The first step involved with the TCM is the creation of a trip-generating function in order to provide a model of site’s use。 There are two basic forms of functions, namely zonal TCM (ZTCM) and inpidual TCM (ITCM)。 In ZTCM, visitors are grouped into zones of origin, and a demand function is estimated from the statistical relationship between the aggregate trips from a zone and the cost of travel。 In the ITCM, values are derived from the inpidual’s total trips and the distance that in- pidual travels。 The type of function determines the dependent variable, which is either the number of trips made by inhabitants of a given geographical zone, or the number of trips made by inpiduals。 In either case, the independent variables describe the cost of travel (e。g。 on- site costs, fuel, tolls, etc。), socio-economic characteristics of visitors (income, education, age, gender, etc。), characteristics of the site, substitute sites,  travel time and others。 There is no commonly accepted formula for the independent variables to include。 Everything that affects the dependent variable should be included, without harming the statistical integrity of the function。 In practice, however, both statistical problems and theoretical issues arise。 The trip-generating function is statistically determined by means of multiple regression analysis。 Then, this function is used in order to derive the relationship between visits and the price of admission, which is also referred to as surrogate demand curve [51]。 Once the demand curve has been established, the consumer surplus can be estimated, which is the total area below the demand curve given that, in fact, the price for entering the site is zero。

However, the method faces some difficulties, such as the following:

1。

It raises serious debates as far as the treatment of travel time is concerned。 Empirical evidences show that decisions about recreation are based not just on the cost of travel but also on the availability of time。 A usual approach is to value time at a fixed per- centage of the wage rate [52e54]。 However, there is considerable debate about this issue [44,55e58,59], which exceeds the scope of this analysis。

2。The alternative choices of visitors as well as the allocation of travel costs in the case of multipurpose trips may bias the results with respect to the recreation value of the site [4,60]。

3。In many cases, the value of the site is underestimated due to ‘‘zero cost’’ visitors (e。g。 children, elderly people, etc。) who usually travel with other members of their families。

4。The method is inapplicable when prospective changes in environmental quality are to be measured [41]。

Nevertheless, the method remains, for practical reasons, one of the most popular approaches for environmental valuation, and is frequently used by government agencies especially in the US and UK [52,53]。

3。4。2。Hedonic Pricing Method

The Hedonic Pricing Method (HPM) is based on the assumption that environmental attributes influence an inpidual’s decision on the consumption of the com- modity, such as buying or selling a house [61,62]。 Since its earliest uses in estimating the effect of environmental quality on housing values, HPM has been proven to be an effective tool towards this direction。 Hedonic models have been used in a variety of applications, such as: clean air, proximity to green areas, to view lakes and forests, and noise levels  [2,9,62e67]。

The theoretical framework of the method is simple。 Dwelling prices differ with respect to: (a) housing characteristics (square footage of the home, number of rooms, quality of accommodation, etc。), (b) neighbour- hood characteristics (level and quality of social in- frastructure, housing density, traffic, and presence of other facilities) and (c) the quality of the environment (air pollution, noise level, view, etc。)。 Other character- istics being equal, it would be reasonable to expect that properties in areas with better environmental quality enjoy higher prices。 Hence, if data on such characteristics are available, it is possible to isolate the contribution of the environmental factor to the market price of a house by means of econometric techniques。 In order to apply the method, data must first be collected on residential property  sales  in  the  region  under  investigation   for a specific time period。 The data required are as  follows: