Transport systems consume scarce natural resources of land and energy, kill many people in accidents, and cause severe local pollution of air and water. New advances in the field of transport have exposed users to a whole series of hazards that, although not new, are becoming more and more striking because of the constantly increasing number of persons affected by them.
Transport is a great consumer of energy, accounting in the USA for about 55% of liquid fuels (petroleum products); in Europe, about 31%; and in Kenya, more than 65% (reflecting the fact that in the developing countries the bulk of liquid fuels is used in the transport sector). The efficiency of energy utilization is variable. Railroads and waterways, for example, are more efficient than aircraft or automobiles. The latter are the least efficient, and they account for the bulk of energy consumption in the transport sector.
The most familiar environmental impacts of road transport are those from air pollution. Petrol-burning vehicles emit carbon monoxide, hydrocarbons, and oxides of nitrogen. Diesel engines emit relatively little of these, but produce more particulates and smoke. Alkyl lead is still added to vehicle fuel in many countries, and is emitted in the exhaust as very small particles. In confined spaces (like tunnels or very narrow streets) carbon monoxide concentrations can rise to levels hazardous to health, especially to people with heart or lung weakness. Nitrogen oxides and hydrocarbons, on the other hand, are not directly toxic, but interact in the presence of sunlight to produce an oxidant smog which irritates the eyes and lungs and damages sensitive plants. This kind of smog made Los Angeles notorious before emissions were controlled, and remains a problem in many large cities such as Tokyo.
Aircraft and railway locomotives together emit a far smaller volume of air pollutants than road vehicles do. However, high-flying aircraft release oxides of nitrogen directly into the lower stratosphere where they may become involved in chemical reactions which could reduce the concentration of ozone, important as a screen against ultraviolet radiation from the sun. Aircraft also contribute carbon monoxide, hydrocarbons, particulates, and nitrogen oxides at lower levels, locally, around airports. The best available evidence suggests, however, that none of these impacts is significant.
Noise is one of the most widely recognized and resented environmental consequences of the increase in road and air transport. It interferes with work, prevents sleep, has psychological effects, and can even damage hearing.
The expansion of rail and marine transport in the past, and more recently of road and air transport, has obviously consumed much land, both for new tracks and highways and around ports and airports. New transport corridors also have a secondary effect on land use because they attract industrial development. New highways can sever wildlife habitats and stimulate uncontrolled and inappropriate rural development, while new seaports are often placed in coastal areas also valued for wildlife and recreation. Because the density of development is lower in cities built to accommodate road traffic, public services may be more costly to operate there. But there are also problems when poorly designed roads are overburdened with vehicles. Drainage systems can be interfered with, and costs of maintenance of both roads and vehicles can become unacceptably high.