The expansion of cities over the past decades has engendered considerable traffic flows. In light of the twin constraints of limited urban space and energy savings, only mass-transit systems of the railway type running on their own rights-of-way are able to cope reasonably with the rising traffic demand. Moreover, problems relating to incorporating the transit systems in urban surrounding and their environmental impact are factors acting in favor of underground installation. However, the promotion of rights-of-way below ground level necessarily calls for specific measures to be taken during the various phases of design, construction and operation of the various projects parallel with a striving to cut costs.

TRENDS OBSERVED IN URBAN TRANSPORTATION

Rising Demand for Transport in Urban Environments Transportation in cities is faced with an ever increasing demand, generated by the inter-dependence between transportation and use of the ground. Access to jobs depends on the available transport, but the nature and distribution of the jobs condition the transit systems. In the past decades there has been a veritable explosion of the suburbs of the big cities due to the three-fold impact of population growth, the widespread diffusion of the automobile, and the lessening, cost of homes. The growth in peripheral urbanization has brought about an increasing, demand for transport and analyses conducted in a number of large cities have shown that the growth is steeper than the population growth alone. This indicates that mobility has increased. This phenomenon, observed in most of the large urbanized areas of the world, is mainly ascribable to the imbalance between home and jobs, between city centers and their peripheral zones. The main transportation modes in use fall into two categories _ mass-transit means, such as typically trains, metros and buses -individual modes typified by private cars The studies conducted in the Paris region have concluded that the 31 % of the displacements are guarantied by the public transit system which plays the major role for links in Paris and for those between Paris and the suburbs. Nor do all these displacements follow the same pattern in the course of the day. There are very distinct peak periods at the start and end of the day's work. And it is precisely to handle the peak traffic loads that the mass-transit modes have to be dimensioned. This trend and the major features of such displacements are reflected in most of the world's great urban centers, and it is highly probable that the increased populations in cities between now and the year 2000 will contribute still further to the growth in the peripheral urbanization of large urban centers. This will entail increased travel, mainly inwards towards the city centers, which in turn will call for providing an hourly transport capacity of several tens of thousands of persons per direction of displacement. Advantages of Public Transportation over Private Cars Given the current restrictions in the matter of protection of the environment and energy saving, the "consumption" of space and of energy by the various conceivable transit modes becomes a paramount criterion of choice.

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