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The transport sector consumed slightly over 63 EJ, or about 20% of global primary energy, in 1990. Transport sector primary energy use grew at a relatively rapid average annual rate of 2.8% between 1971 and 1990, slowing to 1.7% per year between 1990 and 1995. Industrialized countries clearly dominate energy consumption in this sector, using 62% of the world's transport energy in 1990, followed by REF (16%), ALM (12%), and ASIA (10%) regions. The most rapid growth was seen in the ASIA countries (5.9% per year) and the ALM region (4.6% per year). Transport energy use dropped dramatically in the REF region after 1990; by 1995 this region only consumed 11% of global transport energy use. Growth in transport primary energy use also declined slightly in the IND region, dropping from an average of 2.2% per year between 1971 and 1990 to 1.9% per year between 1990 and 1995. High growth continued in the ASIA and ALM regions, with the ASIA countries increasing to an average of 7.6% per year between 1990 and 1995 (BP, 1997; IEA, 1997a; IEA, 1997b).
Influences on GHG emissions from the transport sector are often divided into those that affect activity levels (travel and freight movements) and those that affect technology (energy efficiency, carbon intensity of fuel, emission factors for nitrous oxide (N2O), etc.). The various driving forces and their effects are reviewed in detail in the IPCC Working Group II (WGII) Second Assessment Report (SAR) (Michaelis et al., 1996).
In aggregate, transport patterns are closely related to economic activity, infrastructure, settlement patterns, and prices of fuels and vehicles. They are also related to communication links. At the household level, travel is affected by transport costs, income, household size, local settlement patterns, the occupation of the head of the household, household make-up, and location (Jansson, 1989; Hensher et al., 1990; Walls et al., 1993). People in higher-skilled occupations that require higher levels of education are more price- and income-responsive in their transport energy demand than people in lower-skilled occupations (Greening and Jeng, 1994; Greening et al., 1994).
Urban layout both affects and is affected by the predominant transport systems. It is also strongly influenced by other factors such as people's preference for living in low-density areas, close to parks or other green spaces, away from industry, and close to schools and other services. Travel patterns may be influenced by many factors, including the size of the settlements, proximity to other settlements, location of workplaces, provision of local facilities, and car ownership. A survey of cities around the world (Newman and Kenworthy, 1990) found that population density strongly and inversely correlates with transport energy use.
Many studies have examined the response of car travel and gasoline demand to gasoline price, and are reviewed, for example, in Michaelis (1996) and Michaelis et al. (1996). Such studies typically find a measurable reduction in fuel demand, distance traveled, car sales, and energy intensity in response to fuel price increases. Studies of freight transport found relatively small short-term impacts of diesel price increases, and often produced results that were inconclusive or statistically insignificant. Over the longer term, price responsiveness is generally assumed to be larger because of possible technology responses.
An important influence on future travel may be the development of telecommunication technologies. In some instances, improved communication can substitute for travel as people can work at home or shop via the internet. In others, communication can help to increase travel by enabling friendships and working relationships to develop over long distances, and by permitting people to stay in touch with their homes and offices while traveling. To the extent that improvements in telecommunication technology stimulate the economy, they are likely to result in increased freight transport.
Energy intensity in the transport sector is measured as energy used per passenger-km for passenger transport and per ton-km for freight transport. Transport energy projections typically incorporate a reduction in fleet energy intensity in the range 0.5 to 2% per year (Gr�bler et al., 1993b; IEA, 1993; Walsh, 1993). On-road energy intensity (fuel consumption per kilometer driven) of light-duty passenger vehicles in North America fell by nearly 2% per year between 1970 and 1990, to about 13 to 14 liters per 100 kilometers, but it is now stationary or rising. In other industrialized countries, changes in on-road fuel consumption from 1970 to the present were quite small. The average on-road energy intensity in North America was 85% higher than that in Europe in 1970, but only 25 to 30% higher by the mid-1990s (Schipper, 1996).
