inability to forecast the precise demand for program services. Many programs, like hospital care, for example, respond to varying volumes of often uncontrolled demand. This has been a problem with Britain's National Health Service, which provides most medical care at little or no cost. Such a program differs from those with controlled demand, where generate largely unpredictable demand for transit in some cities. Municipal bus and rail authorities must try to deliver .fixed levels of service, at high fixed costs, of response is based on programmed resources, such as miles of roads to be maintained (though a harsh winter that destroys roads can change the demand from controlled and forecast to uncontrolled).
Similarly, programs like mass tranSit face both types of demand. The presence of the sub-problem of excess traffic congestion (measured by average road density and speed of traffic), together with widely varying fuel prices, can ten with variable fare structures, to passengers whose demand schedules also respond to price changes in comparison to the apparently cheap option of driving (because the price elasticity of demand is high for transit in contrast to other services such as education).