A number of factor determine the operational effectiveness of urban bus fleets in the Third World Labour costs are low, but fuel costs usually represent a high proportion of total costs, with oil often having to be imported. The intensity of operation high loadings, congested road space and environmental factor such as temperatures, rainfall, dust and road damage, all increase operational and maintenance costs in situation where foregin exchange may be in short supply and lead times for obtaining essential spares and equipment are often measured in months or years rather than days or weeks. Many vehicles in the fleet may be out of operation for long periods of time.