TABLE OF CONTENTS - The Professional Green Building Council

Theme B: Creating a livable, healthy and environmentally viable citiesmobility and density. Using Zahavi’s temporal conjecture, the reduction inpopulation density is due to increased travel speed.He hypothesised that the daily time-budget average is constant and equal toapproximately one hour in all cities and that this time factor will remain unchangedover time. The impact of this time-budget constant on the relationship betweeneconomic growth and mobility is particularly important as it indicates that, shouldit continue, the more economic development provides rapid modes of transport, themore these will be used within the limit of just over one hour a day. According tothis logic, it is the access to higher speeds that has permitted the expansion of theurban sprawl. The time-budget constant means that the more we can travel at highspeed, the more distance we consume. Speed is therefore a “pivotal variable”linking the number of kilometres covered and the transport time-budget anddefines the population density of agglomerations.The other constant proposed by Zahavi is that of monetary budgets. The monetarybudget of households also represents a limit to the city’s expansion. The temporaland economic limits of the city make it necessary to consider the cost of speed todefine the mobility behaviour patterns of city inhabitants. The movement distancesaccessible within a one hour per day time limit and within a budget proportional toincome define the urban land take. However, they provide little informationconcerning the structure of the city. Inherited from the past, the various fabricsforming the city structure result from successive modes of mobility 1 and theirspecific speeds, as well as the density of interactions permitted by the urban fabric.Three types of city successively developed to match the dominant mode oftransport, which changed from walking to public transport and then to theautomobile. Cities with a long history present hybrid forms. The history ofEuropean cities implies that they are not totally dependent on the automobile.Other modes of transport continue more or less to operate. In the 1970s, Schaeffer& Sclar [6] established an analysis framework to understand changes in urban416