In some countries, such as Italy and France, where fleet average energy intensity has fallen during the past 20 years, the energy intensity of car travel (MJ/passenger-km) has increased as a result of declining car occupancy and the increasing use of more efficient diesel vehicles (Schipper et al., 1993). However, conversion to diesel has been encouraged by low duties on diesel fuel relative to those on gasoline. The lower costs of driving diesel vehicles may have acted as a significant stimulus to travel by diesel car owners, and so offset much of the energy saved from their high-energy efficiency. A more recent trend, though, is toward higher energy intensity in new cars in countries such as the US, Germany, and Japan (IEA, 1993). Factors in the recent increases in energy intensity include the trend toward larger cars, increasing engine size, and the use of increasingly power-hungry accessories (Martin and Shock, 1989; Difiglio et al., 1990; Greene and Duleep, 1993; IEA, 1993).
Average truck energy use per ton-km of freight moved has shown little sign of reduction during the past 20 years in countries for which data are available (Schipper et al., 1993). Energy use is typically in the region 0.7 to 1.4 MJ/ton-km for the heaviest trucks but can be in excess of 5 MJ/ton-km for smaller trucks. In countries where services and light industry are growing faster than heavy industry, the share of small trucks or vans in road freight is increasing. Along with the increasing power-to-weight ratios of goods vehicles, these trends offset, and in some cases outweigh, the benefits of improved engine and vehicle technology (Delsey, 1991). Energy intensity tends to be lower in countries with large heavy-industry sectors, because a high proportion of goods traffic is made up of bulk materials or primary commodities.
Air traffic grew about three times as fast as GDP in the early 1970s, but only about twice as fast since the early 1980s. After allowing for the effects of continually falling prices, the elasticity of the 10-year average growth rate with respect to the 10-year average GDP growth is not much more than 1.0 (Michaelis, 1997a). Over the 30 years to 1990, the average energy intensity of the civil aircraft fleet fell by about 2.7% per year. The fastest reduction, of about 4% per year, was in the period 1974 to 1988. The large reductions in energy intensity during the 1970s and 1980s resulted partly from developments in the technology used for new aircraft in the rapidly expanding civil aircraft fleet and partly from increases in aircraft load factor (passengers per seat or percentage of cargo capacity filled). The aircraft weight load factor increased from 49% in 1972 to 59% in 1990, but nearly all of this rise occurred during the 1970s (ICAO, 1995a, 1995b).
Transport sector carbon intensities for personal travel, measured as the ratio of emissions to passenger-km traveled, increased in most European countries and Japan between 1972 and 1994. This increase resulted from falling load factors (persons per vehicle), which were greater than improvements in vehicle energy intensity. The only exception among industrialized countries was the US, where carbon intensities dropped from 55 kgC/passenger-km in 1972 to 46 kgC/passenger-km in 1994 (IEA, 1997c; Schipper et al., 1997a). Carbon intensity of freight travel, measured as the ratio of emissions to ton-km transported, rose slightly in a number of industrialized countries between 1972 and 1994, mostly because of modal shifts to more carbon-intensive trucks (Schipper et al., 1997b). As mobility increases in developing countries, transport emissions could rise dramatically. Ramanathan and Parikh (1999) indicate passenger traffic growth at 8% per year and train traffic growth at 5% per year for India. They found that efficiency improvements could reduce future energy demand by 26%. If, in addition, the modal split changes in favor of public transport modes, these authors estimate a 45% reduction in energy demand (Ramanathan and Parikh, 1999).
Fuels used to power transport are typically oil-based, except for rail, for which shifts toward electrified systems can lower carbon intensities depending upon the source of fuel for electricity generation in the country. In France, for example, the move toward electrified rail based on electricity generated by nuclear power led to lower carbon intensities (IEA, 1997c). Increased use of diesel engines can reduce CO2 emissions, but leads to greater emissions of other gaseous pollutants, such as N2O and carbon monoxide (CO). Use of alternative fuels, such as compressed natural gas, LPG, and ethanol, can significantly reduce CO2 emissions from transport (IEA, 1995).
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