Modern city life is a complex jumble of transportation, utility and environmental problems. City planners must address a wide range of issues, including traffic congestion and safety, air pollution and health problems, lack of water and overwhelmed waste management, and the ever-increasing cost of energy.
The advent of motorized private cars radically altered urban life. Automobiles occupy far more street space per person than common carriers such as buses, bicycles, or even pedestrians. As a result, traffic snarls and accidents became commonplace in many cities. Automobiles also tend to concentrate on limited stretches of roads and streets in the center of cities, and traffic volumes increase disproportionately in central city areas.
Attempts to alleviate the traffic problem began in the early 1900s with a series of innovations, such as specialized signals and signs. In the United States, these efforts were augmented by centralized traffic control systems, modeled on railroad practices, that relied on midintersection towers manned by police.
By the 1910s, the traditional city street system was gradually replaced by a network of collectors, arterials, bypasses, ring roads and highways, all designed specifically for automobile use. These roads radically increased urban mobility, but they also contributed to sprawl, and traffic problems remained a major concern. The development of new methods of rail transportation, and the adoption of a focus on green commuting, have helped alleviate some of these problems. The widespread introduction of new, energy-efficient, hybrid, and electric vehicles can further reduce these problems.
