pervades territorial issues (Hansen, 1959). It relates to the ability of individuals to travel and to participate in activities at different locations in an environment. Accessibility can be considered as one of the main determinants of property values, although its influence will differ depending on the type of trip as well as on the configuration of the urban fabric. Previous research show that Euclidean distances to the CBD and various urban amenities, as well as travel times to schools and shopping centres, are appropriate measures of the phenomenon. Such indicators are proxies for accessibility focusing on distances from specific physical features which could be easily assessed using physical network measurements. However, other perceptual dimensions are at stake, which are driven by preferences and actual behaviour of people; these, in turn, depend on self valuation of their time and easy access to various types of services. They define specific utility functions which are somewhat different for each household, but could be differentiated among social groups. These values and willingness are related to specific constraints of individuals and households, including economic status (employment and income), motorization, family structure and life cycle. For instance, a lone person has more freedom for choosing his home than a dual-earner couple with children since the latter must consider the overall needs of all family members while trying to minimise housing and travel costs simultaneously.
Due to the complexity of the urban fabric though, and considering the polycentric pattern of most modern cities, using mere Euclidean distances to the CBD falls short of integrating all relevant aspects of accessibility (Jackson 1979, Dubin and Sung 1987, Niedercorn and Ammari 1987, Hoch and Waddell 1993). Although it could be argued that in modern cities, social gradients are progressively prevailing over access factors as major house price determinants, the faulty specification of accessibility descriptors may well explain their poor performances. Indeed, most of these studies actually resort to Euclidean distances (or their numerical transforms) to measure accessibility-related externalities although minimum travel time and minimum walking distance are also used (Bateman et al. 2001).
Some authors resort to O-D surveys to look at accessibility to jobs and houses. In his study on metropolitan Washington, DC., Levinson (1996) analyzes commuter trips patterns and finds that, by balancing accessibility, the suburbanization of jobs maintains stability in commuting durations despite ising congestion, increasing trip lengths, and increased work and non-work trip making. Helling (1996) explores the effect of residential gravity accessibility by automobile to all employment in the Atlanta metropolitan region on the quantity and nature of travel by men and women, using the Atlanta Regional Commission’s 1990 Household Travel Survey. The author finds that while individuals and households who live in more accessible portions of the metropolitan area spend significantly fewer minutes in travel, this effect is seen primarily for employed men. The research also demonstrates that residential accessibility’s effects on travel, though significant, do not affect everyone and that, while gravity measures of accessibility may help transportation planners predict the results of planned and unplanned changes when used with other information, they do not predict travel behaviour well when used alone.
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