preferences that describe individual i. 

 SUBST_L = a measure of the price and availability of substitutes 

 to area j for individual i. 



The general specification shown is the linear functional 

 form. However, all results discussed below are for a double log 

 model, where dependent and independent variables are transformed 

 to natural log values. Generally, the linear model provides a 

 poor fit to travel cost data, as was the case with this study's 

 data. The double log model is used here because it provides a 

 good fit to the data, may correct for heteroscedasticity , and has 

 an obvious interpretation for differences in net economic values 

 across sites. 



Assumptions of the Trav el Cost Model 



As for any economic model, there are a number of assumptions 

 involved in estimating the Travel Cost Model. These assumptions 

 fall into three groups: the specific assumptions of the TCM, the 

 assumptions related to the basic economic theory of demand, and 

 the statistical assumptions invoked in estimating the demand 

 function . 



The basic assumption of the TCM is that, in a site-based 

 model of recreation demand, travel cost is equivalent to price. 

 Aside from this special assumption, the general problem of 

 modeling recreation demand is similar at the theoretical level to 

 modeling demand for any commodity. Two very general requirements 

 of an economic demand model are that 1) the variables are 

 correctly specified, and 2) the commodity being modeled is 



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