Nature and Analysis of Transportation Demand

Transportation demand, unlike demand for other commodities, like wheat, coffee, housing, clothing, etc., is a derived demand. That is, one demands to be transported not because one just wants to move (except for those rare cases when one goes out for a joy ride!) but because one wants to achieve some other purpose like reaching school, or office or a movie theater. In other words, the need for achieving some goal (like reaching office or a shop) creates the need to travel. Hence, travel demand is primarily affected by the population's need to work, entertain (themselves), socialize, study, etc. Hence, it is not surprising that two of the major aspects in travel demand analysis are land use and trip purpose.

Land use refers to the pattern of land usage in an area. Land use affects transportation demand through generation and distribution of trips. The effect of land use on transportation demand is not necessarily a one-way effect but is rather a part of cycle in which land use changes transportation needs which in turn change land use. Figure  shows a simple schematic of how land use and transportation demand are related.

図: Relationship between land use and transportation demand.

Trip purpose refers to the purpose for which the trip is being undertaken. Travel demand behaviour changes with trip purpose. For example, one hardly exercises any choice for work trips; i.e., one generally does not decide every time whether to go to work or not, one obviously does not decide where to go to work (generally it is fixed over a period of time for a large section of the population), even the choice of route and mode are not daily decisions. On the other hand, for recreational trips, an individual makes a large number of decisions, like whether to go, where to go, how to go. Consequently, the travel demand behaviour for work trips vary considerably from recreational trips. This example, can obviously be extended to other types of trips like shopping trips, etc. Given the effect of trip purpose on travel demand behaviour, the analysis of travel demand is done separately for different trip purposes.

Although, the above discussion throws light on some of the factors which affect travel demand some more understanding of travel demand is necessary before one can analyze the demand and can, with some degree of confidence, predict the volume on various links of a network. Generally a trip (which is the basic quantity in travel demand) materializes after the trip maker makes certain decisions. These decisions can be broadly classified as follows:

• The decision to travel: The trip maker, given his/her requirements, makes a conscious decision to travel so that the requirements can be met.
• The decision on the choice of destination: The trip maker also makes a decision as to where he/she wants to go; for certain kinds of trip purpose, like work trips this decision may not exist; yet for other kinds of trips like shopping trips one may have certain alternative locations to choose from.
• The decision on the choice of mode: The trip maker also takes a decision as to what mode of transport he/she will use for a given trip. This decision, however, is only available to those who have access to different modes and are not captive users of any particular mode.
• The decision on the choice of route: The trip maker on any given trip takes a definite decision on which route to take so as to reach the destination. Again this decision is available to trip makers who have access to modes which can use different routes as per the wishes of the trip maker. Such modes would generally include personal automobiles or two wheelers.

Although, there is unanimity on the fact that the above decisions can aptly capture the entire trip making behaviour of an individual and hence can be used to analyze travel demand pattern of an area, it is difficult to ascertain whether there exists any definite sequence in which these decisions are made. Generally it is assumed, primarily for the ease of analysis rather than anything else, that the decisions are made in a strict sequence as shown in Figure . Analysis techniques which assume that such a sequence exists are referred to as sequential demand analysis techniques.

図: Schematic representation of the assumption of sequential decision making.

Although, even today transportation demand is analyzed sequentially, the assumption that the four major decisions of a trip maker follow a strict sequence (i.e., are in a series) is possibly not the most appealing. Quite often the decision to travel is changed because an appropriate destination does not exist; or an initial choice of destination is changed because one cannot reach the destination in ones desirable mode of transport. It is possibly a truer picture of reality if the decision making framework is assumed to have feedback loops. One such possible structure is shown in Figure . In this structure, unlike in Figure , there are feedback loops indicating that decisions taken earlier can be changed based on a latter decision. For example, the decision to travel may be aborted because at the mode choice stage one realizes that none of the available modes suits ones requirements.

図: An example of the assumption of non-sequential decision making.