Abstract:It is a truism repeated countless times in the course of a transportation professional's career: "Travel is a derived demand"–that is, derived from the demand for spatially separated activities. Belief in this truism underlies a number of transportation policies designed to reduce motorized travel (whether to reduce congestion, improve air quality, or reduce the consumption of non-renewable energy). For example, much attention has been given to land use policies designed to bring origins (residences) closer to destinations (work, shopping, entertainment). "Neo-traditional" developments, which mix diverse land uses and maintain higher densities than the typical suburban sprawl, are often suggested as a potential scheme to reduce motorized travel.

But what if a significant segment of the population enjoys traveling and would therefore be inclined to evade policies designed to facilitate less motorized travel? In fact, there are a number of indications to support the hypothesis that some people assign positive utilities to travel, independently of the utility of performing the activity at the trip destination.

There are two forms of travel that raise some doubts about the validity and utility of the derived demand assumption. The first is the phenomenon of joyriding, in which the activity itself is the travel, and consequently, it could in principle be analyzed under the derived demand assumption (where the activity is not confined to a specific location as it is in other cases). This type of travel has received little if any attention in trip generation models, implying that its magnitude is too small to be of importance, or that we lack the ability to model it because of its complexity and variation. The second type of travel that poses a problem vis-à-vis the derived demand assumption is the excess travel that is embedded within routine trips to work, shopping, or leisure activities. Research suggests that some excess travel can be attributed to the desire to travel and the benefits of travel aside from getting to the destination.

In recent years there has been a growing quest among transportation planners and environmentalists to address transportation problems through improvements in accessibility rather than mobility. Presumably (given travel as a derived demand), if changes in the spatial distribution could significantly enhance access to activities, the amount of travel could be reduced. This quest is part of a broader debate about transportation/land use interactions in which a central theme is whether increased density should be a policy objective for achieving transportation goals.

Improvements in accessibility can be accomplished through many different policy instruments. In addition to land use policies, which take a long time to implement and may involve high capital costs, there are some other, less costly options. For example, telecommunication-based versions of various activities (telecommuting, teleconferencing, teleshopping) are promoted in the hope that they will substitute for a trip to engage in the "equivalent" activity–that is, that they will increase accessibility by offering "virtual mobility". The implicit assumption that travelers are cost-minimizers also underlies. various pricing strategies (congestion pricing, higher fuel taxes, higher parking fees) designed to reduce the net attractiveness of more distant destinations by increasing the cost to get there. While pricing policies are generally geared toward reducing mobility, they may also affect accessibility. In fact, from a political perspective, pricing policies may be more attractive if they are supplemented by changes in accessibility. Pricing policies differ in their spatial effects: congestion pricing and parking fees are usually applied to a specific area, whereas increased fuel taxes do not affect a specific location. Consequently, the latter type of pricing policy only reduces mobility, whereas the former alters the relative accessibilities of affected and unaffected locations.

The context of transatlantic comparative studies is uniquely relevant for researching the relationship between mobility and accessibility. The difference in urban structure, travel patterns, culture, and policy processes offers an opportunity to view the role of some of these factors and, through the understanding of the differences, provide important input to policy-making in both the North American and European contexts.

Note:Published in Social Change and Sustainable Transport, Indiana University Press, chapter 22.