Vehicle stock modelling of highway energy use Tunisian and US applications David L. Greene, Nabil Meddeb and Jin-Tan Liu Road transport is the major petroleum- using sector in many developing coun- tries. Influencing the size and efficiency of the vehicle stock is probably the most important area of energy policy for transport. This paper describes a model of vehicle stock evolution suit- able for estimating the effects of poli- cies directed at efficiency improve- ments, the volume of new registrations and fuel substitution. These are illus- trated by a projection of light trucks in Tunisia and an analysis of hypothetical conservation and fuel-switching initia- tives. The transferability of the model is demonstrated by an estimation of fuel savings due to automobile fuel eco- nomy gains since 1975 in the USA. The present value of savings is put at $92 billion (1984 $). Keywords: Energy demand; Road trans- port; Modelling David L. Greene is with the Transportation Group Energy Division, Oak Ridge Nation- al Laboratory, PO Box X, Oak Ridge, TN 37831, USA, Nabil Meddeb is at the Societe de Matrise de I'Energie, Tunis, Tunisia and Jin-Tan Liu is at Vanderbilt University, Nashville, TN, USA. This article arises from research spon- sored by the Office of Energy, US Agency for International Development and the Office of Transportation Systems, US De- partment of Energy. 1j. Dunkerley, I. Hoch and C. Bouhdili, Transport Energy: Determinants and Poli- cy, unpublished manuscript, Resources for the Future, Washington, DC, June 1985. 2D. Geltner, 'Transport and energy in continued on paqe 438 The major consumer of petroleum in developing countries is most often the transport sector. Transport alone typically accounts for 30%-50% of petroleum use, although the range of variation is considerable. Road transport usually comprises 80% of the sector's energy use (Table 1). Despite sharply higher fuel prices and periods of economic recession, transport energy use in developing countries increased over the decade 1971-1981, at rates from 1% a year for the lowest income countries to 3.5% a year for middle- and high-income developing countries, l Because vehicle ownership rates in the developing world are very low, their potential for further increase in conjunction with income growth is enormous. This significant and growing demand for petroleum poses economic problems for developing countries, whether they are impor- ters or exporters of petroleum. The development of effective energy conservation policies for the transport sector in developing countries has proved difficult because of the sector's diversity and dispersion. Policies considered have included short-run strategies for improving operating efficiencies, such as traffic control or vehicle maintenance, as well as long-run strategies for shifting traffic from more to less energy-intensive modes. 2'3 Perhaps the most important area of conservation policy, influencing the size, composi- tion, and efficiency of the vehicle stock, has not been adequately analysed. Policies to increase new car fuel economy have been particularly effective in the USA. The average miles per gallon of new cars nearly doubled over the 1974-1984 decade, 4 producing an estimated 20% increase in the efficiency of the total stock between 1980-1985 alone. 5 This paper presents a general tool for evaluating the effects of energy policies aimed at the highway vehicle stock and applies that tool using the stock of light trucks in Tunisia and cars in the USA. The two applications are intended to illustrate the versatility of the modelling approach. The essence of the method is a model of the evolution of vehicle stock which projects the distribution of vehicles by age. The key to the utility of the approach is the very large inertia in the capital stock of motor vehicles. 0301-4215/86/050437-10503.00 © 1986 Butterworth & Co (Publishers) Ltd 437