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X. 1. Summary and Conclusions

The main goal of this report and the 1996 report is to analyze and measure the contribution of highway capital to private sector productivity growth. The approach developed here explicitly incorporates demand and supply forces, including the contribution of highway capital, which may affect productivity performance. We estimate the model using disaggregated data composed of 35-sectors in the U.S. economy for the period 1950-1991. The data include measures of gross output, material inputs (inclusive of energy), and private capital and labor. We also estimate demand and supply (cost) functions for each industry. We identify the determinants of productivity growth for each industry, including the contribution of highway capital, and we measure specifically the marginal benefit of highway capital to each industry.

To generate aggregate measures for the whole economy, we use a weighted sum of individual industry elasticities to obtain the aggregate elasticity measures for the whole economy. Using these estimates, we decompose total TFP growth into its various components and calculate the net social rate of return to highway capital. The rates of return on highway capital are compared to that of private sector capital.

In this report, we modify our 1996 analysis in several ways. First, we extend the sample period to cover the period 1950-91. The data set for the 35 sectors which comprises the entire US economy were revised significantly and in principle lead to a new set of estimates and results. Second, we explicitly introduce other infrastructure capital as another unpaid input in the production structure. We explicitly take into account the interaction of this type of capital with private sector inputs and highway capital at the industry and macroeconomy levels. Again, this innovation may lead to results different from those in our 1996 report.23 Finally, we use a different functional form for the cost function, i.e. a translog instead of the Normalized Symmetric MacFadden functional form. We do this because it is now easier to empirically evaluate a more complicated and expanded model.

The specific quantitative results of this report are briefly summarized as follows:

X. 2. Directions for Future Research

There are a number of important issues that require further research:

  1. In this study, we estimate the demand and cost function for each industry separately. A step forward would be to estimate the two functions jointly and explicitly account for cross equation restrictions.
  2. Finding measures to account for quality changes in highway capital stock and intensity of use of the capital stock are the other considerations for future research. The quality adjustments require a careful analysis of the highway capital stock series taking into account the vintage and depreciation of different components of the highway capital. As far as the utilization of the highway capital is concerned, the challenge is to find ways to convert the stock of highway capital to service measures. This requires adjustments for utilization of the highway capital taking account of factors such as congestion, intensity of use of highway capital by industry, and level of business activity. These adjustments could have important effects on the results reported. Finding proper adjustments for the degree of utilization of the highway network is challenging work. There are conceptual and measurement issues that need to be addressed. However, it is likely that the averages and time profile of the industry marginal benefits as well as the rate of return on highway capital will be substantially affected by adjustments for the degree of utilization of the highway capital stock.
  3. Another issue is to examine more closely the depreciation rate estimates that are used to generate the total highway capital. If the depreciation rates are not well measured then the results on marginal benefit, net social rate of return and productivity contribution of highway capital reported here will be affected. Analytical models are available to estimate the depreciation rate from available investment data.24 In addition, availability of data on maintenance expenditures and other relevant data may allow estimating a more precise measure of the depreciation rate and thus better measures of highway capital stock.
  4. A careful analysis is required to examine further the size and pattern of the implied taxes. What is important to note is that the benefits of highway capital vary across industries. The needs of different industries for highway services diverge over time and the degree of benefits of new highway capital expansion may differ considerably across industries. That is, public highway capital creates important distributional effects across industries. These effects need to be further examined to ascertain the sign and magnitudes of the industry marginal benefits. There are a number of possibilities to explore in the future. One possibility is to classify industries into more detailed categories to capture the inherent diversity within each sector. Another possibility is to refine highway capital data in such a way to incorporate the adjustments for quality and degree of congestion.
  5. Finally, in this study we have concentrated on the benefits of highway capital to the production side of the economy. The welfare benefits of highway capital services to the consumers have not been addressed in this report. The magnitude of these benefits are likely to be significant. Efforts to account for the total effect of highway capital requires modeling the consumption sector of the economy and integrating it with the production sector in a general equilibrium model. Such an attempt, though extremely important, at present remain outside the scope of our research.

Chapter 9 | Appendix 1

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