economic conditions, and economic conditions influence transportation. Furthermore, all of these relationships shift with changes in technology, economic development, geographic changes, and many other factors.

Pioneering work by Ishaq Nadiri, Randall Eberts, David Aschauer, Alicia Munnell, Dale Jorgenson, and others has contributed important understandings about these relationships, but we are far from having an accepted, comprehensive model of the key relationships and how they work. As a result, any indicator of transportation and economic growth poses difficult issues of interpretation. Any index of transportation and economic growth must reflect this context of change and uncertainty.

In making public policy, it would be very useful to know the value of an extra dollar invested in transportation. It would be useful to know where, geographically and modally, to invest that dollar. It would also be useful to be able to track the transportation sector’s contribution to the gross domestic product (GDP). Consumers are probably most interested in price indexes—the price of gas, the price of cars, or the fares for different modes. Financial and business interests are interested in productivity measures.

Indexes of accessibility, impedance, bottlenecks, or congestion may have great value, and these features are closely tied to the economic impact of transportation. The economic indicators subgroup did not explore such physical measures in depth, however, although they may warrant consideration as mobility indexes. Perhaps transportation capacity utilization could be the basis of an index. Manufacturing capacity utilization has proven to be a closely watched macro indicator, and a similar measure for transportation may prove useful as well.

A good index should be simple, policy relevant, reliable, and timely. It is useful to compare several possible indexes in these respects. Table 4-1 compares these key attributes for seven indicators that the subgroup considered:

• Transportation prices (an index of the aggregate price of transportation service, possibly subdividable by mode or commodity),

• Transportation productivity (labor productivity or total-factor productivity),

• Contribution of transportation to economic growth,