Over the past 25 years, transport activity has grown at approximately twice the rate of energy efficiency improvements. Because the world’s transportation system continued to rely overwhelmingly on petroleum as an energy source, transport energy use and GHG emissions grew in excess of 2% per year. Projections to 2010 and beyond reviewed above reflect the belief that transport growth will continue to outpace efficiency improvements and that without significant policy interventions, global transport GHG emissions will be 50%–100% greater in 2020 than in 1995. Largely as a result of this anticipated growth, studies of the technical and economic potential for reducing GHG emissions from transport generally conclude that while significant reductions from business-as-usual projections are attainable, it is probably not practical to reduce transport emissions below 1990 levels by the 2010–2015 time period. On the other hand, the studies reviewed generally indicate that cost-effective reductions on the order of 10%–20% versus baseline appear to be achievable. In addition, more rapid than expected advances in key technologies such as hybrid and fuel cell vehicles, should they continue, hold out the prospect of dramatic reductions in GHG emission from road passenger vehicles beyond 2020. Most analyses project slower rates of GHG reductions for freight and air passenger modes, to a large extent reflecting expectations of faster rates of growth in activity.

Assessing the total global potential for reducing GHG emissions from transportation is hindered by the relatively small number of studies (especially for non-OECD countries) and by the lack of consistency in methods and conventions across studies. Not all studies shown in Table 3.16 cover the entire transportation sector, even of the countries included in the study. Most consider a limited set of policy options, (e.g., only motor vehicle fuel economy improvement). In general, the studies do not report marginal costs of GHG mitigation, but rather average costs versus a base case. Keeping all of these limitations in mind, Table 3.16 summarizes the findings of several major studies. For 2010, the average low GHG reduction estimate is just under 7% of baseline total transport sector emissions in 2010, with the higher estimates averaging a 17% reduction. There is, however, considerable dispersion around both numbers, indicative both of uncertainty and differences in methodology and assumptions. For studies looking ahead to 2020, the average low estimate is 15% and the average high estimate is 34% of baseline 2020 transport sector emissions. Estimated (average rather than marginal) costs are generally negative (as much as -US$200/tC), indicating that fuel savings are expected to outweigh incremental costs. There are some positive cost estimates as high as US$200/tC, however. The majority of the studies cited in Table 3.16 are based on engineering-economic analyses. Some argue that this method tends to underestimate welfare costs because trade-offs between CO₂ mitigation and non-price attributes (e.g., performance, comfort, reliability) are rarely explicitly considered (Sierra Research, Inc., 1999).

Table 3.16: Estimates of the costs of reducing carbon emissions from transport based on various studies, 2010-2030 (Brown et al., 1998; ECMT, 1997; US DOE/EIA, 1998; DeCicco and Mark, 1998; Worrell et al., 1997b; Michaelis, 1997; Denis and Koopman, 1998)
Study	Year of publication	Application	Year of scenario	Years in future	Country	Low (MtC)	High (MtC)	Low (%)	High (%)	Low	High
OECD Working Paper 1	1997	Light-duty road vehicle efficiency	2010	13	OECD	50	150	2.5	7.5	US$0	US$0
US National Academy of Sciences	1992 1992	Vehicle efficiency System efficiency	2010 2010	18 18	USA USA	20 3	79 13	3.2 0.5	12.7 2.1	-US$275 -US$183	-US$77 US$18
US DOE 5-Lab Study	1997	Transport sector	2010	13	USA	82	103	13.2	16.6	-US$157	US$6
US Energy Information Administration	1998	Transportation sector	2010	12	USA	41	55	6.6	8.9	-US$121	US$163
Tellus Institute	1997 1997	Transportation efficiency Transportation demand reduction	2010 2010	13 13	USA USA	90 61	90 61	14.5 9.8	14.5 9.8	-US$465 US$0	-US$465 US$0
ACEEE	1998	Transport sector	2010	12	USA		125		22.6	-US$139
US DOE, Clean Energy Futures	2000	Transport sector	2010	10	USA	20	66	3.2	10.5	-US$280	-US$144
European Council of Ministers of Transport	1997 1997 1997 1997 1997 1997 1997 1997	Transport sector Transport sector Transport sector Transport sector Transport sector Transport sector Transport sector Transport sector	2010 2010 2010 2010 2010 2010 2010 2010	13 13 13 13 13 13 13 13	Austria Belgium Czech R. Netherl. Poland Slovakia Sweden UK	2 4 6 11 5 1 4 22			8.3 13.3 57.1 37.2 12.8 16.3 23.2 14.3
Summary for 2010					Minimum/maximum average			0.5 6.7	57.1 16.9	-US$465 -US$153	US$163 -US$62
Denis and Koopman	1998 1998 1998	Road pricing CO2 tax Purchase subsidy + CO2 tax	2015 2015 2015	17 17 17	EU EU EU				25.0 13.0 14.0	US$0	US$0
US Congress OTA	1991	Transportation efficiency	2015	24	USA		195		29.2	-US$180	US$195
Summary for 2015					Minimum/maximum average			13.0	29.2 20.3	-US$180	US$195
US DOE, Clean Energy Futures	2000	Transport sector	2020	20	USA	58	163	8.3	23.4	-US$234	-US$153
ACEEE	1998	Transport sector	2020	22	USA		260		42.4	-US$164
United Nations	1997 1997 1997	Transport sector Transport sector Transport sector	2020 2020 2020	23 23 23	Industrialized Transitional Developing	153 72 297	423 126 450	14.9 18.2 28.4	41.2 31.8 43.1
OECD Working Paper 1	1997	Light-duty road vehicle efficiency	2020	23	OECD	100	500	4.3	21.7	US$0	US$0
Summary for 2020					Minimum/maximum average			4.3 14.8	43.1 34.0	-US$234
ACEEE	1998		2030	32	USA		401		58.8	-US$192

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