InterVISTAS experimented with the use of two-stage
least squares techniques to improve the consistency of
elasticity estimates. The natural logarithm of distance
and the natural logarithm of fuel prices were used
separately and combined as potential IVs. In some
data sets, distance was found to be a worthwhile IV,
exhibiting high correlations with travel prices and low
correlations with traffic. However, there is some concern
that distance should be used as an explanatory variable
instead of an instrument (if route distance is believed
to have an impact on traffic). Fuel prices were found to
be poor IVs. Fuel prices exhibited low correlations with
traffic and travel prices.