Travel Demand

Travel demand is generated by activity—people going places for word, shopping, recreation, and other purposes. Thus, much attention in transportation planning is focused on:

• Spatial distribution of residences and activity centers
• Temporal distribution of trips
• Mode selection for the trip

The process of estimating the “true” demand is complicated by the fact that the demand is (1) frequently distorted or liited by the available capacity, and (2) subject to variations due to the profile of the population, including such factors as gender, age, and personal income. The same factors influence mode selection.

The process of estimating and assigning demand is the subject of transportation. planning and demand estimation. However, because the distribution and redistribution of trips becomes important in many traffic studies, some observations are in order here.

Proxies and Disaggregation

Much of the historic information on travel demand and mode split has been expressed in terms of such “proxies” as (1) auto ownership, (2) household income, (3) household size, (4) trip purpose, and, more recently, (5) gender, because the data was available in these forms or could be surveyed and related to such parameters, (The word “proxy” is used intentionally, because the individual traveler’s decision making process reflect these parameters but is not explained by them).

In recent years there has been a greater emphasis on a disaggregated approach to demand estimation, so that it can be built up from a fundamental understanding of the transportation needs of the constituent groups. For instance, the transportation needs might logically differ based upon (1) age, (2) gender, and (3) employment category. These characteristies might influence both the lwvel of travel demand and the mode selection for all trip purposes.

Selection of Origins or Dstinations

Most models of demand are based on a known residential pattern and a known distribution or jobs and other opportunities, with trips form the residences being “distributed” (matched) destinations, and then “assigned” to transportation network in some repeated fashion. The match and the assignment (including mode split) are frequently checked against such sources as the Census journey-work data, implicitly, the residences are fixed, and people reach out and find jobs and ways to get to the jobs according to the rules built into the model.