Across the entire globe, motor vehicle traffic has increased tremendously. In 1950, there were about 53 million cars on the worldÃ¢â‚¬â„¢s roads; 44 years later, the global automotive fleet has grown to 460 million, a nine fold increase. On average, the fleet has grown by 9.5 million units per year over this period. Simultaneously, as illustrated in Figure 1, the truck and bus fleet has been growing by about 3.6 million vehicles per year (Motor Vehicle Manufacturers' Association, 1991). While the growth rate has slowed considerably in the industrialized countries, population growth and increased urbanization and industrialization are accelerating the use of motor vehicles elsewhere.
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