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An Analysis of Urban Travel Demands
By Walter Y. Oi, Paul W. Shuldiner
The increasing importance of urban transportation stems basically from the rapid growth of urban populations. Over the past century, urban populations have grown from 6.2 million to 110.4 million; an 18-fold increase as compared to an increase in non-urban populations of only 2.7-fold. This growth combines both the normal population growth and a migration of persons from rural to urban areas. Whatever the reasons for it, it has led to substantially larger demands for urban transportation.
The urgency of the urban transportation problem apparently arose from a recognition of the economic nature of travel, especially as it is manifested in the time costs of travel. Increased travel demands, coupled with inadequate adjustments in supplies, have often led to congestion in a city's transportation system. The longer travel times, associated with congestion, seem to have prompted the research in urban travel patterns.
The spending habits of urban families also show the importance of travel and transportation. These expenditure patterns can be found in the periodic surveys conducted by the Bureau of Labor Statistics, the latest of which was conducted in 1950. Except for New York City, more money is spent on travel and transportation by the average urban family than on either housing or clothing. Indeed, only food consumes a larger share of the budget than travel and transportation. In addition, the bulk of transportation expenditures is ascribable to auto transportation.
Reference to earlier budget surveys reveals that travel is consuming an increasingly larger share of the urban family's budget. In 1917-19, the typical urban family--defined as a median income family covered by the survey--spent 2.4 per cent of all consumption expenditures on travel. However, in 1935-36 and 1950, the respective percentages were 11.2 and 13.3 per cent. This trend is largely attributable to the rapid growth of automobile expenditures; if anything, the relative importance of other transportation expenditures has declined over time. Thus, from a viewpoint of dollar outlays, travel is a significant economic good, consuming over one-eighth of the typical urban family's income.
Urban travel is, therefore, simply one aspect of the economic and social behavior of a city's residents. Families do differ widely in the volume and kinds of travel they consume. Yet, the social sciences accept these differences and attempt to explain them. Human behavior can be subjected to analytic techniques similar to those applied in non-human behavior. The inability to conduct controlled experiments and the presence of random elements often confound the social scientist's task. The differences from the physical sciences are, however, those of degree rather than kind.
The historical development of scientific research in the area of urban transportation may be divided into three periods: (1) the period prior to 1944; (2) the period from 1944 to 1951 or 1952; and (3) the period since 1952. This separation into three periods is somewhat arbitrary, based as it is on the authors' judgments. Although the lines of demarcation are vague--particularly between the second and third periods-they do, none the less, mark identifiable shifts in thought and approach. Prior to 1944, traffic engineers dominated the field and primarily devoted their attention to such technological design problems as street capacity, traffic signals, etc. The first origin-and-destination study was conducted by the Bureau of Public Roads in 1944. This step represented a significant shift in emphasis. The O-D ("origin-and-destination") study reflected the need for more information concerning travel activity within urban areas. By this time, it had become evident that such information constituted the major gap, whereas the technological knowledge was substantially superior. Consequently, it is not surprising to find that the O-D studies conducted in the ensuing six or seven years were largely devoted to data collections in the form of various travel inventories. The last and current period coincides with an invasion of the field by sociologists, economists, city planners, and others. This period is characterized by analytic examination of the data collected in the various O-D studies. Around this time, traffic engineers also turned their energies toward analyses of travel demands and behavioral patterns. The application of more powerful and sophisticated mathematical and statistical tools of analysis has also led to significant advances in our understanding of urban travel behavior.
The ultimate purpose of all this research is to provide a more rational and scientific basis for the planning of an urban transportation system. Although private individuals spend considerable sums on transportation, the investments in roads are ordinarily left to the public sector of the economy. Despite some imaginative plans for private financing--schemes for toll collections, differential tolls depending on congestion, etc.--it seems highly unlikely that reliance will be placed on private enterprise to provide a city's street system. The social capital invested in roads can, however, be appraised by various economic criteria of efficiency. Indeed, social investments in public power, highways, irrigation projects, etc., have been evaluated by criteria such as benefit-cost ratios. However, the application of benefit-cost ratios to urban freeway systems has certain troublesome shortcomings. In general, the benefits far exceed the costs due to the high implicit valuation of average time savings. The inattention accorded such benefit-cost ratios may indicate an uneasiness with this particular criterion of economic efficiency. In many cases, efficiency is judged by a comparison of street capacities to traffic volumes. Where heavy congestion or bottlenecks are observed, the system is deemed inadequate. The question of how one measures the economic efficiency of an entire urban transportation system is an important one, but well beyond the scope of this study. It should be noted that unless some cost can be attached to an inefficient system (taking account of both inadequate and excessive capacities), it is virtually impossible to assess the economic efficiency of alternative urban transportation systems.
From our perusal of the literature, it appears to be a maxim that road construction in anticipation of future traffic demands is somehow superior to corrective street construction that remedies obvious inadequacies and congestion points. Indeed, the appropriation of funds to aid city planners in designing future transportation systems is itself evidence of at least partial acceptance of this maxim. If this proposition is granted, then, as a first step, forecasts of future traffic volumes must be developed.
Based on research reported in the literature, it seems safe to draw two broad conclusions. First, the vast majority of research is concerned with forecasts. Furthermore, these forecasts are usually given in terms of point estimates which rarely report the confidence intervals associated with each estimate. In other words, the forecasts presume the existence of precise relationships which are not subject to random errors. This is not surprising, since modern statistical analysis has had its greatest impact on the academic sciences and not on such applied sciences as medicine and engineering. The second conclusion is that the analysis accorded the spatial distribution of trips is both more abundant and considerably more sophisticated than that devoted to the volume of trip generation. Although the two types of analyses are interrelated, they are, in general, analyzed separately. The spatial distribution models--the traffic or so-called gravity models as well as the growth factor models--are primarily concerned with the allocation of trips over the parts of the system or urban area. Even though the final results are usually expressed in terms of interchange volumes, they basically refer to ratios or proportions; that is, if all trip generation rates were uniformly doubled, all interchange volumes would also double. Furthermore, virtually all of the spatial distribution models either explicitly or implicitly presume prior knowledge of the volume of trip generation rates.
In recent O-D studies, notably those conducted in Detroit and Chicago, substantially greater attention has been given to analyses of trip generation rates. In addition, other trip generation studies were reported at around the same time ( 1952 to date) in various publications of the Bureau of Public Roads and of the Highway Research Board. These earlier studies fall into two major categories: (1) those which refer to land use; and (2) those which apply to home-based travel. Both types of studies exhibit an overriding concern with forecasts. As a result, some of the trip generation relationships seem highly implausible, even though they reveal close fits to the data. Furthermore, the statistical techniques employed in some studies were of questionable value. This is not to say that these earlier studies lack merit; on the contrary, they provide an excellent point of departure.
Source: An Analysis of Urban Travel Demands
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