• When the network is operating, or is expected to operate, close to capacity,

• Where elasticity of demand with respect to travel cost is high, and

• Where implementation of a project causes large changes in travel costs.

Two other areas of interest to this study were mentioned in the report. First, with respect to freight travel, the Standing Committee noted the limited studies of the effects of road improvements on the distribution of freight (SACTRA 1994, 158). The available research suggests that the impact of new road capacity on freight is likely to be greater where transport costs constitute a large share of the total distribution expenditures of a company (SACTRA 1994, 159). However, the researchers caution that the role of highway improvements should not be “exaggerated ” because other factors also have a considerable impact on distribution system costs (SACTRA 1994, 159).

Second, new travel resulting from development induced by highway capacity additions is identified in the report as a major source of induced travel. However, the Standing Committee notes the considerable difficulty of separating development that can be attributed to the accessibility provided by the new or improved facility from development that would have occurred anyway because of other economic conditions (SACTRA 1994, 25). It also acknowledges the difficulty of distinguishing development that has simply been transferred from elsewhere in the region (thus resulting in a decline in traffic in those areas) from development that represents net new growth (and thus net new travel) in the region (SACTRA 1994, 25).

A number of researchers have used data for entire metropolitan areas (or large districts within them) to obtain models that predict vehicle miles of travel within these areas as a function of transportation system supply. Ruiter et al. (1979, 2-34) summarized results from several of these studies in the form of elasticities (see Table B-1).